INDEX
S. No
Topic
Week 1
Page No
1
Introduction of welding
1
2
Classification of welding and joints
31
3
Parts of weld joint
62
4
Welding Symbol
103
5
welding power source 1
132
6
Welding power source 2
166
7
Welding Power sources characteristics-1
193
8
Welding Power sources characteristics-2
210
9
Physics of welding-1
227
Week 2
10
Physics of welding-2
255
11
Physics of welding-3
285
12
Physics of welding-4 (Arc Stability and Arc Blow)
313
13
Physics of welding-5 (Metal Transfer-1 )
327
14
Physics of welding-6 (Metal Transfer-2 )
365
Week 3
15
Physics of welding-7 (Metal Transfer-3 )
375
16
Physics of welding-8 (Metal Transfer-4 )
404
17
Physics of welding-9 (Metal Transfer-5 )
421
18
Physics of welding-10 ( Metalting Efficiency )
439
Week 4
19
Oxy-Fuel Gas Welding
475
20
Shielded Metal Arc Welding
507
21
Gas Tungsten Arc Welding
534
22
Gas Metal Arc Welding
558
23
Submerged Arc Welding
593
24
Welding Defects and Inspection
618
Fundamental of Welding Science and Technology
Module-1
Lecture 1: Introduction of Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
1
1
Contents
 Syllabus of the course
 Definition of welding
 History of welding
 Common welding base material
 General advantage of welding
 General disadvantage of welding
 Welding as compared to casting
 Welding as compared to riveting
 Practical applications of welding
2
2
Syllabus of the Course
Week
1
2
3
Module name and contents to be covered
Introduction and classification of welding:
i. Introduction ii. Classification of welding processes
iii. Type of welding joints iv. Type of edge preparation.
Nomenclature and symbol of welding joints:
i. Welding joint design ii. Different types of
nomenclature of welding joints ii. Welding symbols
No. of
lectures
planned
2
2-3
Power source of welding:
i. Types of power source and their characteristics.
2
4
Physics and principle of arc welding:
i. Welding heat sources ii. Arc initiation iii. Type of arc
iv. Forces affecting the arc and metal transfer v. Arc
blow.
3
3
3
Syllabus of the Course cont...
Week
5
6
7
8
Module name and contents to be covered
No. of
lectures
planned
Different type of welding methods and their details:
i. Oxy fuel gas welding ii. SMAW iii. GTAG
2-3
Different type of welding methods their details:
i. GMAG ii. SAW iii. ESW
3
Different type of welding methods their details:
i. EGW ii. Resistance spot welding, iii. Friction welding,
iv. PAW
3
Welding defects and inspection:
i. Different types of welding defects ii. Destructive & non
destructive testing.
2
4
4
Reference/Text Books
 V. M. Radhakrishnan, Welding Technology and Design, New age.
2002.
 J. A. Goldak, Computational Welding Mechanics, Springer 2005.
 O. Grong, Metallurgical Modelling of Welding, 2nd Ed. IOM
publication , 1997.
 L-E Lindgren, Computational Welding Mechanics, Woodhead
Publishing Limited, 2007.
 Dr. O. P. Khanna, Welding Technology, Reprint: 2002.
 A. O. Brien, Welding Handbook: Welding Processes, Part 1, Vol.2,
AWS,2004.
 J. F. Lancaster (Ed), The Physics of welding, Pergamon, 1986.
 R.W. Messler, Principles of Welding, John Wiley and Sons,1999.
5
5
Introduction
 Welding: In general, it is a process of joining two material plates
and make an integrated one.
 The large bulk of materials that are welded are metals and their
alloys. The welding is also applied to the joining of other material
such as thermoplastics.
 In welding heat is supplied either by electrical arc or by a gas
torch or by some other means.
 The most essential requirement is Heat but in some processes
Pressure is also employed.
6
6
History of Welding
 Middle Ages:
Blacksmiths of the Middle Ages welded various types of iron tools by
hammering. The welding methods remained more or less unchanged
until the dawn of the 19th century.
 Late 19th Century
 Engineers/scientists apply advances of electricity to heat and join
metals (Joule, Le Chatelier, etc.)
 Early 20th Century
 Prior to 1st World War welding was not trusted as a method to
join two metals due to crack issues.
 1930’s & 40’s
 Industrial welding gains acceptance and is used extensively in the war
effort to build tanks, aircraft, ships etc. The use of welding in today’s
technology is extensive. It is a remarkable rise since about 1930. 7
7
History of Welding
 19th Century (1800): In this century, major weldings were made.
1830:
Englishman Edmund Davy discovered acetylene in 1836 and
acetylene was soon utilized by the welding industry.
1880:
In 1881, French scientist Auguste De Meritens succeeded in fusing
lead plates by using the heat generated from an arc.
1890:
During the 1890's, one of the most popular welding methods was
invented i.e. carbon arc welding. In this time, thermite welding
was also invented in 1893.
8
8
History of Welding (cont.)
 20th Century (1900):
1900:
 Coated metal electrode was first introduced by Strohmenger. A
coating helped the arc to be much more stable.
 A number of other welding processes were developed during this
period i.e. seam welding, spot welding, flash butt welding, and
projection welding.
1919:
 After the end of World War I, the American Welding Society was
established by Comfort Avery Adams. The aim of the society was
the advancement of welding processes.
1920:
 Automatic welding was first introduced which was invented by P. O.
Nobel. In 1920, an early predecessor of GMAW was invented by P.
9
O. Nobel of General Electric.
9
History of Welding (cont.)
1930: The New York Navy Yard developed stud welding. Stud
welding was increasingly used for the construction industry and also
for shipbuilding.
1940: The GTAW was another significant milestone in the history
of welding which was developed in Battelle Memorial Institute in
1948.
1960: There were several advancements in the welding industry
during the 1960's. Electroslag welding and Plasma arc welding
were invented during this time.
1990: In 1991, Welding Institute invented FSW. It is a solid state
joining process which utilizes frictional heat of a rotating tool and
stirring effect of the tool probe for solid state joining.
 The use of welding in today’s technology is extensive. This growth
10
is faster than the general industrial growth.
10
Common Welding Base Material
 Metals can be classified as:
1. Ferrous
2. Non-ferrous Material
1. Ferrous materials finding day-to-day welding application are:
i) Wrought Iron (Less than 0.035% Carbon)
ii) Cast Iron [Carbon and Silicon % are: 2.3 to 4.5% and 0.5 to 3%
respectively)
iii) Carbon Steel [Low (0.05–0.3%), Medium (0.30–0.59%) and
High (0.6–1.5%)]
iv) Cast Steels [Carbon content between 0.2 to 2.1% by weight,
depending on the grade, also other alloying elements manganese,
chromium, vanadium, and tungsten]
v) Stainless steel [More than 11.5% chromium], etc.
11
11
Common Welding Base Material cont..
2. Non-Ferrous materials finding day-to-day welding application
are:
i) Aluminium and its alloys
ii) Copper and its alloys
iii) Magnesium and its alloys
iv) Nickel and its alloys
v) Zinc and its alloys,
12
12
General Advantage of Welding
 Advantages:
A good weld is as strong as the base metal.
General welding equipment is not very costly.
Portable welding equipment is also available.
Welding permits considerable freedom in design.
A large number of metals/ alloys both similar and dissimilar can
be joined by welding.
Welding can join workpieces by spots, as continuous pressure
tight seams, end-to-end and in a number of other configurations.
Welding can be mechanized.
13
13
General Disadvantage of Welding
 Disadvantages:
 Welding gives out harmful radiations (light), fumes and spatter.
 Welding results in residual stresses and distortion of the workpieces.
 Jigs and fixtures are generally required to hold and position the parts
to be welded.
 A skilled welder is a must to produce a good welding job.
 Welding heat produces metallurgical changes. The structure of the
welded joint is not same as that of the parent metal.
 A welded joint, for many reasons, needs stress-relief heat-treatment.
14
14
Welding as compared to casting
 Machine tool beds which were earlier cast are now fabricated
using welding. In many fields welding has replaced casting
processes.
 Some of the reasons for the same as follow:
 Welding is more economical and is a much faster process as
compared to casting.
 Fabricated mild steel structures are lighter as compared to cast ones.
 Fabricated mild steel structures have more tensile strength and
rigidity as compared to cast ones.
 Cost of pattern and storing is eliminated.
15
15
Welding as compared to casting cont.
 As compared to casting fewer persons are involved in a welding
fabrication.
 Structural shapes not easily obtainable with casting can be
produced by welding without much difficulty.
 Welding design involves low costs and it is very flexible also.
 Fabrication by welding saves machining cost involved in cast part.
16
16
Welding as compared to riveting
 Bridges, ships and boilers which were previously riveted are now
welded.
 Welding is more economical and is a much faster process as
compared to riveting.
 Welded pressure vessels are more pressure tight as compared to
riveted ones.
 Ratio between weight of weld metal and the entire weight of
structure is much lesser than the ratio between the weight of rivets
and entire weight of the structure.
 Cover plates, connecting angles, gusset plates, etc., needed in
riveted construction are not required when welding the structures.
17
17
Welding as compared to riveting cont.
 Members of such shape that present difficulty for riveting can
be easily welded.
Welding can be carried out at any point on a structure, but,
riveting always requires enough clearance to be done.
A welded structure possesses a better finish and appearance than
the corresponding riveted structure.
Layout for punching and drilling of holes is not required in
welding.
18
18
Welding as compared to riveting cont.
Drilling holes in the plate in order to accommodate rivets, breaks
material continuity and weakens a riveted structure.
Making changes in an already cast or riveted structure is extremely
difficult, if not impossible. On the other hand a welded structure can
be repaired without much difficulty.
Welding can produce a 100% efficient joint which is difficult to
make by riveting.
Riveting high strength steels presents the problems of acquiring high
strength steels rivets.
19
19
Practical Applications of Welding
 Welding has been employed in industry as a tool for:
 Regular fabrication of automobile cars, air-crafts, refrigerators,
ships, offshore structure etc.
 Repair and maintenance work, e.g., joining broken parts, rebuilding
worn out components, etc.
 A few important applications of welding are listed below:
i. Aircraft construction :
(a) Welded engine mounts.
(b) Turbine frame for jet engine.
(c) Rocket motor fuel thanks.
(d) Fittings, etc.
ii. Automobile construction :
(a) Arc welded car wheels.
(b) Frame side rails.
(c) Automobile frame, brackets etc.
20
20
Practical Applications of Welding (cont.)
iii. Bridges:
(a) Pier construction.
(b) Section lengths etc.
iv. Buildings:
(a) Column base plates.
(b) Trusses.
(c) Erection of structure, etc.
v. Pressure vessels and tanks:
(a) Clad and lined steel plates.
(b) Shell construction.
(c) Joining of nozzles to the shell, etc.
vi. Storage tanks:
(a) Oil, gas and water storage tanks.
vii. Rail road equipment:
(a) Rail
(b) Under frame
(c) Air receiver
(d) Engine etc.
21
21
Practical Applications of Welding (cont.)
viii. Piping and pipelines:
(a) Rolled plate pipings.
(b) Open pipe joints.
(c) Oil, gas and gasoline pipe lines, etc.
ix. Ships:
(a) Shell frames.
(b) Deck and bulkhead stiffeners.
(c) Girders to shells etc.
x.Trucks and trailers.
xi. Machines tool frames, cutting tools and dies.
xii. Household and office furniture.
22
22
Practical Applications of Welding (cont.)
 In addition, arc welding finds following applications in repair
and maintenance work:
 Repair of broken & damaged components and machinery such as
tools, punches, dies, gears, press and machine tools frames.
 Fabrication of jigs, fixtures, clamps and other work holding
devices.
 Being noiseless as compared to riveting, welding find extensive
use, when making modifications, addition or extension in hospital
buildings.
23
23
Next Lecture Outline
• Classification of Welding and Joints
24
24
Classification of Welding Process
25
25
Joining Process
 There are basically two types of joining process:
1. Mechanical Bonding
2. Atomic Bonding
1. Mechanical Bonding classification:
a) Temporary (With Screw Elements)
b) Permanent/ Semi-permanent
i) Rivets
ii) Stitches
iii) Staples
iv) Shrink-fit
2. Atomic Bonding classification: Welding
26
26
Classification of Welding
 Welding classification can be done based on the following aspects:
1. Depending upon the source of heat
2. Depending upon the application of pressure
3. Depending upon the different phages of base and filler material
4. Depending upon the composition of the joint
5. Depending upon the position of electrode
6. Depending upon the mechanism
27
27
Types of Welding
 Depending upon the source of heat:
(i). Arc welding
(ii). Gas welding
(iii). Resistance welding
(iv) Thermo-chemical welding process
(v) Mechanical energy welding process
(vi) Radiant energy welding process
28
28
Types of Welding
 Different welding techniques name (depending on source of heat):
•
Projection
(i). Arc welding
•
Percussion
•
•
•
•
•
•
•
•
Carbon arc (CAW)
Metal arc (SMAW)
Tungsten inert gas(TIG/GTAW)
Metal inert gas (MIG/GMAW)
Plasma arc (PAW)
Submerged arc (SAW)
Electro-slag (ESW)
Electro gas(ESW)
•
(iv) Thermo-chemical welding
process
•
•
•
•
•
•
•
•
•
•
•
Oxy-acetylene
Air-acetylene
Oxy-hydrogen
Pressure gas
(iii). Resistance Welding
Butt
Spot
Seam
Thermit welding
Atomic hydrogen welding
(v) Mechanical energy welding
process
(ii). Gas Welding
•
•
•
•
Flash Butt
Friction
Ultrasonic
Diffusion
Forge
Roll
Explosive
(vi) Radiant energy welding
process
29
•
•
Electron-beam (EBW)
Laser (LBM)
29
Next Lecture Outline
• Classification of Welding and Joints
30
30
Fundamental of Welding Science and Technology
Module-1
Lecture 2: Classification of Welding and Joints
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
31
Contents
 Classification of welding
 Types of weld joints
 Types of edge preparations
 Shape & name of different weld
32
Joining Process
 There are basically two types of joining process:
1. Mechanical Bonding
2. Atomic Bonding
1. Mechanical Bonding classification:
a) Temporary (With Screw Elements)
b) Permanent/ Semi-permanent
i) Rivets
ii) Stitches
iii) Staples
iv) Shrink-fit
2. Atomic Bonding classification: Welding
33
Classification of Welding
 Welding classification can be done based on the following aspect:
1. Depending upon the source of heat
2. Depending upon the application of pressure
3. Depending upon the different phages of base and filler material
4. Depending upon the composition of the joint
5. Depending upon the position of electrode
6. Depending upon the mechanism
34
Types of Welding
 Depending upon the source of heat:
(i). Arc welding
(ii). Gas Welding
(iii). Resistance Welding
(iv) Thermo-chemical welding process
(v) Mechanical energy welding process
(vi) Radiant energy welding process
35
Types of Welding
 Different welding techniques name (depending on source of heat):
•
Projection
(i). Arc welding
•
Percussion
•
•
•
•
•
•
•
•
Carbon arc (CAW)
Metal arc (SMAW)
Tungsten inert gas(TIG/GTAW)
Metal inert gas (MIG/GMAW)
Plasma arc (PAW)
Submerged arc (SAW)
Electro-slag (ESW)
Electro gas(ESW)
•
(iv) Thermo-chemical welding
process
•
•
•
•
•
•
•
•
•
•
•
Oxy-acetylene
Air-acetylene
Oxy-hydrogen
Pressure gas
(iii). Resistance Welding
Butt
Spot
Seam
Thermit welding
Atomic hydrogen welding
(v) Mechanical energy welding
process
(ii). Gas Welding
•
•
•
•
Flash Butt
Friction
Ultrasonic
Diffusion
Forge
Roll
Explosive
(vi) Radiant energy welding
process
36
•
•
Electron-beam (EBW)
Laser (LBM)
Types of Welding (cont.)
 Depending upon the application of pressure (2-catagories):
• Pressure welding
The pieces of metal to be joined are heated to a plastic state
and forced together by external pressure. This is also known as
Plastic Welding.
• Non-Pressure welding or Fusion welding
The material at the joint is heated to a molten state and
allowed to solidify.
37
Types of Welding (Cont.)
38
Types of Welding (cont.)
 Depending upon the different phages of base and filler material
(3-catagories):
Welding
Liquid state
Welding
Solid state
Welding
Solid-liquid state
Welding
Soldering &
All fusion welding






Friction
Ultrasonic
Diffusion
Forge
Roll
Explosive
39
Brazing
Types of Welding (Cont.)
 Depending upon the composition of the joint (3-catagories):
1. Autogeneous
2. Homogeneous
3. Heterogeneous
1. Autogeneous welding: No filler material is added during this joining.
Ex.: All type of solid phase welding, resistance welding and nonconsumable welding.
2. Homogeneous welding: The composition of filler material used during
this joining is same as the parent material.
Ex.: Arc, Gas and Thermit welding.
3. Heterogeneous welding: The composition of filler material used
during this joining is different as the parent material.
Ex.: Soldering and Brazing.
40
Types of Welding (Cont.)
 Depending upon the position of welding (5-catagories):
i) Downhand or flat
ii) Horizontal
iii) Vertical down
(a) For Butt welding
iv) Vertical up
v) Overhead
41
(b) For Fillet welding
Types of Welding (Cont.)
 Depending upon the mechanism (3-catagories):
1. Manual Welding:
In manual welding both feeding of filler material and
welding speed are controlled manually. Example: SMAW.
2. Semi-automatic:
In this welding either feeding of filler material or welding
speed is controlled automatically. Example: MIG, TIG etc.
3. Fully automatic:
In this welding both feeding of filler material and welding
speed is controlled automatically. Example: SAW, Laser, EB
etc
42
Type of Weld Joints
43
Type of Weld Joints
 There are five basic joints:
i) Butt joint
ii) Tee joint
iii) Lap joint
iv) Corner joint
v) Edge joint
44
Edge Preparation of Weld Joints
 Edge preparation or edge shaping may be applied to each piece
(joint member) in the same way, or combinations of the joint
preparations may be used.
 The edge preparation for welding these joints depends on the
strength requirements and other design considerations.
 The most common edge preparations as shown below:
 Square Edge
 Bevel Edge
 Double Bevel Edge
 Chamfer Edge
 'J' Groove Edge
 Double 'J' Groove Edge
45
Fig. Common edge preparations
Different Butt Joints and Edge Preparation
 A butt joint is formed when the pieces to be welded are laid side by
side.
 This is one of the most widely used types of joint.
Note: The bevel groove and V grooves are easier to prepare and are
used more than the J or U groves.
46
T Joints
 A T joint is formed when one piece of metal to be welded is placed
vertically on another piece lying horizontally, to form the shape of an
inverted ‘T’.
 The vertical member is usually placed at 90 deg. to the horizontal
member.
 The most widely used types of weld applied to a T joint is the fillet
weld. The weld resembles a Triangle when viewed from the side.
47
Different T Joints and Edge Preparation (cont.)
 The tee joint’s vertical piece can be also prepared using a Bevel, or J
shape.
48
Different Lap Joints
 A Lap joint is formed when one piece to be welded is laid down and
another piece is overlapped to form an edge for fillet welding or an
area to allow plug or slot welding.
 A plug weld is made by welding holes evenly spaced across the
length of one or both sides of the joint.
 A slot weld is similar except slots are made instead of holes.
 Other welds may be applied to laps such as projection, or seam
welding.
49
Different Corner Joints and Edge Preparation
 A corner joint is formed by placing one piece to be welded on the
other so that a corner is formed. It may be Flush; Half Open; or
Fully Open.
 For proper penetration and strength, an edge preparation may be
applied to one, or both of the pieces of the joint.
50
Different Edge Joints and Edge Preparation
 An Edge joint is formed when the two edges of the pieces to be
welded come together.
 This joint may be formed as a result of another structural shape and
is not as widely used as like the other type of joints.
51
Shape & name of different weld
52
Shape & name of different weld cont.
53
Shape & name of different weld cont.
54
Shape & name of different weld cont.
55
Shape & name of different weld cont.
56
End
57
Next Lecture Outline
Different Parts of a Joint and Joint Design
58
Introduction
 It may be highly essential to describe the exact joint design.
• Once it is possible to identify the types of joints then can be
able to identify individual features that make up the joint
geometry for a particular joint.
• The features and elements of the welded joint are often
necessary variables in welding procedure.
• Welding expert may required to apply this knowledge once in
the industry for manufacturing of welded product.
59
Parts of a weld
• Joint root
• Groove face
• Root face
• Root edge
• Root opening
• Bevel angle
• Groove angle
• Groove radius
60
Detail Nomenclature of Butt & Fillet Joint
 Nomenclature of Butt Joints:
 Nomenclature of T-Joints:
61
Fundamental of Welding Science and Technology
Module 1
Lecture 3: Parts of weld joints
and welding symbol
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
62
Contents
 Different parts of a weld joint
 Extra necessary information of weld joint
 Welding symbol
63
2
Introduction
 It may be highly essential to describe the exact joint design.
• Once it is possible to identify the types of joints then can be
able to identify individual features that make up the joint
geometry for a particular joint.
• The elements of the welded joint are highly necessary
variables in welding procedure.
• Welding expert may required to apply this knowledge once in
the industry for manufacturing of welded product.
64
3
Parts of a weld
• Joint root
• Groove face
• Root face
• Root edge
• Root opening
• Bevel angle
• Groove angle
• Groove radius
65
4
Joint root
 Joint root: It is that portion of a joint to be welded where the
members are closest to each other.
• The joint root may be either an area, or a line.
Joint roots as shaded areas
Joint roots as line
66
5
Groove Face, Root Face and Root Edge
• Groove face: It is all the
surface of a member
included in the groove.
• Root face is that portion of
the groove face within the
joint root.
• Root edge: It is a root face
of zero width.
67
6
Bevel angle and Groove angle
•Bevel: It is an angular edge
preparation.
• Bevel angle: It is the angle
between the bevel of a joint
member
and
a
plane
perpendicular to the surface of
the member.
• Groove angle: It the total
included angle of the groove
between two members.
68
7
Root opening and Groove Radius
•Root opening: It is the separation
between the work pieces at the joint
root.
• Groove radius: It is an arc radius
of a weld joint which applies only to
J & U-groove welds. It is “the
radius used to form the shape of a Jor U- groove weld.”
69
8
Detail Parts of Butt Joint
 Parts of Butt Joints (Nomenclature ) :
70
9
Different Parts of T Joint
 Parts of T-Joints (Nomenclature ) :
71
10
Extra necessary information of weld joint
72
11
Extra necessary information
 Extra information is necessary to describe the exact joint design:
• For a single-bevel-groove-weld, the bevel angle and the
groove angle are equal.
•
In case of a J- or U- groove weld”, the weld configuration is
normally specified by both an angle and a radius.
73
12
Single and double sided edge preparations
Note: Single sided preparations
are normally made on thinner
materials, or when access form
both sides is restricted.
 A single-V butt joint is also
more simpler to prepare.
Note: Double sided preparations
are normally made on thicker
materials, or when access form
both sides is unrestricted.
74
13
Single and double sided edge preparations
 Note: A double-V butt joint will cut down the amount of welding
by almost one half in comparison to a single-V butt joint.
 A double-V butt joint reduces the amount of distortion.
 A double-V butt may require the joint to be turned over to
complete the other side and access to both sides is required.
75
14
Advantages and disadvantages J & U groove
 Advantages: ‘J’ and ‘U’ preparations give a more uniform and even
distribution of weld metal throughout the depth of the joint and
reduce distortion and residual stresses.
• On thicker sections the ‘U’ and ‘J’ preparations require less weld
metal again reducing distortion and saving welding cost. It may also
be used as double preparations.
 The disadvantages of these preparations is that they require costly
machining and may suffer from lack of side wall fusion.
76
15
Root gap vs. the groove angle
 The root gap is used for electrode accessibility to the root of the
joint, the smaller the groove angle larger the root gap is required to
achieve good penetration.
 The root gap must be increased as the groove/bevel angle decreases.
77
16
Single-V with backing bar/strip
• A backing strip or backing bar is used mainly to support the root
and to prevent burn through of weld joint.
• May be used for large root gaps and reduced groove angles.
• It can also allow for a feather edge to be used, no root face required.
• A backing strip usually forms part of the weld and a backing bar is
usually removable.
78
17
Transitioning
• Transitioning is carried out to reduce the wall thickness on a
joint that has two different plate/pipe thickness to match the
thickness of the thinner plate/pipe.
• The transition may be applied by a pneumatic beveling machine
or by a disc grinder.
• The transition may be applied to the inside or out side of the
joint, in the case of a pipeline it is normally applied to the
inside.
79
18
Transitioning
• Abrupt changes in material thickness, causes stress concentrations
and low fatigue strength.
• A smooth transition is required to reduce the chances of fatigue
cracking.
• A taper of less than 1 in 4 is recommended for maximum fatigue
strength.
Note: 1 in 4 taper means1/4 unit reduction of radius per 1 unit length
Ex.: 1/4 inch reduction of radius per 1 inch length 19
80
Transitioning
• Joints a) and b) are the most common types of transitioning.
81
20
Generalized Welding Symbol
82
21
Generalized Welding Symbol
83
22
Generalized Welding Symbol Contd….
84
23
Weld Symbols (Butt Joints)
85
24
Weld Symbol (T- Joints)
86
25
Weld Symbol (Corner Joints)
87
26
A few extra symbol parts
Some other welding symbols
88
27
Basic Weld Symbols
89
28
Example Welding Symbol
90
29
Example Problem
5/9 inch Fillet Weld
as shown
Start with Arrow Side
91
30
Example Problem Cont.…
3/5 inch Fillet Weld
as shown
Start with Arrow Side
92
31
Example Problem Cont…
 Reinforcement with Melt - Through
93
32
Multiple Reference Lines
 Indicates a sequence of operations
 First operation is closest to the arrow
94
33
Example Problem Contd…
 Depth of penetration (Arrow side) = 3/5’’
 Depth of penetration (Arrow side) = 3/5’’
Root opening = 1/14’’
 Groove angle (Arrow side) = 65°
 Groove angle (Other side) = 60°
 Fist weld specification is CJP
 2nd weld specification is GTSM
95
34
Extra Important Notes
 The Arrow
• A straight arrow is used for weld locations.
• A broken-arrow line is used for joint preparation and breaks
toward the piece that is to be beveled.
96
35
Example Problem Cont…
97
36
Extra Important Notes
No measurement on depth, the Bevel goes all the way to the other side
The throat measurement will be given in the left of the weld symbol
The effective throat measurement will be given in parentheses, left of the
37
weld symbol
98
Extra Important Notes
Surfacing and Hard facing Welds – Welds that are applied to
areas that need to be built up or need hard facing to prevent wear.
 The height of the weld will be indicated to the left of the weld
symbol.
99
38
Extra Important Notes
Back and Backing Welds:
 A Backing weld will be made on the opposite side of a groove
before the groove weld is made and will also appear on the
opposite side of the reference line. It will also be noted in the tail
as to be a Backing weld.
 A Back weld will be made on the opposite side of a groove weld
after the groove weld and will also appear on the opposite side of
the reference line. It will also be noted in the tail as to be a Back
weld.
100
39
Extra Important Notes
 Additional reference lines are used to present a sequence of
welds or operations to be preformed. Additional references can be
made in two ways, fist drawing another reference line or stacking
symbols.
101
40
End
102
41
Fundamental of Welding Science and Technology
Lecture 4:Welding symbol
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
103
Contents
 Generalized Welding Symbol
 Different examples of weld symbol
 Some important notes
104
2
Generalized Welding Symbol
105
3
Generalized Welding Symbol Contd….
106
4
Weld Symbols (Butt Joints)
107
5
Weld Symbol (T- Joints)
108
6
Weld Symbol (Corner Joints)
109
7
A few extra symbol parts
Some other welding symbols
110
8
Basic Weld Symbols
111
9
Example Welding Symbol
112
10
Example Problem
5/9 inch Fillet Weld
as shown
Start with Arrow Side
113
11
Example Problem Cont.…
3/5 inch Fillet Weld
as shown
Start with Arrow Side
114
12
Example Problem Cont…
 Reinforcement with Melt - Through
115
13
Multiple Reference Lines
 In case of a sequence of operations
 First operation is closest to the arrow
116
14
Example Problem Contd…
 Depth of penetration (Arrow side) = 3/5’’
 Depth of penetration (Arrow side) = 3/5’’
Root opening = 1/14’’
 Groove angle (Arrow side) = 65°
 Groove angle (Other side) = 60°
 Fist weld specification is CJP
 2nd weld specification is GTSM
117
15
Extra Important Notes
 The Arrow
• A straight arrow is used for weld locations.
• A broken-arrow line is used for joint preparation and breaks
toward the piece that is to be beveled.
118
16
Example Problem Cont…
119
17
Extra Important Notes
No measurement on depth, the Bevel goes all the way to the other side
The throat measurement will be given in the left of the weld symbol
The effective throat measurement will be given in parentheses, left of the
18
weld symbol
120
Extra Important Notes
Surfacing and Hard facing Welds – Welds that are applied to areas
that need to be built up or need hard facing to prevent wear.
 The height of the weld will be indicated to the left of the weld
symbol.
121
19
Extra Important Notes
Back and Backing Welds:
 A Backing weld will be made on the opposite side of a groove
before the groove weld is made and will also appear on the
opposite side of the reference line. It will also be noted in the tail
as to be a Backing weld.
 A Back weld will be made on the opposite side of a groove weld
after the groove weld and will also appear on the opposite side of
the reference line. It will also be noted in the tail as to be a Back
weld.
122
20
Extra Important Notes
 Additional reference lines are used to present a sequence of
welds or operations to be preformed. Additional references can be
made in two ways, like by drawing another reference line or
stacking symbols.
123
21
Welding Power Sources
124
22
Types of electric discharges
NON SUSTAINABLE
SUSTAINABLE
125
SUSTAINABLE
126
24
Introduction
 POWER SOURCES are apparatuses that are used to supply
current and voltage that are suitable for particular welding processes.
 Arc Welding Power Sources
 Arc welding requires that an electric arc be established between an
electrode and the workpiece to produce the heat needed for melting the
base plate.
 Because utility energy is not delivered at the proper voltage and
current, it must be converted to the required levels by the welding
power source.
 Arc power sources convert the customary 240 or 480 V alternating
current (ac) utility power to a range from 20 to 80 V and simultaneously
increase the current proportionately.
 Motor- or engine-driven welding generators are wound to deliver the
correct voltage and current directly; therefore, no transformer is
necessary.
127
Categories of Power Sources
 The conventional welding power sources (based on power supply):
Power Source
(i) Welding Transformer
(ii) Welding Rectifier
Supply
AC
DC
AC or DC (Depending on
generator)
(iii) Welding Generators
DC
(iv) Inverter type
128
Open-Circuit Voltage
 Open-Circuit Voltage (OCV): When no load is connected to the
output terminals of a welding power source, the voltage that appears at
the terminals is at its maximum.
 A high OCV value generally uses in arc starting and stability.
 In transformer-type power sources, OCV is established by the
incoming utility line voltage and the transformer primary-to
secondary turns ratio.
 The open circuit voltage normally ranges between 70-90 V in case
of welding transformers.
 In case of rectifiers it is 50-80 V.
 However, welding voltages are lower as compared to open circuit
voltage of the power source.
129
History of welding power units
 The welding power unit converts the high voltage of the mains
supply to a nonhazardous level. Figure below shows the historical
development of different welding power units.
130
Fig: Development of different welding power units.
End
131
29
Fundamental of Welding Science and Technology
Module 2
Lecture 4: Welding Power Sources
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
132
Contents
 Introduction
 Categories of power sources (for arc welding)
 Open circuit voltage
 History of power sources
 Brief discussion about power sources
133
2
Types of electric discharges
NON SUSTAINABLE
SUSTAINABLE
134
SUSTAINABLE
135
4
Introduction
 POWER SOURCES are apparatuses that are used to supply
current and voltage that are suitable for particular welding processes.
 Arc Welding Power Sources
 Arc welding requires that an electric arc be established between an
electrode and the workpiece to produce the heat needed for melting the
base plate.
 Because utility energy is not delivered at the proper voltage and
current, it must be converted to the required levels by the welding
power source.
 Arc power sources convert the customary 240 or 480 V alternating
current (ac) utility power to a range from 20 to 80 V and simultaneously
increase the current proportionately.
 Motor- or engine-driven welding generators are wound to deliver the
correct voltage and current directly; therefore, no transformer is
necessary.
136
Categories of Power Sources
 The conventional welding power sources (based on power supply):
Power Source
(i) Welding Generators
Supply
AC or DC (Depending on
generator)
(ii) Welding Transformer
DC
(iii) Welding Rectifier
AC
(iv) Inverter
DC
137
Open-Circuit Voltage
 Open-Circuit Voltage (OCV): When no load is connected to the
output terminals of a welding power source, the voltage that appears at
the terminals is at its maximum.
 A high OCV value generally uses in arc starting and stability.
 In transformer-type power sources, OCV is established by the
incoming utility line voltage and the transformer primary-to
secondary turns ratio.
 The open circuit voltage normally ranges between 70-90 V in case
of welding transformers.
 In case of rectifiers it is 50-80 V.
 However, welding voltages are lower as compared to open circuit
voltage of the power source.
138
History of welding power units
 The welding power unit converts the high voltage of the mains
supply to a nonhazardous level. Figure below shows the historical
development of different welding power units.
139
Fig: Development of different welding power units.
Motor-generator sets
 Motor-generator sets were popular for many years, and are still
sometimes used.
 High cost and poor efficiency made it difficult for them to compete
with modern welding power units.
 However, their welding characteristics can be excellent.
 They consist of a (3-phase) motor, directly coupled to a DC
generator.
 Welding generator power units driven by petrol or diesel engines are
still made, and fill a need: they are used at sites without a supply of
mains electricity.
140
Fig. Motor-Generator set.
Welding with AC power source
 AC is a popular choice for welding due to the fact that it uses a
simple and inexpensive power unit.
 Introducing AC does however lead to complications because unless
special steps are taken, the arc will extinguish on each zero
crossing.
 The need to re-ignite the arc also requires a sufficiently high opencircuit voltage, of at least 50 V, or more.
 The advantages of alternating current are reduced risk of magnetic
arc blow effect and good oxide-breaking performance when TIGwelding of aluminium.
141
The welding transformer
 Welding transformers provide alternating current (AC), and are
the cheapest and perhaps the simplest type of power unit.
 AC power sources for shielded metal arc welding (SMAW) can be
as simple as a single transformer.
142
Welding with AC power source cont.
 For A.C. welding, the power is always a transformer with a control
for current adjustment either by varying the inductance or by
changing the magnetic coupling between primary and secondary
windings of a transformer.
 For changing the inductance 3 different types of reactors are
available:
i) Tapped reactors
ii) Moving core reactors and
iii) Saturable reactors
 All the above designs provide good control of current and a suitable
output for SMAW and GTAW. The choice depends upon cost and
performances.
143
Welding with AC power source cont.
 Tapped reactors: These reactors consist of a copper cable wound
on a laminated core. The windings are provided with tapping
circuit. Here only a limited number of setting can be
accommodated.
144
Welding with AC power source cont.
 Moving core reactors: Here the increasing and decreasing of the
inductance of winding is done by moving a laminated core in or
out through a reactor coil.
145
Welding with AC power source cont.
 Saturable reactors: In this design, the welding current control is
achieved by putting saturable reactor unit in the secondary circuit.
 These reactors have better control and can be remotely controlled
also. These reactors are costly.
146
Welding with AC power source cont.
 Multi operator sets: Here one transformer provides 3 or 6 outlets.
 In this case, the current in each secondary circuit should be
independently controlled and a separate reactor must be included
in each lead.
147
AC power source cont.
 More advanced power units, for use with TIG, submerged arc and
occasionally MIG welding, can be controlled by thyristors or
transistors using square-wave switching technology.
 Newer technologies deliver a square wave output at line
frequency. A square wave eliminates peaking and provides a rapid
transition through zero, which is important to cyclic reignition of
the arc.
 Thyristors are employed
square current waveform.
with magnetic cores to generate the
Fig.(a) Square wave, line
frequency, and equal dwell.
148
AC power source cont.
 Adjustable imbalance permits the operator to control the ratio of
electrode positive (EP) to electrode negative (EN) current by dwell
extension.
Fig. Square wave with unbalanced dwells.
149
Welding with DC power source cont.
 Mainly 3 different types of Direct Current (DC) welding power
sources are there i.e.:
i. Rectifiers and
ii. Generators
iii. Inverters
150
DC power source: Rectifiers
 A traditional welding rectifier power source produces DC.
 A full-wave rectifier is used to convert the AC output from a
transformer into DC for welding.
 For a single-phase input in most of the applications some form of
smoothing is required.
 A three-phase input is usually preferred as it gives more uniform
load on main supply and smoothens the ripples, eliminating the
smoothening circuit.
 This can be given varying slopes, from straight to drooping, so that
the unit can be used with several different welding methods.
151
Fig. Ripple voltage from a full-wave
rectifier, before and after the
application of a smoothing capacitor.
DC power source: Rectifiers
 Simple three-phase full wave rectifier unit for welding:
Fig. Circuit diagram
Fig. Block diagram
152
Welding inverters
Inverter: In a inverter unit, the 50 Hz mains supply is first rectified
and then, using power semiconductors, is turned back into AC at a
higher frequency, usually in the range 5-100 kHz. Finally this higher
frequency AC convert to DC.
 This reduces the weight of the transformer and inductor,
making the power unit small and portable.
 Low losses result in high efficiency, to the order of 80-90 %.
 This units have excellent performance.
153
Fig. Inverter Block Diagram
Welding inverters (contd.)
 Inverter units offer the following advantages:
 Low weight and small size
 Good welding performance
 Several weld with high efficiency
The size of the transformer and inductor depend on the number of turns (N) and the
cross-sectional core area (A), both of which can be reduced.
This reduces the weight of the transformer and inductor to a fraction of what is
needed for a 50 Hz unit, making the power unit small and portable.
154
Reference/Text Books
 V. M. Radhakrishnan, Welding Technology and Design, New age.
2002.
 J. A. Goldak, Computational Welding Mechanics, Springer 2005.
 O. Grong, Metallurgical Modelling of Welding, 2nd Ed. IOM
publication , 1997.
 L-E Lindgren, Computational Welding Mechanics, Woodhead
Publishing Limited, 2007.
 Dr. O. P. Khanna, Welding Technology, Reprint: 2002.
 A. O. Brien, Welding Handbook: Welding Processes, Part 1, Vol.2,
AWS,2004.
 Md. Ibrahim Khan, Welding Science and Technology, New Age In.
 J. F. Lancaster (Ed), The Physics of welding, Pergamon, 1986.
 R.W. Messler, Principles of Welding, John Wiley and Sons,1999.
155
SOME NOTES
156
Criteria for selection of welding power source
 The following factors must be considered for selection a welding
power source:
 Initial cost of the power source.
 Periodic maintenance and repair cost.
 Availability of mains power supply: 220 V or 440 V.
 Steady output current even with input voltage fluctuation.
 Type of current needed AC or DC or both.
 Current rating required to accommodate all sizes of electrodes
needed for the jobs.
157
Criteria for selection of welding power source
 Machine’s ability to strike and maintain stable arc for the type of
electrodes to be used.
 Type of V-I characteristics (CC or CV) needed for the process
employed.
 Whether machine is required to give radiographic quality welds
and impact strength with the type of electrodes used.
 Whether the machine needs to serve several welding processes
expected to be used in the shop.
 Machine’s ability to stand shop environment (corrosive gases,
dust, moisture etc.).
 Need for remote current control.
158
Duty Cycle
 Duty cycle is the ratio of arcing time to the weld cycle time
multiplied by 100.
 Welding cycle time is either 5 minutes as per European
standards or 10 minutes as per American standard and
accordingly power sources are designed.
 If arcing time is continuously 5 minutes then as per European
standard it is 100% duty cycle and 50% as per American standard.
 At 100% duty cycle minimum current is to be drawn i.e. with the
reduction of duty cycle current drawn can be of higher level.
 The welding current which can be drawn at a duty cycle can be
evaluated from the following equation:
159
Duty Cycle
 Duty cycle and associated currents are important as it ensures that
power source remains safe and its windings are not getting
damaged due to increase in temperature beyond specified limit.
 The power source rating is also determined by its duty cycle,
which indicates for what proportion of a period of ten minutes
that the power source can be operated at the specified load.
 For example: 400 A at 35 % duty factor, means that the power
source can supply 400 A for 3.5 minutes in every ten minutes without
overheating.
160
Classification of Power Source on the basis of duty cycle
 The National Electrical Manufacturers Association (NEMA)
categorizes arc welding power sources into three classes on the
basis of duty cycle:
 CLASS I: Rated output at 60 (at 300A), 80, or 100% duty cycle
 CLASS II: Rated output at 30, 40, or 50 % (at 250A) duty cycle
 CLASS III: Rated output at 20% (at 225A) duty cycle
161
Classification of Power Source on the basis of duty cycle
 In Fig. curve A shows a NEMA
Class I (60%) 300 A rated machine
that is capable of a maximum 375 A
at reduced duty cycle (38%) and 232
A at 100% (continuous).
 Curve B represents a NEMA Class
II (50%) 250 A machine with a
continuous duty of 176 A.
 Curve C represents an engine-driven
machine rated at 225 A and 20%
duty. It does not offer output in
excess of its rating because of a
horsepower limitation of the engine.
162
Fig. Selected duty cycle (i) Curves.
A, 300 A, 60% Machine; (ii) B, 250
A, 50% Machine; (iii) C, 225 A,
20%
(Note: C-machine is Enginedriven machine)
Rated Current & Rated Voltage
 Rated current:
 The rated current is the current for which the power source is
designed.
 Note: Always check the technical data or the rating plate to make
sure what the actual value of rated current is.
 Rated voltage:
 IEC 974 (International electro-technical commission) specifies a standard
load line, for each value of rated current, there is a voltage which is known
as rated voltage.
 The relationships specified by IEC 974 differ from one welding method to another:
for currents up to 600 A, the voltages are as follows:
• MMA and SAW: U= 20 + 0.04 . I ,for currents up to 600 A &
For currents above 600 A: U = 44 V
• TIG: U= 10+0.04. I, for currents up to 600 A &
For currents above 600 A: U = 34 V
• MIG/MAG: U= 14+0.05. I, for currents
up to 600 A
163
For currents above 600 A: U = 44 V
Reference/Text Books
 V. M. Radhakrishnan, Welding Technology and Design, New age.
2002.
 J. A. Goldak, Computational Welding Mechanics, Springer 2005.
 O. Grong, Metallurgical Modelling of Welding, 2nd Ed. IOM
publication , 1997.
 L-E Lindgren, Computational Welding Mechanics, Woodhead
Publishing Limited, 2007.
 Dr. O. P. Khanna, Welding Technology, Reprint: 2002.
 A. O. Brien, Welding Handbook: Welding Processes, Part 1, Vol.2,
AWS,2004.
 Md. Ibrahim Khan, Welding Science and Technology, New Age In.
 J. F. Lancaster (Ed), The Physics of welding, Pergamon, 1986.
 R.W. Messler, Principles of Welding, John Wiley and Sons,1999.
164
End
165
Fundamental of Welding Science and Technology
Module 2
Lecture 5: Welding Power Sources
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
166
Categories of Power Sources Characteristics
 Power sources characteristics can be classified into two main
categories :
1. Static characteristics power sources
2. Dynamic or pulse characteristics power sources
 Based on the static characteristics power sources can be classified as
below:
 Constant current (CC) or
characteristic power source.
drooping
or
falling
 Constant potential or constant voltage (CV) or flat
characteristic power source.
 Combined characteristic power source.
167
Constant Current Power Supply
 The volt ampere output curves for constant current power source are called ‘drooper‘
because of substantial downward or negative slope of the curves.
 The power source may have open circuit voltage adjustment in addition to output current
control.
 With a change in arc voltage, the change in current is small.
 So, with a consumable electrode welding process, electrode melting rate would
remain fairly constant with a change in arc length.
168
Fig. Drooping or Constant
current or Falling
Characteristic
Constant Current Power Supply
 These power sources are required for processes using relatively thicker consumable
electrodes which may sometimes get stubbed to workpiece or
with non-consumable tungsten electrode where during touching of electrode for
starting of arc may lead to damage of electrode if current is unlimited.
 A drooping characteristic, as compared with a straight characteristic, also permits a
higher no-load voltage, which is in order to prevent the arc from extinguishing too
easily.
 These method generally used in manual metal arc welding.
169
Constant Voltage Power Supply
 If the voltage remains almost constant when it is loaded it is known as a constant
voltage or flat characteristic. Typically a voltage drop of 2-5 V is normal.
 A straight characteristic maintains good control of the arc length when welding with
methods involving a continuously fed filler wire, such as MIG or SAW.
 In this case, the current is determined by the speed of the filler wire (i.e. the quantity
of filler material being fed into the weld).
Fig : Constant Potential or 170
Constant Voltage or Flat
Characteristic.
Constant Voltage Power Supply (Contd.)
 With constant voltage power supply the arc voltage is established by setting the output
voltage on the source.
 The speed of electrode drive is used to control the average welding current.
 The use of such power source in conjunction with a constant electrode wire feed results in
a self regulating or self adjusting arc length system.
 Due to some internal or external fluctuation if the change in welding current occurs, it will
automatically increase or decrease the electrode melting rate to regain the desired arc length.
171
Self-regulation of the arc
 The point of intersection between the arc characteristic and the power unit load
characteristic is referred to as the working point.
 The working point at any particular time represents the welding current and voltage at
that time.
 If the arc length is to be stable, the power source characteristic must not slope too much.
Fig. How the slope of the power unit characteristic affects
the welding current if the arc length is altered.
Ex. MIG
172
Self-regulation of the arc
 Sometime it happens that, if the length of the arc is reduced, the voltage drops and
the current increases.
 The current increases from working point 1 to working point 2 if the slope of the
characteristic is slight (as in Fig. below).
 The increase in current raises the rate of melting of the electrode, and the arc
length is restored.
 This is known as the self-regulation characteristic of the arc length.
173
 Note: But only to working point 3 if the characteristic has a steep slope.
Combined characteristic power source
 This power source can provide both constant current (CC) and
constant voltage (CV) characteristic.
 Here the high voltage portion is CC characteristic
 Below a certain threshold voltage, the power source characteristic
switched to CV characteristic as shown in Fig. below.
 This power source is useful for SMAW process to assist the arc
stating and to avoid electrode sticking in the weld pool.
174
Fig. Combined characteristic power source
Power Source Selection
 Because no single power source is right for all welding situations,
it is necessary to know the processes to be used before selecting the
best power source.
Table: Power source selection relative to welding process
175
Dynamic characteristic
 With relatively slow changes in the arc, one can assume that the
working point follows the power unit static characteristic.
 However, in the case of more rapid changes, the dynamic
characteristics of the power unit increasingly determine how quickly
the current can change to suit.
 This is important, particularly when welding with short-circuiting
drop transfer.
 Power units for short arc welding usually incorporate an inductor in
their output.
 The action of the inductor is, if the voltage changes instantaneously,
as when a droplet of molten metal short-circuits the arc, the current
will rise much slower.
176
Dynamic characteristic
 Dynamic characteristic is the rapid transient variation of output
current and voltage.
 It occurs or this is important, particularly when welding with shortcircuiting drop transfer, arc starting and arc re-ignition.
 To cope up with these above conditions, power source should have
good dynamic characteristics to obtain stable and smooth arc.
 Power units for short-circuits arc welding usually incorporate an
inductor in their output.
 The action of the inductor is, if the voltage changes
instantaneously, as when a droplet of molten metal short-circuits the
arc, the current will rise much slower.
177
Dynamic characteristic cont..
 Therefore it is important that there should not be a current surge
during the short circuit, as this would result in high electromagnetic
forces that would cause spatter and oscillations on the surface of the
weld pool.
Fig. Welding current in short arc welding with
low inductance (top) and with high inductance
(bottom).
 Sometime it can be referred as Pulsed mode Power Supply.
178
Pulsed mode Power Supply
 It is a DC power source but in pulsing mode. Here current fluctuate in
a predetermined fashion not random manner.
 By applying pulse the metal deposition can be controlled.
 At background current no metal transfer occur.
 The background time is set for keeping the arc ignited.
 The background current is set in such a fashion so that the weld pool remain in
molten state.
 The peak pulse current is set based on the requirement of transition current.
179
 In AC arc get extinguished but here arc always on.
SOME NOTES
180
Duty Cycle
 Duty cycle is the ratio of arcing time to the weld cycle time
multiplied by 100.
 Welding cycle time is either 5 minutes as per European
standards or 10 minutes as per American standard and
accordingly power sources are designed.
 If arcing time is continuously 5 minutes then as per European
standard it is 100% duty cycle and 50% as per American standard.
 At 100% duty cycle minimum current is to be drawn i.e. with the
reduction of duty cycle current drawn can be of higher level.
 The welding current which can be drawn at a duty cycle can be
evaluated from the following equation:
181
Duty Cycle
 Duty cycle and associated currents are important as it ensures that
power source remains safe and its windings are not getting
damaged due to increase in temperature beyond specified limit.
 The power source rating is also determined by its duty cycle,
which indicates for what proportion of a period of ten minutes
that the power source can be operated at the specified load.
 For example: 400 A at 35 % duty factor, means that the power
source can supply 400 A for 3.5 minutes in every ten minutes
without overheating.
182
Classification of Power Source on the basis of duty cycle
 The National Electrical Manufacturers Association (NEMA)
categorizes arc welding power sources into three classes on the
basis of duty cycle:
 CLASS I: Rated output at 60 (at 300A), 80, or 100% duty cycle
 CLASS II: Rated output at 30, 40, or 50 % (at 250A) duty cycle
 CLASS III: Rated output at 20% (at 225A) duty cycle
183
Classification of Power Source on the basis of duty cycle
 In Fig. curve A shows a NEMA
Class I (60%) 300 A rated machine
that is capable of a maximum 375 A
at reduced duty cycle (38%) and 232
A at 100% (continuous).
 Curve B represents a NEMA Class
II (50%) 250 A machine with a
continuous duty of 176 A.
 Curve C represents an engine-driven
machine rated at 225 A and 20%
duty. It does not offer output in
excess of its rating because of a
horsepower limitation of the engine.
184
Fig. Selected duty cycle (i) Curves.
A, 300 A, 60% Machine; (ii) B, 250
A, 50% Machine; (iii) C, 225 A,
20%
(Note: C-machine is Enginedriven machine)
Rated Current & Rated Voltage
 Rated current:
 The rated current is the current for which the power source is
designed.
 Note: Always check the technical data or the rating plate to make
sure what the actual value of rated current is.
 Rated voltage:
 IEC 974 (International electro-technical commission) specifies a standard
load line, for each value of rated current, there is a voltage which is known
as rated voltage.
185
Rated Current & Rated Voltage
 The relationships specified by IEC 974 differ from one welding
method to another: for currents up to 600 A, the voltages are as
follows:
186
Wire, cooling system and insulation type
 The maximum current which can be drawn from a power source at
given a duty cycle depends upon size of winding wire, type of
insulation and cooling system of the power source.
 Generally at a given duty cycle, large diameter cable wire, high
temperature resistant insulation and force cooling system allow high
welding current drawn from the welding source.
 Insulation type
 The maximum allowable temperature of various components (primary
and secondary coils, cables, connectors etc.), depends on the quality
and type of insulation and materials of coils used for manufacturing of
power source. The insulation is classified as below:
 A, B, E, F and G in increase order of their maximum allowable
temperature 60, 75, 80, 100 and 125 deg. centigrade respectively.
187
Wire feed systems for constant arc length
 There are generally two types of feed systems are used to maintain
the arc length:
i) Constant speed feed drive system &
ii) Variable speed feed drive system.
 Constant speed feed drive system:
 Here the feed rollers rotating at fixed
speed are used for pushing/pulling
wire to feed into the weld so as to
maintain the arc length during welding.
 It is normally used with constant
voltage power sources in combination
with small diameter electrodes where
self regulating arc helps to attain the
constancy in arc length.
188
Wire feed systems cont…
 Variable speed feed drive system:
 In this case the feed rollers used for
feeding electrode wire are rotated at
varying speed as per need to maintain
the arc length during welding. Like:
SAW and GMAW processes.
 Fluctuation in arc length due to any
reason is compensated by increasing
or decreasing the electrode feed rate.
 Here the electrode feed rate is controlled by regulating the speed of
feed rollers powered by electric motor.
 Input power to the variable speed motor is regulated with the help of
sensor which takes inputs from fluctuations in the welding arc gap.
189
9.2.2 Power factor (pf)
Power factor of a power source is defined as a ratio of actual power (KW) used to
produce the rated load (which is registered on the power meter) and apparent
power drawn from the supply line (KVA) during welding. It is always desired to
have high power factor (pf). Low power factor indicates unnecessary wastage of
power and less efficient utilization of power for welding. Welding transformers
usually offer higher power factor than other power sources. However, sometimes
low power factor is intentionally used with welding transformers to increase the
stability of AC welding arc. The basic principle of using low power factor for better
arc stability has been explained in section 6.2.2. Application of a welding power
source with high power factor offers many advantages such as:
Reduction of the reactive power in a system, which in turn reduces the
power consumption and so drop in cost of power
More economic operations at an electrical installation (higher effective
power for the same apparent power)
Improved voltage quality and fewer voltage drops
Use of low cable cross-section
Smaller transmission losses
190
Setting the current and voltage
 When welding with coated electrodes, or when performing TIG welding, it is the
current that is set on the power unit. In this case the arc voltage depends on the
arc length of the welding.
 When welding with a continuously supplied filler wire, e.g. MIG/ SAW welding, it is
the voltage that is set on the power unit. The voltage then determines the length
of the arc.
 This is a result of the arc's self-regulation characteristic: if the welder raises the
welding torch, the arc length does not alter: instead, it is the wire stickout that alters.
 The current cannot be set directly: instead, it depends on the wire feed speed (and wire
diameter) used.
 The current, in other words, sets itself so that it is at just the value needed to melt the
filler wire at the same rate as the wire is fed out.
191
Relationship between current and rate of melting
Fig: The relationship between current and rate
of melting for MIG/MAG welding with normal
stickout.
Note: As self-regulation does not work very well with a drooping characteristic, an arc
voltage regulator is used to control the wire feed speed. As a result, the arc and the
192
arc length are kept constant.
Fundamental of Welding Science and Technology
Module 2
Lecture 7: Welding Power Sources
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
193
Categories of Power Sources
 The conventional welding power sources (based on power supply):
Power Source
(i) Welding Generators
Supply
AC or DC (Depending on
generator)
(ii) Welding Transformer
DC
(iii) Welding Rectifier
AC
(iv) Inverter
DC
194
AC power source cont.
 More advanced power units, for use with TIG, submerged arc and
occasionally MIG welding, can be controlled by thyristors or
transistors using square-wave switching technology.
 Newer technologies deliver a square wave output at line
frequency. A square wave eliminates peaking and provides a rapid
transition through zero, which is important to cyclic reignition of
the arc.
 Thyristors are employed
square current waveform.
with magnetic cores to generate the
Fig.(a) Square wave, line
frequency, and equal dwell.
195
AC power source cont.
 Adjustable imbalance permits the operator to control the ratio of
electrode positive (EP) to electrode negative (EN) current by dwell
extension.
Fig. Square wave with unbalanced dwells.
196
Welding with DC power source cont.
 Mainly 3 different types of Direct Current (DC) welding power
sources are there i.e.:
i. Rectifiers and
ii. Generators
iii. Inverters
197
DC power source: Rectifiers
 A traditional welding rectifier power source produces DC.
 A full-wave rectifier is used to convert the AC output from a
transformer into DC for welding.
 For a single-phase input in most of the applications some form of
smoothing is required.
 A three-phase input is usually preferred as it gives more uniform
load on main supply and smoothens the ripples, eliminating the
smoothening circuit.
 This can be given varying slopes, from straight to drooping, so that
the unit can be used with several different welding methods.
198
Fig. Ripple voltage from a full-wave
rectifier, before and after the
application of a smoothing capacitor.
DC power source: Rectifiers
 Simple three-phase full wave rectifier unit for welding:
Fig. Circuit diagram
Fig. Block diagram
199
Welding inverters
Inverter: In a inverter unit, the 50 Hz mains supply is first rectified
and then, using power semiconductors, is turned back into AC at a
higher frequency, usually in the range 5-100 kHz. Finally this higher
frequency AC convert to DC.
 This reduces the weight of the transformer and inductor,
making the power unit small and portable.
 Low losses result in high efficiency, to the order of 80-90 %.
 This units have excellent performance.
200
Fig. Inverter Block Diagram
Welding inverters (contd.)
 Inverter units offer the following advantages:
 Low weight and small size
 Good welding performance
 Several weld with high efficiency
The size of the transformer and inductor depend on the number of turns (N) and the
cross-sectional core area (A), both of which can be reduced.
This reduces the weight of the transformer and inductor to a fraction of what is
needed for a 50 Hz unit, making the power unit small and portable.
201
Classification of Power Source on the basis of duty cycle
202
Duty Cycle
 Duty cycle is the ratio of arcing time to the weld cycle time
multiplied by 100.
 Welding cycle time is either 5 minutes as per European
standards or 10 minutes as per American standard and
accordingly power sources are designed.
 If arcing time is continuously 5 minutes then as per European
standard it is 100% duty cycle and 50% as per American standard.
 At 100% duty cycle minimum current is to be drawn i.e. with the
reduction of duty cycle current drawn can be of higher level.
 The welding current which can be drawn at a duty cycle can be
evaluated from the following equation:
203
Duty Cycle
 Duty cycle and associated currents are important as it ensures that
power source remains safe and its windings are not getting
damaged due to increase in temperature beyond specified limit.
 The power source rating is also determined by its duty cycle,
which indicates for what proportion of a period of ten minutes
that the power source can be operated at the specified load.
 For example: 400 A at 35 % duty factor, means that the power
source can supply 400 A for 3.5 minutes in every ten minutes without
overheating.
204
Classification of Power Source on the basis of duty cycle
 The National Electrical Manufacturers Association (NEMA)
categorizes arc welding power sources into three classes on the
basis of duty cycle:
 CLASS I: Rated output at 60 (at 300A), 80, or 100% duty cycle
 CLASS II: Rated output at 30, 40, or 50 % (at 250A) duty cycle
 CLASS III: Rated output at 20% (at 225A) duty cycle
205
Classification of Power Source on the basis of duty cycle
 In Fig. curve A shows a NEMA
Class I (60%) 300 A rated machine
that is capable of a maximum 375 A
at reduced duty cycle (38%) and 232
A at 100% (continuous).
 Curve B represents a NEMA Class
II (50%) 250 A machine with a
continuous duty of 176 A.
 Curve C represents an engine-driven
machine rated at 225 A and 20%
duty. It does not offer output in
excess of its rating because of a
horsepower limitation of the engine.
206
Fig. Selected duty cycle (i) Curves.
A, 300 A, 60% Machine; (ii) B, 250
A, 50% Machine; (iii) C, 225 A,
20%
(Note: C-machine is Enginedriven machine)
Rated Current & Rated Voltage
 Rated current:
 The rated current is the current for which the power source is
designed.
 Note: Always check the technical data or the rating plate to make
sure what the actual value of rated current is.
 Rated voltage:
 IEC 974 (International electro-technical commission) specifies a standard
load line, for each value of rated current, there is a voltage which is known
as rated voltage.
 The relationships specified by IEC 974 differ from one welding method to another:
for currents up to 600 A, the voltages are as follows:
• MMA and SAW: U= 20 + 0.04 . I ,for currents up to 600 A &
For currents above 600 A: U = 44 V
• TIG: U= 10+0.04. I, for currents up to 600 A &
For currents above 600 A: U = 34 V
• MIG/MAG: U= 14+0.05. I, for currents
up to 600 A
207
For currents above 600 A: U = 44 V
Reference/Text Books
 V. M. Radhakrishnan, Welding Technology and Design, New age.
2002.
 J. A. Goldak, Computational Welding Mechanics, Springer 2005.
 O. Grong, Metallurgical Modelling of Welding, 2nd Ed. IOM
publication , 1997.
 L-E Lindgren, Computational Welding Mechanics, Woodhead
Publishing Limited, 2007.
 Dr. O. P. Khanna, Welding Technology, Reprint: 2002.
 A. O. Brien, Welding Handbook: Welding Processes, Part 1, Vol.2,
AWS,2004.
 Md. Ibrahim Khan, Welding Science and Technology, New Age In.
 J. F. Lancaster (Ed), The Physics of welding, Pergamon, 1986.
 R.W. Messler, Principles of Welding, John Wiley and Sons,1999.
208
End
209
Fundamental of Welding Science and Technology
Lecture 8: Welding Power Sources
Characteristics
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
210
Categories of Power Sources Characteristics
 Power sources characteristics can be classified into two main
categories:
1. Static characteristics of power sources
2. Dynamic or pulse characteristics of power sources
211
212
Power Source Selection
 Because no single power source is right for all welding situations,
it is necessary to know the processes to be used before selecting
the best power source.
Table: Power source selection relative to welding process
213
Dynamic characteristic
 Dynamic characteristic is the rapid transient variation of output
current and voltage.
 It occurs, particularly when welding with short-circuiting drop
transfer, arc starting and arc re-ignition.
 To cope up with these above conditions, power source should have
good dynamic characteristics to obtain stable and smooth arc.
 Power units for short-circuits or short arc welding usually
incorporate an inductor in their output.
 The function of the inductor is, if the voltage changes
instantaneously then the current will rise much slower. (i.e.
particularly when a droplet of molten metal short-circuits the arc
then the voltage changes instantaneously).
214
Dynamic characteristic cont..
 Therefore it is important that there should not be a current surge
during the short circuit, as this would result in high electromagnetic
forces that would cause spatter and oscillations on the surface of the
weld pool.
Fig. Welding current in short arc welding with
low inductance (top) and with high inductance
(bottom).
 Sometime it can be referred as Pulsed mode Power Supply.
215
Pulsed mode Power Supply
 It is a DC power source but in pulsing mode. Here current fluctuate in
a predetermined fashion not random manner.
 By applying pulse the metal deposition can be controlled.
 At background current no metal transfer occur.
 The background time is set for keeping the arc ignited.
 The background current is set in such a fashion so that the weld pool remain in
molten state.
 The peak pulse current is set based on the requirement of transition current.
216
 In AC arc get extinguished but here arc always on.
SOME IMPORTANT NOTES
217
Transisterised power supply unit
 Several modern arc welding power supply units contain
transisterised/solid-state circuit power supply unit for regulating the
output or replacing the reactors found in conventional system.
 It consist a feedback system for regulating the welding parameters.
 These transistors can be made to behave as variable resistance in
response to command signals.
 Thus the same supply unit can be made to work as CV source for
GMAW and then, simply by changing the command signals, it can
be made to give CC output to GTAW process.
218
Transisterised power supply unit cont.
 In some cases this is also used for compensating the fluctuations in
mains output voltage. This provides a stable and consistent operation
of arc in GMAW process.
 As per the requirement, it can provide accurately controlled pulses.
So, it can also be used in pulsed GTAW or GMAW process.
219
Wire feed systems for constant arc length
 There are generally two types of feed systems are used to maintain
the arc length:
i) Constant speed feed drive system &
ii) Variable speed feed drive system.
 Constant speed feed drive system:
 Here the feed rollers rotating at fixed
speed are used for pushing/pulling
wire to feed into the weld so as to
maintain the arc length during welding.
 It is normally used with constant
voltage power sources in combination
with small diameter electrodes where
self regulating arc helps to attain the
constancy in arc length.
220
Wire feed systems cont…
 Variable speed feed drive system:
 In this case the feed rollers used for
feeding electrode wire are rotated at
varying speed as per need to maintain
the arc length during welding. Like:
SAW and GMAW processes.
 Fluctuation in arc length due to any
reason is compensated by increasing
or decreasing the electrode feed rate.
 Here the electrode feed rate is controlled by regulating the speed of
feed rollers powered by electric motor.
 Input power to the variable speed motor is regulated with the help of
sensor which takes inputs from fluctuations in the welding arc gap.
221
Wire, cooling system and insulation type
 The maximum current which can be drawn from a power source at
given a duty cycle depends upon size of winding wire, type of
insulation and force cooling system of the power source.
 Welding cable:
 Generally, the sizes varying from 6 AWG (0.380inch O.D.) to 500
MCM (1 MCM = 0.5067 square milimeters). It consists of bare
annealed copper as per ASTM-B3 standard.
 The wire wrapped inside a non-conductive, durable jacket. The jacket
on most welding cable is thermoset, typically EPDM or Neoprene.
 Insulation type:
 The maximum allowable temperature of various components (primary
and secondary coils, cables, connectors etc.), depends on the quality
and type of insulation and materials of coils used for manufacturing of
power source. The insulation is classified as below:
 A, B, E, F, G and H are the different categories of insulation.
222
Welding power sources based on capacity
 Small Power-Sources: It has single phase ac input and the output
which are available for users with limited requirements.
 These may be small welding shops, hobby shops, schools rated at a
small duty cycle i.e. 20% duty cycle.
 Slightly larger or medium power-sources: It has single phase ac
input which may have selectable ac or dc output, with additional
controls useful for GTAW.
 Larger Power-Sources: It is used for industrial applications and
greater current requirements, most of the Power-Sources have three
phase input.
223
224
Criteria for selection of welding power source
 The following factors must be considered for selection a welding
power source:
 Initial cost of the power source.
 Periodic maintenance and repair cost.
 Availability of mains power supply: 220 V or 440 V.
 Steady output current even with input voltage fluctuation.
 Type of current needed AC or DC or both.
 Current rating required to accommodate all sizes of electrodes
needed for the jobs.
225
Criteria for selection of welding power source
 Machine’s ability to strike and maintain stable arc for the type of
electrodes to be used.
 Type of V-I characteristics (CC or CV) needed for the process
employed.
 Whether machine is required to give radiographic quality welds
and impact strength with the type of electrodes used.
 Whether the machine needs to serve several welding processes
expected to be used in the shop.
 Need for remote current control.
 Machine’s ability to stand shop environment (corrosive gases,
dust, moisture etc.).
226
Fundamental of Welding Science and Technology
Lecture 8: Principle & Physics of Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
227
Introduction
 In fusion welding process, the material around the joints is melted in
both parts to be joined.
 The most important factors governing a fusion welding process are:
 The characteristics of the heat source.
 The characteristics of the arc.
 The nature of deposition of filler material in the Fusion Zone (FZ),
known as weld pool.
 The heat flow characteristics in the joints.
228
General characteristics of heat sources
 Heat Source:
A heat source, suitable for welding, should release the heat
in sharply defined isolated zone. Moreover, the heat should be
produced at high temperature and at a high rate.
 First of all, let us see how an electric arc is created and maintained
between 2 opposing polarity.
229
General characteristics of heat sources cont..
 Welding Arc: It has been defined as a sustained electrical
discharge through an ionized gas.
 The discharge is initiated by an avalanche of electrons emitted from
hot cathode and maintained by the thermal ionization of the hot gas.
 This electrical discharge through an ionized gas produces a good
amount of heat energy.
 A welding arc is a high current (upto 2000 amp) and low voltage
(10 to 50) discharge.
 Electric welding arc besides being a heat source, transfer
material, create turbulence in weld pool.
230
Thermionic emission and Ionization
231
Thermionic emission
 Initially, a good contact is made between the electrode and work.
 Thereafter, the electrode is withdrawn. As a result, the metallic
bridges starts breaking, thus increasing current density per bridge.
 Finally, the current density rises to such a high value that the
bridges start boiling.
 Under such conditions, the electrons come out of both the
surfaces by a process known as thermionic emission.
 Note: Obviously, the electrons ( having (–) ve charge ) coming
out of the anode (+ve terminal) are pulled back, whereas those
coming out of the cathode (-ve terminal ) are also attracted
towards the anode.
232
Thermionic Emission
233
General characteristics of heat sources (cont.)
 According to the Richardson-Dushman equation the emitted
electron current density, I (A/m2), is related to the absolute
temperature T by the equation:
Ie= AT2 exp(- Ø /(kT)) amp/m2
 where ‘T’ = the absolute temperature (K), k = Boltzmann’s constant
Ø = The thermionic work function of the cathode material (in electron
volts) and ‘A’ = Richardson's constant (in A/m2K2).
Note:
 The Ø, in fact, represents the kinetic energy necessary to ‘boil’ out
an electron.
 From above equation shows that a low value of Ø, together with a
high value of T, makes the emission of electron easier.
A = 4*πmek2/h3 ~ 1202 mA/mm2K2, where m is the mass of electron,
e is elementary charge, and h is Plank's constant.
234
k = 8.6173324×10−5 eV K−1
General characteristics of heat sources (cont.)
 Work function :The minimum amount of energy needed for an
electron to leave a surface is called the work function.
 The work function is characteristic of the material and for most
metals is on the order of several electron-volts.
Table: Thermionic work function
Metal type
Ø
(eV)
Aliminium
Cu
Fe (iron)
Tungsten
K(Potassium)
Nickel
4.1
4.4
4.4
4.5
2.2
5.0
235
One electron volt is equal to 1.602×10−19 J
Ionization
The inter-particle collisions, taking place in the gap between
electrodes, give rise to a process called, ‘thermal ionization’.
 Ionization Potentials of some commonly used gas:
N 15.6 eV
Ar 15.8
He 24.6
236
Conduction of Current in the Arc
Cathode (-)
Ion
Electrons Emitted
Thermal
Ionization
Free Electron
Plasma
T>10,000 K
Recombination
Neutral
Gas Atom
Anode (+)
Electrons Absorbed
237
General characteristics of heat sources (cont..)
 Once arc started, the arc itself becomes a source of ions through a
process of ionization.
 These ions are attracted by the cathode (-ve terminal) and the
resulting collisions keep the cathode hot.
 The total current in the arc is carried by 2 sets of electrons
• Primary electrons: It is emitted by cathode (-ve terminal).
• Secondary electrons: It is produced as a result of the ionization the
arc gap.
238
Arc Structure
 The conditions in the arc column are quite different from the
region where the arc comes in contact with electrode (i.e., cathode)
and the workpiece (i.e., anode in the DCSP).
 In the immediate vicinity of the electrode or the job, the plasma can
no longer maintain its high temperature because it comes in contact
with comparatively much colder workpiece and electrode.
 High temperature gradients exist on both the ends of the arc
column and naturally the arc gets divided into 3 distinct zones i.e.:
i) The most concentrated source of heat is the cathode spot.
ii) Hottest region is the arc column
iii) The largest quantity of heat is produced at the anode.
 The cathode is negative, anode is positive and arc column is
electrically neutral as it contains equal number of ions and electrons.
239
Regions of Welding Arc
240
Arc Structure
•Cathode spot: This is relatively a very small area on the cathode
surface, emitting the electrons.
•Cathode space: It is a gaseous region adjacent to the cathode and has a
thickness of the order of 10-2 mm. This region has the +ve space charge, so
the voltage drop is necessary as the electrons are to be pulled from this
region.
•Arc column: This is the visible portion of the arc consisting of the plasma (
hot ionized gas) where the voltage drop is not sharp.
•Anode space: This is gaseous region (thickness 10-2 mm ) and adjacent to
the anode surface when a sharp drop of voltage takes place.
 This is because the electrons have to penetrate the anode surface after
overcoming the repulsion of the thermionically emitted electrons from anode
surface.
•Anode spot: This is the area on the anode surface where the electrons are
absorbed. This one is longer the cathode spot.
241
Potential Drop Characteristics
 The potential/voltage drop changes if the material or spacing
between electrode changes.
 However, a change in spacing and the current essentially changes
only the drop in arc column.
 It has been experimentally found that, for given spacing, the
voltage reduces up to a current value of 50 amp and increases
thereafter, as shown in below:
242
Potential drop Characteristics
 Upto 50 amp of current, the shape of the arc is almost cylindrical.
This results in a higher conductivity (and consequently lower
resistance).
 However, beyond 50 amp of current, the arc bulges out and the
current path becomes more than the arc gap which again increases
the resistance of the arc.
 Due to these opposite effects, i.e., higher temperature and longer
current path, the voltage drop remains almost constant over a wide
range of current values.
• The electrode drops are also independent of the arc length.
 Here, we can write voltage drop across the entire arc as:
V=A+Bl (As voltage drop line depends on arc length)
where A is the electrode drop and Bl represents the column drop.
243
Arc power
 The power of an arc varies with its length and there is an
optimum length for which the arc power is maximum. This
optimum arc length (lopt) can be determined as follows:
244
Arc power
 This procedure can be repeated for various values of arc length and
a plot of arc power (P) versus the arc length (l) can be plotted as
shown fig. below.
 Now, the optimum arc length lopt can be easily determined from the
fig. below.
245
Problem
 The voltage – length characteristics of a direct current arc is given by
V= (20+40L) volts.
Where l is the arc length in cm. the power source characteristics is approximated by
a straight line with an open circuit voltage = 80 V and short circuit current =1000
amp. Determine the optimum arc length and corresponding power.
246
247
End
248
Ionization: Let an electron of charge ‘e’, moving in electric field of gradient E (volt/
distance), experiences a force of magnitude (eE). Charge of an electron = 1.60217662 ×
10-19 coulombs.
In other words, it accelerates at a rate of (eE/m), where ‘m’ is its mass. m = 9.10938356 ×
10-31 kilograms
So, if it travels through a distance ‘d’ before colliding with another particle (a neutral atom
or another electron) it has a kinetic energy (Eed) .
This kinetic energy is nothing but heat and manifests itself through increased temperature.
The inter-particle collisions, taking place in the gap between electrodes, give rise to a
process called, ‘thermal ionization’.
Note: The SI derived unit of electric charge is the coulomb (C). In electrical
engineering, it is also common to use the ampere-hour (Ah),
Electric charge is the physical property of matter that causes it to experience a
force when placed in an electromagnetic field. There are two types of electric
charges; positive and negative (commonly carried by protons and electrons
respectively).
 Coulomb : It is the charge (symbol: Q or q) transported by a constant
current of one ampere in one second:
249
 −1 C is equivalent to the charge of approximately 6.242×1018 electrons.
Conduction of Current in the Arc
•In a welding arc, the electrons are emitted from the cathode, get accelerated in
the cathode drop region and gain energy.
• As they entered arc column , they lose their energy by colliding with gas atoms
and molecules which in turn get ionized , i.e. electrons and +ve ions are
suspended .
• The ions and electrons then move towards cathode & anode respectively and
get concentrated over there.
• Due to this concentration of charge carriers (i.e. electrons and ions ) in the
anode and cathode drop zones, a nonlinear voltage distribution is prevalent
along the arc length , and high electric arc field strengths are found in cathode
and anode drop zones .
250
Regions of Welding Arc
• In a welding arc, the electrons are emitted from the cathode, get accelerated in the cathode
drop region and gain energy.
• As they entered arc column , they lose their energy by colliding with gas atoms and
molecules which in turn get ionized , i.e. electrons and +ve ions are
suspended .
• The ions and electrons then move towards cathode & anode respectively and
concentrated over there.
get
• Due to this concentration of charge carriers (i.e. electrons and ions ) in the anode and
cathode drop zones, a nonlinear voltage distribution is prevalent
along the arc length , and high electric arc field strengths are found in cathode and anode
drop zones .
251
Potential Drop Characteristics
 The potential/voltage drop changes if the material or spacing
between electrode changes.
 However, a change in spacing and the current essentially changes
only the drop in arc column.
 It has been experimentally found that, for given spacing, the
voltage reduces up to a current value of 50 amp and increases
thereafter, as shown in below:
252
Ex.:
The voltage – length characteristics of a DC arc is given by
V= (20+5l) volts.
Where l is the arc length in cm which varied between 5mm to 7mm. Here the current
varied between 500 A to 400 A The power source characteristics is approximated by
a straight line. Find the open circuit voltage and short circuit current. Also
determine the optimum arc length and corresponding power.
The coulomb (unit symbol: C) is the International System of Units (SI) unit of
electric charge. It is the charge (symbol: Q or q) transported by a constant current of
one ampere in one second:
Thus, it is also the amount of excess charge on a capacitor of one farad charged to
a potential difference of one volt:
It is equivalent to the charge of approximately 6.242×1018 (1.036×10−5 mol) protons,
and −1 C is equivalent to the charge of approximately 6.242×1018 electrons.
253
254
Fundamental of Welding Science and Technology
Lecture 10: Physics of Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
255
Contents
• Arc initiation
• Type of welding arc
• Arc stability & arc blow
• Metal transfer
• Forces affecting metal transfer
256
Arc initiation
Arc initiation:
 Arc is initiated by providing a conducting path between the
electrode and job. Or Ionizing the gap between the two.
 It can be done by following method:
 By tapping method
 By scratching the electrode with the job
 By steel wool
 By a carbon rod
 By a high frequency unit
257
Arc initiation cont.
 To begin the SMAW Process, first of all an arc must be struck. This
can be done using one of the following techniques:
 Tap Start – tap the rod against the base metal. By momentarily
touching the electrode with job and taking it away.
 Scratch start – scratch the electrode on the base metal like a
match
Fig. Tapping method of arc starting
258
Fig. Scratch method of arc starting
4
Arc initiation cont.
 By steel wool: Another case where the steel wool kept pressed
between the electrode and the job.
 When the welding current is switched on , steel wool provides a
conducting path for the arc to establish.
 Ex.: SAW and automatic MIG welding.
259
Arc initiation cont.
 By a carbon rod: Arc can be initiated with help of a carbon rod.
 Suitable arc gap is kept between the electrode and workpiece, current
is witched on, then the electrode and the job simultaneously are
momentarily touched with a carbon rod.
 Ex.: Automatic metal arc welding
260
Arc initiation cont.
H. F. (high frequency) unit: In order to eliminate the chances of
electrode contamination a H. F. ( high frequency ) unit is inserted in
the circuit to initiate the arc.
Some power sources need high frequency unit to start the arc, which
may be requirement of processes like TIG and plasma arc. High
frequency unit is introduced in the welding circuit but in between the
control circuit and HF unit, filters are required so that high frequency
may not flow through control circuit and damage it. High frequency
unit is a device which supplies high voltage of the order of few kV
along with high frequency of few kHz with low current. This high
voltage ionizes the medium between electrode and
workpiece/nozzle starting pilot arc which ultimately leads to the
start of main arc. Although high voltage may be fatal for the operator
but when it is associated with high frequencies then current does not
enter body but it causes only skin effect i.e. current passes through the
skin of operator causing no damage to the operator.
261
Arc initiation cont.
The arc is struck by using a high frequency unit:
In this method, a high frequency circuit is superimposed on the
welding current. The welding torch (which holding the electrode) is
brought nearer to the job. When electrode tip reaches within 3 to 2
mm from the job/workpiece, a spark jumps across the air gap
between the electrode and the job. This air path gets ionized and arc
is established.
262
Types of welding arc
 The welding arcs may be categorized of the following types:
i)
Steady arc (electrical discharge between two electrodes)
ii) Unsteady arc (this is due to electrical short circuiting metal
transfer where the arc interrupted)
iii) Continuously non-steady arc (this is due to AC current flow)
iv) Pulsed arc (intermittent current pulses are superimposed on a
regular arc to obtained spray metal transfer during pulse interval)
263
Types of welding arc cont..
 Depending upon the geometry of the electrode (cathode) tip and
plasma in the region of the cathode 2 different modes of welding
arcs:
• Cathode spot mode &
• Normal mode
Fig. Modes of welding arc
 Note: The cathode spot mode exhibits a constriction of the plasma at
the cathode and is accompanied by a higher voltage for a given arc
length.
264
Types of welding arc
 The temperature also shows for an argon arc operating at 300 amps
with same
arc gap in 2 modes. The arc voltage measured in
cathode spot mode is 14.8 volts and normal mode 12.8 volts.
Fig. Temperature distribution in arcs
265
Temperature measurement of welding arcs
 Spectroscopic techniques: These are sophisticated and require
accurate optical alignment.
 Electrostatic probes: In designing the probe, the most important
consideration is that the measurement should not affect the arc
properties.
 This requirement restricts the size of the probe.
 The probe should be sufficiently rigid.
 Like molybdenum of dia. 0.15mm used to measure temperature,
length 100 mm , but only 10-20mm passes through the arc.
 Note: The probe has to move fast enough (through the arc) to
prevent the wire becoming so hot that it cannot emit electrons or
vaporize but not so fast that physically disturbs the arc column.
266
Functions of current flow
 In an arc, current density, magnetic field strength and pressure all
decrease from cathode drop region towards the arc column, because
arc c/s increases rapidly in the arc column.
 The current flow through the arc gives rise to self
induced magnetic field which compresses the arc plasma
resulting in appreciable axial and radial pressure
gradient in the arc.
 The radial pressure gradient constrict the arc (pinch
effect) and raises the temperature of the arc discharge.
 Whereas axial pressure gradient give rise to plasma
streaming which transports material (metal and slag
particles) and heat from electrode to the work piece.
 Plasma streams stablise the arc and exert a pressure on
the molten pool which helps increase penetration .
267
 Note: Both these effects are proportional to square of arc current.
Arc stability
 Arc is said to be stable if it is uniform and steady.
 A stable arc will produce good weld bead & defect free nuggets.
 The stability of a welding arc is governed by so many factors, as
mentioned below:
 Suitable matching of arc and power source characteristics, a little
variation in arc length, i.e., arc voltage should not extinguish the arc.
 Continuous and proper emission of electrons from electrode (say
cathode) and thermal ionization in the arc column.
 Arc length.
 Electrode tip geometry. Electrodes with smaller tip diameter have
more stable arc.
 Presence of dampness, oil grease etc. on the surface of workpiece
increase arc instability.
 Limited practice on the part of the welder.
268
Arc Blow
 The unwanted deflection or the wandering of a welding arc from its
intended path is termed as arc blow or arc bow.
 Arc blow is the result of magnetic disturbances which unbalance the
symmetry of self–induced magnetic field around the electrode, arc
and workpiece.
 Arc blow becomes severe when welding is carried out in confined
spaces and corners on a heavy metal plate, using DC power source.
269
Arc Blow cont.
 AC arcs are less susceptible to arc blow than DC arcs.
•
Because AC reverses direction which in turn, reverses the
magnetic field build up, collapses and rebuilds as current reverses
from +ve to –ve.
• This phenomenon does not permit the magnetic field strength to
build up to a value so as to cause arc blow.
 On the other hand in DC welding, the magnetic field set up in
workpiece (adjacent to the arc) continuously builds up and the arc
blow occurs.
270
Factors affecting arc blow
 Magnetic fields produced in the workpiece adjacent to the welding
arc, due to current flow through the arc.
 With multiple welding heads, arc at one electrode may be affected by
the magnetic field of the arc at the other electrode.
 The magnetic field produced in the workpiece around the earth
connection may tend to drive the arc away from the point from
where this connection is made.
 This magnetic field is also produced because of current flow from
the earth connection to the workpiece.
271
Types of arc blow
 There are three kinds of arc blow depending upon the above factors:
i.
A forward blow at the starting end of a weld and backward
blow at the finishing end of a weld.
ii.
A sideward deflection.
iii.
Arc rotate.
272
Mechanism of Forward and Backward arc blow
 Magnetic flux lines get crowded near the starting and finishing end of the
workpiece because they find an easier path through the workpiece than that
through the air.
 The arc seeks the path of least resistance and deflects towards the weak flux side.
 Which causes the forward arc blow (deflection of arc and welding direction is
same ) at the starting end and backward arc blow at the finishing end of the weld
bead in a workpiece.
273
Mechanism of Sideward arc blow
• It depends on workpiece earth clamp.
• As the arc comes near the arc clamp, it deflects sidewardly in a direction away from the
clamp, perhaps because of the magnetic flux enacted in the workpiece by the earth
clamp (i.e., ground).
• This magnetic flux is produced by the flow of the current from clamp to workpiece.
• It is also noted that as the arc crosses the earth connection, it has a tendency to come
to the original line of travel .
274
Effects of arc blow
a) Increased arc blow results in an unstable arc
b) Poor weld bead appearance
c) Irregular and erratic weld deposition
d) Undercutting and lack of fusion
e) Spatter
f) Uneven and weak welded joints
g) Slag entrapment
h) Porosity
275
Remedies for arc blow
 Arc blow can be minimized by keeping the following factors in view:
 Changing the position of the earth clamp or current return lead and
welding away from the earth connection.
Avoiding the presence of rearranging magnetic material around the
workpiece and arc.
 Storing workpiece away from the magnetic sources, such as welding
power sources.
Employing ground connection more than one.
Using a short arc.
 Lowering arc current.
 Using smaller diameter electrodes.
 Decreasing arc travel speed.
Superimposing a counteracting externally applied longitudinal magnetic field.
276
End
277
Introduction
 The physics of welding deals with complex physical phenomenon associated
with weld induced heat, electricity, magnetism, light etc.
 Majority of welding process require application of heat which is obtained
through :
• Flame
• Arc
• Contact resistance
• Electron beam
• Laser etc.
• Magnetic fields set up due to flow of current through the electrode and the arc
generate pinch effect and influence the welding arcs.
• Some of the welding arc features strongly influenced by the presence of
magnetic fields i.e.
 Arc blow
 Plasma streaming and
 Metal transfer.
278
Arc Structure Mechanism
 Cathode drop zone:
 It is constrained within two imaginary planes, one just at the
end of cathode spot and other at the beginning of the arc plasma
column.
Cathode tip appears darker as compared to arc column.
 This region is very important because electrons are produced
here, and the arc stability depends on regular supply of electrons.
 There are 3 different cathode drop mechanisms:
1. Cathode drop mechanism for electrodes made up of high melting
point and low work function material.
2. Cathode drop mechanism for low melting point electrodes
279
3. Cathode drop mechanism due to plasma
emission
Arc Structure Mechanism
 Cathode drop mechanism for electrodes made up of high melting point:
• Here at high temperature, the electrons are emitted from the cathode by
 ‘thermionic emission’,
 get accelerated in cathode drop zone,
 gain kinetic energy, which is lost in arc column when the electrons collide with the
gas atoms and molecules.
• These ions so produced travel towards cathode ( being attracted by it), strike it and
give up their kinetic energy.
• This produces high heat it is maintained at high temperature necessary for further
emission of electrons.
• In this case cathode spot is not well defined.
280
Arc Structure Mechanism
Cathode drop mechanism for low melting point electrodes:
• There is a relatively larger cathode area containing many active well defined
small cathode spots which move around with velocity of about 104 cm/sec.
• These cathode spots, constantly formed, are vanished and get reformed
elsewhere.
 Cathode drop mechanism due to plasma emission:
• It is related to high pressure and low current (through high current density)
arcs.
• In this case cathode is stationary and well marked.
281
Arc Structure Mechanism
 Arc plasma column:
 It is that portion of the welding arc which is situated between anode and
cathode drop regions.
• Arc column consists of a radiating mixture of electrons, ions(+ve) and highly
exited neutral atoms and molecules.
• It maintain a regular supply of ions and electrons to keep current flowing
between cathode and anode( in DCSP).
• Arc column temperature ranging from 5000K to 50000K.
• This temperarure is achieved from the cathodically emitted electrons which
collide with the gas atoms raise their temperature and ionize them and in turn
producing more electrons which again collide with neutral atoms and thus the
degree of ionization increases .
282
Arc Structure Mechanism
 Anode drop region:
It is situated between the anode spot and the place where the arc column finishes.
 This region forms the electrical connection between arc plasma column and the
anode.
 The potential drop here is due to concentration of electrons which enter in this arc
column.
 3 phenomena in the anode drop zone are:
• Temperature falls (from that of arc column)
• Ions are produced
• Ions accelerated towards arc column.
• The chances of +ve ions formation increases as the anode plasma temperature
rises.
283
Note: High frequency unit
Some power sources need high frequency unit to start the arc, which may be
requirement of processes like TIG and plasma arc. High frequency unit is
introduced in the welding circuit but in between the control circuit and HF unit, filters
are required so that high frequency may not flow through control circuit and damage
it. High frequency unit is a device which supplies high voltage of the order of
few kV along with high frequency of few kHz with low current. This high
voltage ionizes the medium between electrode and workpiece/nozzle starting
pilot arc which ultimately leads to the start of main arc. Although high voltage
may be fatal for the operator but when it is associated with high frequencies then
current does not enter body but it causes only skin effect i.e. current passes through
the skin of operator causing no damage to the operator.
284
Fundamental of Welding Science and Technology
Lecture 11: Physics of Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
285
Contents
• Arc power
• Arc initiation
• Type of welding arc
• Arc stability & arc blow
• Metal transfer
• Forces affecting metal transfer
286
Problem-2
 The voltage – length characteristics of a DC arc is given by
V= (15+4l) volts.
where l is the arc length in mm which varied between 4mm to 6mm. Here the current
varied between 500 A to 400 A. The power source characteristics is approximated by
a straight line. Find the open circuit voltage and short circuit current. Also
determine the optimum arc length and corresponding power.
287
288
Arc initiation
Arc initiation:
 Arc is initiated by providing a conducting path between the
electrode and job/workpiece.
 Or Ionizing the gap between the two.
 It can be initiated by following ways:
 Tapping method
 Scratching method
 By steel wool
 By a carbon rod
 By a high frequency unit
289
Arc initiation cont.
 Tap Start – tap the rod against the base metal. By momentarily
touching the electrode with job and taking it away.
 Scratch start – scratch the electrode on the base metal like a
match.
Example: SMAW Process
Fig. Tapping method of arc starting
290
Fig. Scratch method of arc starting
6
Arc initiation cont.
 By steel wool: In this case, the steel wool kept pressed between the
electrode and the job.
 When the welding current is switched on, steel wool provides a
conducting path for the arc to establish.
 Ex.: SAW and automatic MIG welding.
291
Arc initiation cont.
 By a carbon rod: Arc can be initiated with help of a carbon rod.
 Suitable arc gap is kept between the electrode and workpiece, current
is switched on, then the electrode and the job simultaneously are
momentarily touched with a carbon rod.
 Ex.: Automatic metal arc welding
292
Arc initiation cont.
 H. F. (high frequency) unit: In order to eliminate the chances of
electrode contamination a H. F. ( high frequency ) unit is inserted
in the circuit to initiate the arc.
 High frequency unit is a device which supplies high voltage of the
order of few kV along with high frequency of few kHz with low
current.
 When electrode tip is brought within 3 to 2 mm from the
job/workpiece, a spark jumps across the air gap between the
electrode and the job. Then this high voltage ionizes the medium
between electrode and workpiece/nozzle starting pilot arc which
ultimately leads to the start of main arc.
 Ex.: GTAW and PAW.
 Note: Although high voltage may be fatal for the operator but when
it is associated with high frequencies then current does not enter
body but it causes only skin effect i.e. current passes through the
skin of operator.
293
294
Types of welding arc
 The welding arcs may be categorized of the following types:
i)
Steady arc (generally in DC, electrical discharge between two
electrodes)
ii) Unsteady arc (this is due to electrical short circuiting metal
transfer where the arc interrupted)
iii) Continuously non-steady arc (this is due to AC current flow)
iv) Pulsed arc (intermittent current pulses are superimposed on a
regular arc to obtained spray metal transfer during pulse interval)
295
Types of welding arc cont..
 Depending upon the geometry of the tungsten electrode (cathode) tip
2 different modes of welding arcs were observed by Olsen:
• Cathode spot mode &
• Normal mode
Fig. Modes of welding arc
 Note: The normal mode is more stable and readily obtainable.
 The cathode spot mode exhibits a constriction of the plasma at the
cathode and is accompanied by a higher voltage for a given arc length.
296
Types of welding arc
 The temperature also shows for an argon arc operating at 300 amps
with same arc gap in 2 modes. The arc voltage measured in
cathode spot mode is 14.8 volts and normal mode 12.8 volts.
Fig. Temperature distribution in arcs
297
Temperature measurement of welding arcs
 Electrostatic probes: In designing the probe, the most important
consideration is that the measurement should not affect the arc
properties.
 This requirement restricts the size of the probe.
 The probe should be sufficiently rigid.
 Like molybdenum of dia. 0.15mm used to measure temperature,
length 100 mm , but only 10-20 mm passes through the arc.
 Note: The probe has to move fast enough (through the arc) to
prevent the wire becoming so hot that it cannot emit electrons or
vaporize but not so fast that physically disturbs the arc column.
 Spectroscopic techniques: These are sophisticated and require
accurate optical alignment.
298
Arc stability & arc blow
299
Functions of current flow
 In an arc, current density, magnetic field strength and pressure all
decrease from cathode drop region towards the arc column, because
arc cross-section increases rapidly in the arc column.
 The current flow through the arc gives rise to self
induced magnetic field which compresses the arc plasma
resulting in appreciable axial and radial pressure
gradient in the arc.
 The radial pressure gradient constrict the arc (pinch
effect) and raises the temperature of the arc discharge.
 Whereas axial pressure gradient give rise to plasma
streaming which transports material and heat from
electrode to the work piece.
 Plasma streams stablise the arc and exert a pressure on
the molten pool which helps increase penetration .
300
 Note: Both these effects are proportional to square of arc current.
Arc stability
 Arc is said to be stable if it is uniform and steady.
 A stable arc will produce good weld bead & defect free nuggets.
 The stability of a welding arc is governed by so many factors, as
mentioned below:
 Suitable matching of arc and power source characteristics, a little
variation in arc length, i.e., arc voltage should not extinguish the arc.
 Continuous and proper emission of electrons from electrode (say
cathode) and thermal ionization in the arc column.
 Arc length.
 Electrode tip geometry.
 Presence of dampness, oil grease etc. on the surface of workpiece
increase arc instability.
 Limited practice on the part of the welder.
301
Arc stability cont.
 Polarity have significant effect on arc stability:
 The welding processes in which electrode is expected to emit free
electrons (i.e. SMAW, GTAW, PAW) required for easy arc initiation
and their stability, selection of polarity affects the arc stability.
 In consumable electrode welding i.e. in SMAW
using covered electrode having low ionization
potential elements provide better stable arc with
DCEN than DCEP.
 However, SMA welding with DCEP gives
smoother metal transfer.
 Similarly, in case of non-consumable electrode
welding i.e. in GTAW welding, tungsten electrode
is expected to emit electrons for providing stable
arc and therefore DCEN is commonly used except
in case of reactive metals e.g. Al, Mg, Ti.
302
Arc Blow
 The unwanted deflection or the wandering of a welding arc from its
intended path is termed as arc blow or arc bow.
 Arc blow is the result of magnetic disturbances which unbalance the
symmetry of self–induced magnetic field around the electrode, arc
and workpiece.
 Arc blow becomes severe when welding is carried out in confined
spaces and corners on a heavy metal plate, using DC power source.
303
Arc Blow cont.
 AC arcs are less susceptible to arc blow than DC arcs.
• Because AC reverses direction which in turn, reverses the
magnetic field build up, collapses and rebuilds as current reverses
from +ve to –ve.
• This phenomenon does not permit the magnetic field strength to
build up to a value so as to cause arc blow.
 On the other hand in DC welding, the magnetic field set up in
workpiece (adjacent to the arc) continuously builds up and the arc
blow occurs.
304
Factors affecting arc blow
 Magnetic fields produced in the workpiece adjacent to the welding
arc, due to current flow through the arc.
 With multiple welding heads, arc at one electrode may be affected by
the magnetic field of the arc at the other electrode.
 The magnetic field produced in the workpiece around the earth
connection may tend to drive the arc away from the point from
where this connection is made.
 This magnetic field is also produced because of current flow from
the earth connection to the workpiece.
305
Types of arc blow
 There are three kinds of arc blow depending upon the above factors:
i.
A forward blow at the starting end of a weld and backward
blow at the finishing end of a weld.
ii.
A sideward deflection.
iii.
Arc rotate.
306
Forward and Backward arc blow
 Magnetic flux lines get crowded near the starting and finishing end
of the workpiece because they find an easier path through the
workpiece than that through the air.
 The arc seeks the path of least resistance and deflects towards the
weak flux side.
 Which causes the forward arc blow at the starting end and
backward arc blow at the finishing end of the weld bead in a
workpiece.
307
Sideward arc blow
 It depends on workpiece earth clamp.
 As the arc comes near the arc clamp, it deflects sidewardly in a
direction away from the clamp, perhaps because of the magnetic
flux enacted in the workpiece by the earth clamp (i.e., ground).
 This magnetic flux is produced by the flow of the current from
clamp to workpiece.
 It is also noted that as the arc crosses the earth connection, it has a
tendency to come to the original line of travel .
308
Arc rotation
 Here the arc deflection as well as rotation occur.
 Arc rotation indicates that under certain conditions of arc blow,
perhaps arc experiences magnetic field lines parallel to the arc axis.
Fig. Arc rotation and deflection
309
Effects of arc blow
 Increased arc blow results in an unstable arc
 Poor weld bead appearance
 Irregular and erratic weld deposition
 Undercutting and lack of fusion
 Spatter
 Uneven and weak welded joints
 Slag entrapment
 Porosity
310
Remedies for arc blow
 It can be minimized by keeping the following factors in view:
 Changing the position of the earth clamp and welding away from
the earth connection.
 Storing workpiece away from the magnetic sources, such as
welding power sources.
 Employing ground connection more than one.
 Using a short arc.
 Lowering arc current.
 Using smaller diameter electrodes.
 Decreasing arc travel speed.
 Superimposing a counteracting externally applied longitudinal
magnetic field.
311
End
312
Fundamental of Welding Science and Technology
Lecture 12: Physics of Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
313
Forces affecting metal transfer
 There are mainly two types forces affecting the drop transfer:
• Aid metal transfer forces
• Retard metal transfer forces
In general various forces involved in the phenomenon are given below:
• Surface tension
• Viscosity of the liquid metal
• High velocity gas jets
• Gravity
• Lorentz forces.
314
Forces affecting metal transfer (cont’d)
 Surface tension force: It is a retarding force which tries to keep
the drop in its position.
 The force of surface tension acting on the drop when it is just to
detach is given by:
 where
d =electrode diameter
σ = surface tension
k = it is a function dependent on electrode diameter and
capillarity constant of the material. Normally it varies
from 0.6 to 1.0.
 Note: The force of surface tension ranges from 400 to 800 dyne
for electrode from 1.5 to 3 mm. At higher temperature the surface
tension is lowered.
315
Forces affecting metal transfer (cont’d)
 Viscosity of liquid metal: It is retaining force.
 The high velocity gas jets striking the job and getting
back may retard the movement of metal drop tending to
fall down in the molten pool.
 Gravity: It acts as a detaching force when welding in flat
position and it is a retarding force when welding overhead.
 The force of gravity (ρvg), which depends upon volume of
globule.
 It is almost negligible on small diameter droplets.
316
Forces affecting metal transfer (cont’d)
 Lorentz force:
 This force is the result of interaction of the arc current with its
self–induced magnetic field.
 This force exercises pinch effect on the globule, aids in the neck
formation and drop detachment.
 The self-induced magnetic field of the arc plasma results in
plasma streaming which carries the detached drop to the
workpiece.
 The pinch force varies from 250 to 1000 dyne for 1.5 to 3 mm
electrode dia.
 Note: Surface tension and viscosity of the liquid metal help
droplet to grow in size. Whereas electromagnetic forces constrict
(i.e., neck) the molten end of the electrode due to that drop
separates from electrode.
317
Metal transfer
Introduction:
 When an electric arc is struck between the job and the consumable electrode,
 The arcing end of the electrode starts melting,
 Takes approximately a spherical shape, hangs towards the job, and
 Ultimately drop down on the same, either with a free flight through the arc or by
short circuiting the job.
• The size of the droplet and the metal (drop) transfer rate affects weld bead geometry, weld
metal microstructure and strength of welded joints etc.
• Metal transfer can be studied by a high speed movie camera (3000-7000frames per sec.).
Like MIG (where metal transfer is well visible).
• But where the metal is not visible like (like SMAW or SAW) X-rays have been used to
study metal transfer.
318
Fig.: Metal Transfer
319
Types of Metal Transfer
 There are 2 main types of metal transfer :
• Free flight transfer and
• Short-circuiting or deep transfer.
 Free flight transfer: In which metal drops get detached from the electrode, pass
through the arc and fall on the job.
 This category of Free flight transfer can further be classified as four categories :
• Sub- threshold metal transfer
• Globular metal transfer
• Spray metal transfer
• Jet metal transfer
320
Fig. Types of metal transfer
Globular type
• Here the drop diameter is approximately twice the electrode wire diameter.
Ex: SMAW, SAW
• It is observed at low arc current or with larger arc.
• The no. of drop transferred per sec. is very less.
• The globules may pass freely through the welding arc or depending upon the
size and gap of the arc they may short circuit the arc.
• This transfer is associated with spatter loss and shallow penetration height.
321
Spray type
• Here the drop diameter is approximately equal to the electrode core wire
diameter.
• The rate of drop transfer is much higher than the globular transfer.
• Here is a continuous spray of drops.
• It occurs at high arc currents and low arc lengths.
• Though associated with some spatter, spray mode of transfer produces
stable arc, good weld bead, deep penetration, a strong joint and is
recommended for thicker plates .
 Jet type: In this case the electrode end becomes tapered and a jet of drops comes
out from the electrode .
322
 In MIG welding spray drop transfer using 1.63 mm dia wire are given below:
Description
Welding of
Steel in Ar
atmosphere
Steel in CO2
atmosphere
Copper in Ar
atmosphere
Al in Ar
atmosphere
Approx. current
range (amp)
Drop transfer rate
( drop /sec)
250-320
14-125
200-300
10-60
200-350
25-150
150-200
25-140
323
Characteristics of free flight metal transfer
• The temperature of the droplet formed from a steel electrode just as it detaches,
ranges from 1800-20000C .
• The size of the droplet ranges in between 0.5-5mm.
• For instance, drops of 0.75mm diameter and 3.5mm dia may posses transfer
velocities of approximately160cm/sec and 40cm/sec respectively.
324
Free flight metal transfer
 It can be seen from above that from A to D, arc gap goes on decreasing and
hence (in the voltage oscillogram ) arc voltage continuously decreases from A to D.
 At point E, the drop detaches from the neck (due to electromagnetic force neck
forms) and transfer from the job.
 Immediately at point F, the original arc gap is restored and arc voltage jumps to
normal value and the next cycles starts.
Fig. Steps of free flight metal transfer
325
 Arc voltage along with various steps in free flight metal transfer
Short-circuiting transfer
 In short- circuiting type of metal transfer:
 The arcing end of the electrode starts melting,
 Develops to a spherical shape,
 Makes contact with molten pool in the base metal and get detach from the
electrode.
• When the hanging drop touches the base metal, the circuit is shorted and arc
extinguishes.
• The moment the drop is detached from the electrode, the circuit again opens and
arc gets reignited.
• The short circuiting frequency is mainly dependent on: electrode wire dia. and arc
voltage, i.e., wire dia. increase and short circuit frequency decrease.
326
Fundamental of Welding Science and Technology
Lecture 13: Physics of Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
327
Forces affecting metal transfer
 There are mainly two types forces affecting the drop transfer:
• Aid metal transfer forces
• Retard metal transfer forces
In general various forces involved in the phenomenon are given below:
• Surface tension
• Viscosity of the liquid metal
• High velocity gas jets
• Gravity
• Lorentz forces.
328
Forces affecting metal transfer (cont’d)
 Surface tension force: It is a retarding force which tries to keep
the drop in its position.
 The force of surface tension acting on the drop when it is just to
detach is given by:
 where
d =electrode diameter
σ = surface tension (Force/distance)
k = it is a function dependent on electrode diameter and
capillarity constant of the material. Normally it varies
from 0.6 to 1.0.
 Note: The force of surface tension ranges from 400 to 800 dyne
for electrode from 1.5 to 3 mm. At higher temperature the surface
tension is lowered.
329
Forces affecting metal transfer (cont’d)
 Viscosity of liquid metal: It is retaining force.
 The high velocity gas jets striking the job and getting
back may retard the movement of metal drop tending to
fall down in the molten pool.
 Gravity: It acts as a detaching force when welding in flat
position and it is a retarding force when welding overhead.
 The force of gravity (ρvg), which depends upon volume of
globule.
 It is almost negligible on small diameter droplets.
330
Forces affecting metal transfer (cont’d)
 Lorentz force:
 This force is the result of interaction of the arc current
with its self–induced magnetic field.
 This force exercises pinch effect on the globule, aids in
the neck formation and drop detachment.
 The self-induced magnetic field of the arc plasma results
in plasma streaming which carries the detached drop to
the workpiece.
 The pinch force varies from 250 to 1000 dyne for 1.5 to 3
mm electrode dia.
 Note: Surface tension and viscosity of the liquid metal
help droplet to grow in size. Whereas electromagnetic
forces constrict (i.e., neck) the molten end of the
electrode due to that drop separates from electrode.
331
Metal transfer
 Introduction:
 When an electric arc is struck between the job and the consumable
electrode,
 The arcing end of the electrode starts melting,
 Takes approximately a spherical shape, hangs towards the job, and
 Ultimately drop down on the same, either with a free flight through
the arc or by short circuiting the job.
 The size of the droplet and the drop transfer rate affects weld bead
geometry, weld metal microstructure and strength of welded joints etc.
 Metal transfer can be studied by a high speed movie camera (30007000 frames/sec). Like GMAW (here metal transfer is well visible).
 But where the metal is not visible like (like SMAW or SAW) X-rays
have been used to study metal transfer.
332
Weld metal transfer
Fig.: Metal Transfer
333
Classification of metal transfer
There are 3 main types of metal transfer (IIW classification ) :
i. Free flight transfer
ii. Bridging transfer
iii. Slag protected transfer
i. Free flight transfer: (a) Globular (b) Spray and (c) Explosive
ii. Bridging transfer: (a) Short-circuiting (i.e. in short-arc GMAW) &
(b) Bridging without interruptions (Welding
with filler wire addition)
iii. Slag protected transfer: (a) Flux wall guided (SAW)
(b) Other modes(SMA, cored wire, ESW)
334
Free Flight Metal Transfer
 Free flight transfer: In which metal drops get detached from the
electrode, pass through the arc and fall on the job.
 Categories of Free flight transfer (depends on approx. size of
droplet):
• Sub-threshold metal transfer
• Globular metal transfer
• Spray metal transfer
• Jet metal transfer
335
Fig. Types of metal transfer
Globular metal transfer
 Here the drop diameter is approximately twice the electrode wire
diameter. Ex: SMAW, SAW.
 It is observed at low arc current or with larger arc.
 The no. of drop transferred per second is very less (1 to 10 drop/s).
 The globules may pass freely through the welding arc or depending
upon the size and gap of the arc they may short circuit the arc.
 This transfer is associated with spatter loss and shallow penetration
height.
336
Spray metal transfer
 Here the drop diameter is approximately equal to the electrode core
wire diameter.
 The rate of drop transfer is much higher than the globular transfer.
 Here is a continuous spray of drops.
 It occurs at high arc currents and low arc lengths.
 Spray mode of transfer produces stable arc, good weld bead, deep
penetration, a strong joint and is recommended for thicker plates .
 Jet type: In this case the electrode end becomes tapered and a jet
of drops comes out from the electrode.
337
Spray Transfer cont.
 In MIG welding spray drop transfer using 1.63 mm dia. wire are
given below:
Description
Welding of
Steel in Ar
atmosphere
Steel in CO2
atmosphere
Copper in Ar
atmosphere
Al in Ar
atmosphere
Approx. current
range (amp)
Drop transfer rate
( drop /sec)
250-320
14-125
200-300
10-60
200-350
25-150
150-200
25-140
338
Categories of Spray Transfer
There are mainly 3 different categories of spray metal transfer
(i) Projected (Due to Intermediate-current in GMAW)
(ii) Streaming (Due to Medium-current in GMAW)
(iii) Rotating (Due to High-current in GMAW)
339
Categories of Spray Transfer cont.
 Projected Spray Metal Transfer:
 Electromagnetic force is the main governing force.
 Projected spray is characterized by small droplets
(close to the electrode diameter) transferring from the
electrode tip to the weld pool at a rate of about
hundreds per second, without short-circuiting the
pool.
 Very regular and no significant amounts of spatter are
observed.
Fig. Projected spray transfer
 Projected spray transfer can only be used in the flat
position, because of the large volume of the molten
metal in the weld pool.
 A projected spray is obtained at high voltage (long arc) and intermediate-current
i.e. just above transition current in GMAW.
 The transition current is dependent on a great number of parameters, such as filler
material, shielding gas composition and electrode extension/ diameter.
340
Categories of Spray Transfer cont.
 Streaming Spray Metal Transfer:
 Electromagnetic
governing force.
force
is
the
main
 With a further increase of the welding
current, projected spray metal transfer
transforms into “streaming spray” transfer.
 Greater heat is produced in the electrode tip.
 The anodic area increases due to higher Fig. Streaming spray transfer
current arriving the wire end. As a result, a
wire volume above the arc-wire coupling is
heated enough to become plastic, resulting in
the “tapered” shape of the electrode end.
 At the tip, very fine droplets are formed and
detached. As long as this tapered end does
not touch the pool, there is no spatter.
341
Categories of Spray Transfer cont.
 Rotating spray Metal Transfer:
 Here also electromagnetic force is the main governing
force.
 This mode of metal transfer takes place by a further
increase in the current level from that of streaming spray.
 The wire electrode tapering effect is more pronounced with
overheating, resulting in an extended metal filament.
 Strong electromagnetic forces, caused by the excessively
high welding current applied, move the column away from
its straight line of flow.
 The combination of asymmetric radial forces and azimuthal forces results in a
spiral motion of the column.
 The droplets (extremely fine) are detached from the tip of the rotational filament in
tangential direction, producing a lot of spatter.
 This process was used to improve the sidewall penetration in the flat
position and prevent the molten pool sagging during the horizontal welding.
342
Explosive type metal transfer
 Explosive Metal Transfer:
 In certain gas and wire compositions, droplets
attached to the electrode tip eject material in
an explosive manner in which small droplets
are expelled from the molten part of the
electrode tip and transferred to the weld pool.
 Here the droplet spattered on the tip of
electrode after detachment.
 This is due to chemical reactions between
gas–metal inside the droplet.
 It is usually accompanied by considerable
amount of fine spatter.
 Electromagnetic force and chemical reactions
are the main governing force.
343
Characteristics of free flight metal transfer
 The temperature of the droplet formed from a steel electrode
just as it detaches, ranges from 1800-2000 oC .
 The size of the droplet ranges in between 0.5-5 mm.
 For instance, drops of 0.75 mm and 3.5 mm diameter may
posses transfer velocities of approximately160 cm/s and 40
cm/s respectively.
344
Characteristics of free flight metal transfer
 The details of formation of neck and detachment of molten
globule in GMAW process are given below:
 It can be seen from below figure that from A to D, arc gap goes on
decreasing and hence arc voltage continuously decreases from A
to D.
 At point E, the drop detaches from the neck (due to
electromagnetic force neck forms) and transfer to the job.
 Immediately at point F, the original arc gap is restored and arc
voltage jumps to normal value and the next cycles starts.
345
Fig. Steps of free flight metal transfer
Characteristics of free flight metal transfer
 Arc voltage along with various steps in free flight metal transfer (i.e.
the voltage oscillogram):
346
Bridging transfer:
(a) Short-circuiting &
(b) Bridging without interruptions
347
Bridging Metal Transfer
 Bridging Metal Transfer:
 Happens when the electrode wire is subject to only low shortcircuit current during the contact drop-pool.
 The surface tension becomes the driving force for metal
transfer, reducing the importance of the pinch effect on droplet
detachment.
 Neither droplet repulsion (low pool and droplet oscillation) nor
spatter generation is observed with bridging transfer leads to a
uniform bead appearance.
 Usually generated with a constant current power source
characteristic and/or very high inductance levels.
 The transfer mode can be properly used for, e.g., joining thin
sheet metals.
348
Short-circuiting transfer
 In short-circuiting type of metal transfer:
 The arcing end of the electrode starts melting
 Develops to a spherical shape
 Makes contact with molten pool in the base
metal and get detach from the electrode.
 When the hanging drop touches the base metal,
the circuit is shorted and arc extinguishes.
 The moment the drop is detached from the
electrode, the circuit again opens and arc gets
reignited.
 The short circuiting frequency is mainly
dependent on: electrode wire dia. and arc
voltage, i.e., wire dia. increase and short circuit
frequency decrease.
349
Short-circuiting transfer cont.
• Normal short circuiting ranges from 20 to 200 per second.
 At CO2 atmosphere of steel welding the short circuiting voltage
is about 20V and maximum short circuiting frequency:
 For 1.5 mm electrode dia. = 75/sec.
 For 0.75 mm electrode dia. = 150/sec.
350
Characteristics of short-circuiting metal transfer
 During period A to B, the drop grows, hangs and thus the arc voltage
decreases.
 At point B short circuit occurs, arc extinguishes and voltage drops down.
 Short circuit remains between C to E, during which current increases
because of reduced resistance between electrode and workpiece.
 Electrode pinch effect increases due to increase of current during shorting,
neck formation (D point) quickens and ultimately at point E drops get
detached from the electrode. At this stage arc reignites and arc voltage shorts
to normal value.
 At point F once again drop formation starts.
351
Fig. Steps of short-circuiting transfer
Characteristics of short-circuiting transfer cont.
Arc voltage along with various steps in which short-circuiting
transfer takes place:
352
 Other type of Metal Transfer
 Repelled type (free flight, globular type)
 Pulsed type (free flight, spray type)
353
Repelled transfer
 This type of metal transfer obtained in MIG welding when using
a) CO2 as shielding gas and
b) Other shielding gases and DCSP.
 Here the droplet appears to be repelled towards the side of the
electrode. Gradually neck formation takes place and drop separates.
 The repelled nature of the droplets may be due to high velocity gas
jet striking the workpiece and getting back.
354
Fig. Repelled mode of metal transfer
Pulsed transfer
 Pulsed arc welding is a controlled method of spray transfer welding
requiring a more sophisticated power source.
 In spray transfer, metal transfer along arc generally occurs at
constant current.
 In short circuit transfer, the current generally irregular in nature.
 But in this case, transfer of metal from the wire tip to molten pool
occurs only at a period of pulse or peak current.
 During the interval between pulses, a low „background‟ current
maintains the arc to keep the wire tip molten but no metal is
transferred.
 Note: Here we can control the deposition rate (by adjusting pulse rate
and peak current and background current).
355
Pulsed transfer (cont.)
Fig. Sequence of events in pulsed metal transfer
Time vs. current during the sequence of events is as below:
356
The drop transfer rate also depends on
 Arc current
 Arc length
 Type of polarity
 Electrode material and
 Electrode extension
 Note: The drop transfer increases with DCEP (i.e. DCRP),
with increase in arc current and electrode extension.
357
Effect of voltage & Current on metal transfer
Fig. Effect of arc current and voltage on drop transfer
 Arc current increase drop transfer rate increases. Because increasing the
current increases the electrode burn off rate.
 At a constant current, drop transfer rate decreases as the voltage increases,
because the heat losses increase as at high voltage, the arc length
increases.
 For longer arc heat loss is more.
358
Effect of Polarity on Metal Transfer
Electrode Positive:
 At low welding currents the size of the droplet in argon develops to
a diameter more than the diameter of the electrode (i.e. globular).
The droplet size is roughly inversely proportional to the current and
only a few droplets are released per second.
 With long arc length, the droplets are transferred without short
circuit.
 In spray transfer, the tip of the electrode becomes pointed and the
drops are transferred at a rate of about a hundred per second. The
current at which this occurs is called transition current.
 Axial spray transfer is stable. There is no spatter, the drops are
transferred in line with electrode. The metal can therefore be
directed where needed for making fillet vertical or overhead welds.
359
Effect of Polarity on Metal Transfer
 Electrode Negative:
 GMAW arc becomes unstable and spattery when electrode
negative is used.
 The drop size is big and due to arc forces the drops are propelled
away from the workpiece as spatter.
 Spray transfer is observed in argon shielded consumable electrode
arc only. It appears that argon provides the unique plasma
properties with the self-magnetic force to develop axial spray
transfer through the arc.
360
Effect other of Gases on Metal Transfer
 Helium gas:
 Helium, although inert gas, does not produce axial spray transfer.
The transfer is globular with both polarities at all current levels.
 Helium arcs are useful, nevertheless, because they provide deep
penetration.
 Spray transfer can be obtained by mixing small quantities of Argon
(about 20 %).
361
Effect other of Gases on Metal Transfer
 Carbon dioxide and Nitrogen:
 Active gases like carbon dioxide and nitrogen do not produce spray
transfer, spatter on the other hand is increased.
 Spatter can be minimised by burying the arc below the plate surface
to trap the spatter in the deep arc crater. This technique is used
when:
 Carbon dioxide is used to shield arcs in mild steel.
 Nitrogen is used mixed with argon to shield aluminium alloys.
 Nitrogen is used to shield copper.
 The amount of spatter, massiveness of the drops and instability of
transfer generally are greater when the electrode is negative.
 Spray transfer can be achieved by painting cesium and sodium on
steel wire surface with carbon dioxide shield using direct current
electrode negative polarity (i.e. DCEN).
362
End
363
transition current
For projected transfer: Below the “transition current” and with moderate to high voltage, the transfer is globular. If the current is set above the
transition current, the spherical droplets become progressively smaller, correspondingly increasing the transfer
The radial, compressing, fraction of the electromagnetic force increases dramatically, subjecting the droplet to a strong pinch effect, limiting its
volume and size and allowing only a small droplet to be formed.
The role of azimuthal forces as a factor hindering the manufacture of quasi-force-free magnet windings is shown. It is established
that these forces are caused by the transverse magnetic field that arises due to deviations of a real winding from the calculated
configuration.
Globular transfer means the weld metal transfers across the arc in large droplets, usually larger than the diameter of the
electrode being used. This mode of transfer generally is used on carbon steel only and uses 100 percent CO2 shielding gas. The
method typically is used to weld in the flat and horizontal positions because the droplet size is large and would be more difficult to
control if used in the vertical and overhead positions compared to the short-circuit arc transfer. This mode generates the most
spatter; however, when higher currents are used with CO2 shielding and a buried arc, spatter can be greatly reduced. You must use
caution with a buried arc because this can result in excessive reinforcement if travel speed isn't controlled.
Stainless steel GMAW electrodes normally aren't used in this mode of transfer because their nickel and chrome content (9 to 14
percent nickel and 19 to 23 percent chromium) creates a higher electrical resistance than carbon steel electrodes. In addition to the
electrical resistance differences, the use of 100 percent CO2 as a shielding gas could be detrimental to the corrosion resistance of
the stainless steel electrodes. Carbon steel ER70S-3 and ER70S-6 generally are the electrodes of choice.
Kink: form or cause to form a sharp twist or curve.





During period A to B, the drop grows, hangs and thus the arc voltage decreases.
At point B short circuit occurs, arc extinguishes and voltage drops down. (due to no arc length gap voltage reduced)
Short circuit remains between C to E, during which current increases because of reduced resistance between electrode and workpiece.
Electrode pinch effect increases due to increase of current during shorting, neck formation (D point ) quickens and ultimately at point E
drops get detached from the electrode. At this stage arc reignites and arc voltage shorts to normal value.
At point F once again drop formation starts.
Newton's viscosity law's states that, the shear stress between adjacent fluid
layers is proportional to the velocity gradients between the two layers. The ratio
of shear stress to shear rate is a constant, for a given temperature and pressure,
and is defined as the viscosity or coefficient of viscosity.
Viscocity unit is N364
S/m2
Fundamental of Welding Science and Technology
Lecture 14: Physics of Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
365
Welding Energy Input
 The energy input, ‘H’ is computed as the ratio of total input power
‘P’ of the heat source to its welding travel velocity, w.
H= P/w
If the source of heat is an electric arc then,
H= VI/w
Preciously speaking the net energy input would be
Hnet = ƞVI/w
where ƞ = the heat transfer efficiency.
366
Efficiency of welding
 In welding there are two different types of efficiency:
i.
Heat source efficiency
ii. Melting or heat transfer efficiency
367
Heat source efficiency
 In the case of arc welding, having a constant voltage E and a
constant current I, the arc efficiency can be expressed as;
where Q
Qnominal
tweld
is the rate of heat transfer from heat source to work piece
is the nominal rate of heat of the power source
is the welding time
 The term, heat input per unit length of weld often refers to
where
V
Qnominal /V
is the welding speed
is heat input per unit length of weld
368
Measurement of Heat source efficiency
 Heat source efficiency can be measured using a calorimeter (by measuring
the heat transfer from the heat source to the workpiece and then to the
calorimeter).
 The temperature rise in the cooling water
(Tout-Tin) can be measured using
thermocouples or thermistors. Heat
transfer from the workpiece to the
calorimeter is given by:
where
W is the mass flow rate of water
C
is the specific heat of water
Tout is the outlet water temperature
Tin is the inlet water temperature
t
is time
Fig. Rise in cooling water
temperature as a function of time.
 Note: The calorimeter can be a round cross section if the workpiece is a pipe
or a rectangular cross section if the workpiece is a sheet.
369
Heat Source Efficiencies
Fig. Heat Source Efficiencies in Various Welding Processes
370
Melting Efficiency
 Melting efficiency is the ability of the heat source to melt the base metal
(as well as the filler metal).
 The melting efficiency of the arc ηm can be defined as follows:
where
V
Hbase
Hfiller
tweld
Abase
Afiller
is the welding speed
is the energy required to raise a unit volume of
base metal to the melting point and melt it.
is the energy required to raise a unit volume of
filler metal to the melting point and melt it.
is the welding time.
is cross-sectional area base metal which is melted
is cross-sectional area filler metal.
Aweld = Afiller +Abase
 Note: The quantity inside the parentheses represents the volume of material melted.
And the denominator represents the heat371transfer from the heat source to the workpiece.
Increase of V and tweld results in increase of melting efficiency of the arc ηm .
Melting Efficiency (contd.)
 With the help of the following equation for determining Afiller,
 where Rfiller and Vfiller are the radius and feeding speed of the filler metal,
respectively.
372
373
374
Fundamental of Welding Science and Technology
Lecture 15: Physics of Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
375
• Metal transfer
• Efficiency of welding
376
Classification of Metal Transfer
There are 3 main types of metal transfer (IIW classification ) :
i. Free flight transfer
ii. Contact transfer
iii. Slag protected transfer
i. Free flight transfer: (a) Globular (b) Spray and (c) Explosive
ii. Contact transfer: (a) Bridging &
(b) Short-circuiting (i.e. in short-arc GMAW)
iii. Slag protected transfer: (a) Flux wall guided
(b) Other modes(SMA, cored wire, ESW)
377
Contact transfer:
(a) Bridging
&
(b) Short-circuiting
378
Short-circuiting Transfer
 Short-circuiting type of metal transfer:
 Here the arcing end of the electrode starts melting
 Develops to a spherical shape
 Makes contact with molten pool in the base metal
and get detach from the electrode.
 When the hanging drop touches the base metal, the
circuit is shorted and arc extinguishes.
 Electrode pinch effect increases due to increase of current during
shorting, neck formation quickens and ultimately the drops get
detached from the electrode.
 The moment the drop is detached from the electrode, the circuit
again opens and arc gets reignited.
379
Short-circuiting Transfer cont.
 Normal short circuiting ranges from 20 to 200 per second.
 The short circuiting frequency is mainly dependent on: electrode
wire dia. and arc voltage, i.e., with increases of wire dia. and
voltage decreases the short circuit frequency.
 At CO2 atmosphere of steel welding the short circuiting voltage
is about 20 V and maximum short circuiting frequency:
 For 1.5 mm electrode dia. = 75/sec.
 For 0.75 mm electrode dia. = 150/sec.
380
Characteristics of short-circuiting metal transfer
 During period A to B, the drop grows, hangs and thus the arc voltage
decreases.
 At point B short circuit occurs, arc extinguishes and voltage drops down.
 Short circuit remains between C to E, during which current increases
because of reduced resistance between electrode and workpiece.
 Electrode pinch effect increases due to increase of current during shorting,
neck formation (D point) quickens and ultimately at point E drops get
detached from the electrode. At this stage arc reignites and arc voltage shorts
to normal value.
 At point F once again drop formation starts.
381
Fig. Steps of short-circuiting transfer
Characteristics of short-circuiting transfer cont.
Arc voltage along with various steps in which short-circuiting
transfer takes place:
382
Slag protected transfer
383
 Some More Type of Metal Transfer
 Repelled type (free flight, globular type)
 Pulsed type (free flight, spray type)
 Metal Transfer from Additional Filler Wire
384
Repelled Transfer
 This type of metal transfer obtained in MIG welding when using
a) CO2 as shielding gas and
b) Other shielding gases and DCSP.
 Excessive vapor can be formed in the pool by some shielding gases,
especially CO2 rich gases.
 Here the droplet appears to be repelled towards the side of the
electrode. Gradually neck formation takes place and drop separates.
 The repelled nature of the droplets may be due to high velocity gas
jet striking the workpiece and getting back.
 Droplet transfer occurs when
gravity and aerodynamic forces
exceed the repelling arc forces.
385
Fig. Repelled mode of metal transfer
Pulsed Transfer
 Pulsed arc welding is a controlled method of spray transfer welding
requiring a more sophisticated power source.
 In spray transfer, metal transfer along arc generally occurs at
constant current.
 In short circuit transfer, the current generally irregular in nature.
 But in this case, transfer of metal from the wire tip to molten pool
occurs only at a period of pulse or peak current.
 During the interval between pulses, a low ‘background’ current
maintains the arc to keep the wire tip molten but no metal is
transferred.
 Note: Here we can control the deposition rate (by adjusting pulse rate
i.e. by controlling the peak current and background current pulse).
386
Pulsed Transfer (cont.)
Fig. Sequence of events in pulsed metal transfer
Time vs. current during the sequence of events is as below:
387
Metal Transfer from Additional Filler Wire
 Metal transfer from additional filler wire takes place when such a
filler wire or filler rod is used as in GTAW, PAW and the oxy-fuel
gas welding.
 In these welding processes the filler wire is melted by the
application of heat without forming a part of the electrical circuit.
 The forces acting on the molten droplet are similar to those in
GMAW and SMAW however the electromagnetic pinch effect does
not play any part by being absent. The transfer, therefore, cannot
approach the spray mode.
 Most often short-circuit (or bridging) mode of metal transfer is
adopted to make the maximum use of heat however, globular
transfer may also be used, if required.
 Globular transfer, when used, results in lower
deposition efficiency due to delayed
detachment of the droplet from the filler wire.
388
Effect of parameters on metal transfer
 The drop transfer rate also depends on
 Arc current
 Arc length
 Type of polarity
 Electrode material and
 Electrode extension
 Shielding gas
 Note: The drop transfer increases with DCEP (i.e. DCRP),
with increase in arc current and electrode extension.
389
Effect of voltage & current on metal transfer
Fig. Effect of arc current and voltage on drop transfer
 Arc current increase drop transfer rate increases. Because increasing the
current increases the electrode burn off rate.
 At a constant current, drop transfer rate decreases as the voltage increases,
because the heat losses increase as at high voltage, the arc length
increases.
 For longer arc heat loss is more.
390
Effect of Polarity on Metal Transfer
 Electrode Positive:
 By this polarity stable arc and drop transfer can be obtained.
 By varying the current the drop transfer rate and type can be varied.
 At low welding currents the size of the droplet in argon develops to
a diameter more than the diameter of the electrode (i.e. globular).
The droplet size is roughly inversely proportional to the current and
only a few droplets are released per second.
 With long arc length, the droplets are transferred without short
circuit.
 In spray transfer, the tip of the electrode becomes pointed and the
drops are transferred at a rate of about a hundred per second. The
current at which this occurs is called transition current.
391
Effect of Polarity on Metal Transfer
 Electrode Negative:
 GMAW arc becomes unstable and spattery when electrode
negative is used.
 The drop size is big and due to arc forces the drops are propelled
away from the workpiece as spatter.
 Spray transfer can be observed in argon shielded consumable
electrode arc only. It appears that argon provides the unique
plasma properties with the self-magnetic force to develop axial
spray transfer through the arc.
392
Effect other of Gases on Metal Transfer
 Helium gas:
 Helium, although inert gas, does not produce axial spray transfer.
The transfer is globular with both polarities at all current levels.
 Helium arcs are useful, nevertheless, because they provide deep
penetration.
 Spray transfer can be obtained by mixing small quantities of Argon
gas (about 20 %).
393
Effect other of Gases on Metal Transfer
 Carbon dioxide and Nitrogen:
 Active gases like carbon dioxide and nitrogen do not produce spray
transfer, spatter on the other hand is increased.
 The amount of spatter, massiveness of the drops and instability of
transfer generally are greater when the electrode is negative.
 Spray transfer can be achieved by painting cesium and sodium on
steel wire surface with carbon dioxide shield using direct current
electrode negative polarity (i.e. DCEN).
 Some applications of these gases as a shielding medium:
 Carbon dioxide can be used to shield arcs in mild steel.
 Nitrogen can be used mixed with argon to shield aluminium
alloys.
 Nitrogen is used to shield copper.
394
Efficiency of welding
395
Welding Energy Input
 The energy input per unit length, ‘H’ is computed as the ratio of
total input power ‘P’ of the heat source to its welding travel
velocity, w.
H= P/v
 If the source of heat is an electric arc then,
H= EI/v
 Preciously speaking the net energy input would be
Hnet = ƞEI/v
 where ƞ = the heat transfer efficiency.
396
Efficiency of welding
 In welding there are two different types of efficiency:
i. Heat source efficiency
ii. Melting or heat transfer efficiency
397
Heat source efficiency
 In the case of arc welding, having a constant voltage E and a
constant current I, the arc/ heat source efficiency can be expressed as;
where Q
Qnominal
tweld
is the rate of heat transfer from heat source to work piece
is the nominal rate of heat of the power source
is the welding time
398
Measurement of Heat source efficiency
 Heat source efficiency can be measured using a calorimeter (by measuring
the heat transfer from the heat source to the workpiece and then to the
calorimeter).
 The temperature rise in the cooling water
(Tout-Tin) can be measured using
thermocouples or thermistors. Heat
transfer from the workpiece to the
calorimeter is given by:
where
W
C
Tout
Tin
t
is the mass flow rate of water
is the specific heat of water
is the outlet water temperature
is the inlet water temperature
is time
Fig. Rise in cooling water
temperature as a function of time.
 Note: The calorimeter can be a round cross section if the workpiece is a pipe
or a rectangular cross section if the workpiece is a sheet.
399
Heat Source Efficiencies
Fig. Heat Source Efficiencies in Various Welding Processes
400
Melting Efficiency
 Melting efficiency is the ability of the heat source to melt the base metal
(as well as the filler metal).
 The melting efficiency of the arc ηm can be defined as follows:
where
V
Hbase
Hfiller
tweld
Abase
Afiller
is the welding speed
is the energy required to raise a unit volume of
base metal to the melting point and melt it.
is the energy required to raise a unit volume of
filler metal to the melting point and melt it.
is the welding time.
is cross-sectional area base metal which is melted
is cross-sectional area filler metal.
Aweld = Afiller +Abase
 Note: The quantity inside the parentheses represents the volume of material melted.
 And the denominator represents the heat401transfer from the heat source to the workpiece.
 Increase of V and tweld results in increase of melting efficiency of the arc ηm .
Melting Efficiency (contd.)
 With the help of the following equation for determining Afiller,
 where Rfiller and Vfiller are the radius and feeding speed of the filler metal,
respectively.
402
End
403
Fundamental of Welding Science and Technology
Lecture 16: Physics of Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
404
• Effect of Parameters on Metal transfer
• Efficiency of welding
405
Effect of parameters on metal transfer
 The drop transfer rate also depends on
 Arc current
 Arc length
 Type of polarity
 Electrode material and
 Electrode extension
 Shielding gas
 Note: The drop transfer increases with DCEP (i.e. DCRP),
with increase in arc current and electrode extension.
406
Effect of voltage & current on metal transfer
Fig. Effect of arc current and voltage on drop transfer
 Arc current increase drop transfer rate increases. Because increasing the
current increases the electrode burn off rate.
 At a constant current, drop transfer rate decreases as the voltage increases,
because the heat losses increase as at high voltage, the arc length
increases.
 For longer arc heat loss is more.
407
Effect of Polarity on Metal Transfer
 Electrode Positive:
 By this polarity stable arc and drop transfer can be obtained.
 By varying the current the drop transfer rate and type can be varied.
 At low welding currents the size of the droplet in argon develops to
a diameter more than the diameter of the electrode (i.e. globular).
The droplet size is roughly inversely proportional to the current and
only a few droplets are released per second.
 With long arc length, the droplets are transferred without short
circuit.
 In spray transfer, the tip of the electrode becomes pointed and the
drops are transferred at a rate of about a hundred per second. The
current at which this occurs is called transition current.
408
Effect of Polarity on Metal Transfer
 Electrode Negative:
 GMAW arc becomes unstable and spattery when electrode
negative is used.
 The drop size is big and due to arc forces the drops are propelled
away from the workpiece as spatter.
 Spray transfer can be observed in argon shielded consumable
electrode arc only. It appears that argon provides the unique
plasma properties with the self-magnetic force to develop axial
spray transfer through the arc.
409
Effect other of Gases on Metal Transfer
 Helium gas:
 Helium, although inert gas, does not produce axial spray transfer.
The transfer is globular with both polarities at all current levels.
 Helium arcs are useful, nevertheless, because they provide deep
penetration.
 Spray transfer can be obtained by mixing small quantities of Argon
gas (about 20 %).
410
Effect other of Gases on Metal Transfer
 Carbon dioxide and Nitrogen:
 Active gases like carbon dioxide and nitrogen do not produce spray
transfer, spatter on the other hand is increased.
 The amount of spatter, massiveness of the drops and instability of
transfer generally are greater when the electrode is negative.
 Spray transfer can be achieved by painting cesium and sodium on
steel wire surface with carbon dioxide shield using direct current
electrode negative polarity (i.e. DCEN).
 Some applications of these gases as a shielding medium:
 Carbon dioxide can be used to shield arcs in mild steel.
 Nitrogen can be used mixed with argon to shield aluminium
alloys.
 Nitrogen is used to shield copper.
411
Efficiency of welding
412
Welding Energy Input
 The energy input per unit length, ‘H’ is computed as the ratio of
total input power ‘P’ of the heat source to its welding travel
velocity, w.
H= P/v
 If the source of heat is an electric arc then,
H= EI/v
 Preciously speaking the net energy input would be
Hnet = ƞEI/v
 where ƞ = the heat transfer efficiency.
413
Efficiency of welding
 In welding there are two different types of efficiency:
i. Heat source efficiency
ii. Melting or heat transfer efficiency
414
Heat source efficiency
 In the case of arc welding, having a constant voltage E and a
constant current I, the arc/ heat source efficiency can be expressed as;
where Q
Qnominal
tweld
is the rate of heat transfer from heat source to work piece
is the nominal rate of heat of the power source
is the welding time
415
Measurement of Heat source efficiency
 Heat source efficiency can be measured using a calorimeter (by measuring
the heat transfer from the heat source to the workpiece and then to the
calorimeter).
 The temperature rise in the cooling water
(Tout-Tin) can be measured using
thermocouples or thermistors. Heat
transfer from the workpiece to the
calorimeter is given by:
where
W
C
Tout
Tin
t
is the mass flow rate of water
is the specific heat of water
is the outlet water temperature
is the inlet water temperature
is time
Fig. Rise in cooling water
temperature as a function of time.
 Note: The calorimeter can be a round cross section if the workpiece is a pipe
or a rectangular cross section if the workpiece is a sheet.
416
Heat Source Efficiencies
Fig. Heat Source Efficiencies in Various Welding Processes
417
Melting Efficiency
 Melting efficiency is the ability of the heat source to melt the base metal
(as well as the filler metal).
 The melting efficiency of the arc ηm can be defined as follows:
where
V
Hbase
Hfiller
tweld
Abase
Afiller
is the welding speed
is the energy required to raise a unit volume of
base metal to the melting point and melt it.
is the energy required to raise a unit volume of
filler metal to the melting point and melt it.
is the welding time.
is cross-sectional area base metal which is melted
is cross-sectional area filler metal.
Aweld = Afiller +Abase
 Note: The quantity inside the parentheses represents the volume of material melted.
 And the denominator represents the heat418transfer from the heat source to the workpiece.
 Increase of V and tweld results in increase of melting efficiency of the arc ηm .
Melting Efficiency (contd.)
 With the help of the following equation for determining Afiller,
 where Rfiller and Vfiller are the radius and feeding speed of the filler metal,
respectively.
419
End
420
Fundamental of Welding Science and Technology
Lecture 17: Physics of Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
421
1
 Melting Efficiency
 Welding Parameters and Their Effects
422
2
Melting Efficiency
 Melting efficiency is the ability of the heat source to melt the base metal
(as well as the filler metal).
 The melting efficiency of the arc ηm can be defined as follows:
where
V
Hbase
Hfiller
tweld
Abase
Afiller
is the welding speed
is the energy required to raise a unit volume of
base metal to the melting point and melt it.
is the energy required to raise a unit volume of
filler metal to the melting point and melt it.
is the welding time.
is cross-sectional area base metal which is melted
is cross-sectional area filler metal.
Aweld = Afiller +Abase
 Note: The quantity inside the parentheses represents the volume of material melted.
 And the denominator represents the heat423transfer from the heat source to the workpiece.
3
 Increase of V results in increase of melting efficiency of the arc ηm .
Melting Efficiency (contd.)
 With the help of the following equation for determining Afiller,
 where Rfiller and Vfiller are the radius and feeding speed of the filler metal,
respectively.
424
4
Welding Parameters and Their Effects
 Weld quality and weld deposition rate both are influenced by
various welding parameters and joint geometry. These parameters
are the process variables as given below:
 Welding current
 Arc voltage
 Welding speed
 Electrode feed rate
 Electrode extension (stick-out)
 Electrode diameter
 Joint geometry
425
5
Welding Parameters and Their Effects cont.
 Each of the above parameters affects, to varying extent, the following:
 Deposition rate
 Weld-bead shape
 Depth of penetration
 Cooling rate
 Weld induced distortion.
 So, a proper understanding of the effects of welding parameters is
important to obtain a sound welded joint with adequate metal
deposition rate and minimum distortion.
426
6
Welding Parameters and Their Effects cont.
 Welding Current:
 For a given electrode and polarity in DC welding, melting rate is
directly proportional to the energy (current and voltage).
 Part of this energy Q is used to melt the base metal (qb), part goes
to melt electrode and flux (qf) rest is dissipated as conduction (qep
+ qce), convection (qv) and radiation (qr).
Q = qb + qf + (qcp + qce) + qv + qr
Here, Q = EI (Watt i.e. J/s)
= I2 Ra (J/s)
 where Q = electrical power consumed
I = welding current
E = arc voltage
Ra= arc resistance
427
7
Welding Parameters and Their Effects cont.
 Welding Current:
 It is most important variable affecting melting rate, the deposition
rate, the depth of penetration and the amount of base metal melted.
 If the current (for a given welding speed) is too high, it will result in:
 Excessive penetration (thinner plates will melt through)
 Excessive melting of electrode-excessive reinforcement
 More heat input to plates being joined increased distortions
 If the welding current is too low, it will result in:
 Inadequate penetration
 Lack of fusion
 Note: Current could be AC or DC. DC provides steady arc, smooth
metal transfer, good wetting action and uniform weld bead size. 8
428
Welding Parameters and Their Effects cont.
 Arc voltage:
 Arc voltage is the voltage between the job and the electrode
during welding. For a given electrode it depends upon the arc
length.
 Open circuit voltage approximately varies between 50-100 V
whereas arc-voltage are between 15 V to 40 V. When the arc is
struck, the open circuit voltage drops to arc voltage and welding
load comes on power supply.
 The arc voltage depends on arc length and type of electrode.
 As the length increases, arc resistance increases, resulting in
higher voltage drop i.e., arc-voltage increases.
429
9
Welding Parameters and Their Effects cont.
 Arc voltage:
 Proper arc length is important in obtaining a sound joint.
 Short arc: It may causes short circuits during metal transfer
 Long arc--lacks direction and intensity, gives heavy spatter and
formation of undercuts.
 Weld-bead appearance depends on arc-voltage.
 Increase in arc-voltage tends to cause porosity, spatter, flatten the
weld bead and increase weld width.
 Reduction in arc-voltage leads
to: narrower weld-bead and
higher crown.
 Trials are, therefore, made to
obtain optimum arc voltage.
430
Fig. Effect of welding voltage on weld
bead shape keeping all other parameters
10
constant
Welding Parameters and Their Effects cont.
 Welding speed: It generally conforms to a given combinations of
welding current and arc voltage.
 If welding speed is more than required:
 Heat input to the joint decreases
 Less filler metal is deposited than requires, less weld reinforcement
height
 Undercut, arc blow, porosity and uneven bead shape may result.
 If welding speed is slow:
 Filler metal deposition per length increases, more weld reinforcement
 Heat input per unit length increases
 Weld width increases and reinforcement height also increases more
convexity
 Penetration decreases beyond a certain decrease in speed.
 A large weld pool, rough bead and possible slag inclusion.
431
11
Welding Parameters and Their Effects cont.
 Welding speed:
 With all variables held constant, weld penetration depth attains a
maximum at a certain intermediate speed.
 At excessively low welding speeds the arc strikes a large molten
pool, the penetrating force get cushioned by the molten pool.
 With excessively high welding speeds, there is substantial drop in
thermal energy per unit length of welded joint resulting in
undercutting along the edges of the weld bead.
 It is because of insufficient backflow of filler metal to fill the
path melted by the arc.
 Note: Welding speed is to be adjusted within limits to control weld
12
size and depth of penetration.
432
Welding Parameters and Their Effects cont.
 Electrode feed speed:
 Electrode feed rate determines the amount of metal
deposited per unit length or per unit time.
 In most welding machines the welding current adjusts itself
with electrode feed speed to maintain proper arc length.
 Electrode Extension:
 Electrode extension, also known as length of stick out, is the distance
between the end of the contact tube and the end of the electrode.
 An increase in electrode extension results in an increase in electrical
resistance.
 This causes resistance heating of electrode extended length, resulting in
additional heat generation and increase of electrode melting rate.
 But the energy so consumed reduces the power delivered to the arc thus
decreases bead width and penetration depth.
433
13
Welding Parameters and Their Effects cont.
Electrode Extension:
 To maintain proper bead geometry along with a desired penetration
and higher melting rate (i.e., large electrode extension), the machines
voltage setting must be increased to maintain proper arc length.
 At current densities above 125 A/mm2, electrode extension becomes
important.
 An increase of upto 50% in deposition rate can be achieved by
using long electrode extensions without increasing welding current.
 This increase in deposition rate is accompanied with decrease with
decrease in penetration.
 Thus when deep penetration is desired long electrode extension is
not desirable.
 On the other hand, for thinner plates, to avoid the possibility of
melting through, a longer electrode extension becomes beneficial.
434
14
Welding Parameters and Their Effects cont.
 Electrode Diameter:
 It affects bead configuration, affecting penetration and deposition
rate.
 At any given current, a smaller diameter electrode will give higher
current density causing a higher deposition rate compared to large
diameter electrode.
 A larger diameter electrode, however requires a higher minimum
current to achieve the same metal transfer characteristics. Thus
larger electrode will produce higher deposition rate at higher
current.
 In case of poor fit-up or thick plates welding larger electrode size is
better to bridge large root openings than smaller ones.
435
Fig: Effect of electrode size on Bead
geometry keeping current voltage and
speed constant
15
Welding Parameters and Their Effects cont.
 Joint geometry
436
16
End
437
17
438
18
Fundamental of Welding Science and Technology
Lecture 18: Oxy-Fuel Gas Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
439
1
Introduction
 Oxyfuel Gas Welding (OFW) is a manual welding process in which
the metal surfaces to be joined are melted progressively by heat from
a gas flame, with or without filler metal, and solidify without the
application of pressure to the parts being joined.
Oxygen + Fuel Gas
Oxy Fuel Gas Welding.
 Gas is used to produced arc.
 Commonly used fuel gas for OFW are H2, CH4, C3H8, C2H2.
 Oxy Acetylene Welding (OAW) is one of the popular Oxyfuel Gas
Welding process in which acetylene is used as a gas to produce arc.
440
2
Principle of the Process
 Heat required for welding is obtained from the flame generated
due to oxy-fuel mixture combustion.
 The flame resulting at the tip of the torch is sufficient to melt and
join the parent metal.
 A filler metal rod is generally added to molten metal pool to build
up the seam slightly for greater strength.
 This welding does not require the components to be forced
together under pressure until the weld forms and solidifies.
441
3
Oxy Fuel Welding Setup
 The simplest and most frequently used OFW system consists of:
 Compressed gas cylinders
 Gas pressure regulators
 Hoses, and
 A welding torch
 The gas regulator attached to each cylinder, whether fuel gas or
oxygen, controls the pressure at which the gas flows to the welding
torch.
442
4
Oxy Fuel Welding Setup (Cont.)
 Oxygen and fuel gases are
stored in separate cylinders.
 At the torch, the gas passes
through an inlet control valve,
through tubes within the
handle, and into the mixing
chamber of the welding nozzle
attached to the welding torch.
 The mixed gases then pass
through the welding tip and
produce the flame at the exit
end of the tip.
 Filler metal, when needed, is provided by a welding filler rod that is melted
progressively along with the surfaces to be joined.
443
5
Gas Torch & Regulator
444
6
Gases
 Oxygen and acetylene are the principal gases used in OFW.
 Oxygen supports combustion of the fuel gases.
 Acetylene supplies both the heat intensity and the atmosphere
needed to weld steel.
 Hydrogen, natural gas (methane), propane and proprietary gases
(i.e. LPG) are used only to a limited extent in oxy-fuel gas welding
or brazing of metals with a low melting temperature.
445
7
Gases (cont.)
 Oxygen is supplied for oxy-fuel gas welding and cutting at a purity
of 99.5% and higher, because small percentages of contaminants
have a noticeable effect on combustion efficiency.
 When the consumption requirement is relatively small, the oxygen is
supplied and stored as a compressed gas in a standard steel cylinder
under an initial pressure of up to 180 MPa.
 The most frequently used cylinder has
a capacity of 6.91 m3.
 When oxygen consumption exceeds approximately 6.91 m3 cylinders
per week, it may be more economical to obtain and store oxygen in
liquid form.
446
8
Gases (cont.)
Acetylene (C2H2): It is a hydrocarbon gas.
 This gas is unstable when it is under a pressure of 203 kPa and
above, and a slight shock can cause it to explode, even in the absence
of oxygen or air.
 Safety rules for the use of acetylene and the handling of acetylene
equipment are extremely important. This gas should not be used at
pressure greater than 105 kPa.
 Acetylene cylinders must not be subjected to sudden shock and
should be stored well away from any source of heat or sparks.
447
9
Gases (cont.)
 Methane, propane and proprietary gases may be used with
oxygen to weld some lower- melting- temperature metals.
 Usually these gas mixtures cannot be applied to the welding of
steel because when they are burned at temperatures high enough for
welding then their flame atmospheres become excessively
oxidizing.
 If the ratios of oxygen to the fuel gas are reduced to a carburizing
condition then the flame temperatures become too low.
 So, these gases are usually limited to heating, brazing and braze
welding.
448
10
Gases (cont.)
 Hydrogen is used mainly for welding lower-melting-temperature
metals, such as aluminum, magnesium and lead.
 It cannot be used to weld common thicknesses of steel sheet, because
it results in a flame temperature that is too low which is not suitable to
produce good fusion.
 However it can be used in welding thin sheet, where its lower
combustion intensity (about 60% of that of acetylene) can be an
advantage.
 It is generally used for brazing and to some extent for braze welding.
 This gas is available in compressed gas cylinders of various sizes.
449
11
Oxyacetylene Combustion
 As the oxygen-acetylene mixture burns from the tip of the welding
torch, it displays several clearly recognizable zones of combustion.
The overall chemical equation for the complete combustion of
acetylene is:
2C2H2 + 5O2 4CO2 + 2H2O ............................................(1)
 Combustion takes place in two stages:
 The first stage: In the first stage the oxygen uses for combustion is
supplied from the oxygen cylinder. The reaction can be seen as the
small inner cone of the flame. The highest temperature is at the point
of this cone.
2C2H2 + 2O2 4CO + 2H2 .................................................(2)
450
12
Oxyacetylene Combustion (cont.)
 The second stage:
4CO + 2H2 + 3O2 4CO2 + 2H2O.....................................(3)
uses the oxygen supplied from the air surrounding the flame. This
combustion zone constitutes the outer envelope of the flame.
 Note: About two-fifths of the oxygen necessary for the complete
combustion of acetylene comes from the oxygen cylinder; the
remainder comes from the air.
 Because of the need for supplemental oxygen from the atmosphere,
the oxygen/acetylene flame cannot be used inside tubes of structures
subject to oxygen depletion.
451
13
 Oxy-Acetylene Flame Adjustment
and
 Types of Flames
452
14
Flame Adjustment
The sequences for setting up a positive-pressure welding outfit are:
 Check all parts of the apparatus, making sure they are free of dirt, oil,
or grease and in proper working condition.
 Open the cylinder valve slowly and carefully. The operator should
never stand in front of the regulator when opening the cylinder valve.
 Wash out the oxygen line while the acetylene line is closed and the
acetylene line while the oxygen line is closed.
 Set the oxygen and fuel gas regulators to the recommended working
pressure with appropriate torch valve open.
 First open the acetylene (or fuel gas) inlet valve and light the
welding torch, using a spark lighter.
 Then open the oxygen inlet valve and adjust the flame, using both
inlet valves.
 Note: Different welding atmospheres and flame temperatures can be produced by
varying the relative amounts of oxygen and fuel gas in the gas flowing to the 15tip of
the torch.
453
Type of Oxy-Acetylene Flames
 The type of flame produced depends upon the ratio of oxygen to
acetylene in the gas mixture which leaves the torch tip.
 There are three distinct types of oxy-acetylene flames, usually termed:
i. Carburizing flame (Excess Acetylene flame)
ii. Neutral flame
iii. Oxidizing flame (Excess Oxygen flame)
454
16
Oxy-Acetylene Flames (cont.)
 Acetylene Flame: When acetylene alone is burned in air, it produces
a flame that varies in color from yellow near the torch tip to orangered at the outer extremity.
 Depending upon the presence of excess acetylene in oxy-acetylene flame it
can be categories as:
(i) Carburizing Flame and (ii) Reducing Flame
 Carburizing Flame: As the oxygen valve in the torch is progressively
opened and the ratio of oxygen to acetylene increases, the flame becomes
generally bright. Then, the bright portion contracts toward the welding tip,
forming a distinct bright zone within a blue outer envelope.
 This is a carburizing flame because it has a large excess of acetylene; it is
sometimes described as a soft flame because it has very little force.
 Application: It has a relatively low temperature and is used in silver brazing
and soldering, as well as in the welding of lead. It is generally used for
carburizing (surface hardening) purposes.
455
Fig. Acetylene Flame
17
Fig. Carburizing Flame
Oxy-Acetylene Flames (cont.)
 Reducing Flame (Max. temperature is about 3040°C): The flame is
as a slightly excess acetylene or reducing flame but less than the
carburizing flame.
 As more oxygen is introduced, the bright zone of the flame contracts
further and is seen to consist of two parts:
• A bright inner cone and
• A pale-green feather,
 The feather is caused by a slight excess of acetylene. It disappears as the
oxygen-to- acetylene ratio approaches 1 to 1.
 For welding steel, the length of the feather should be about one-eighth to
one-quarter, but never more than one-half, the length of the inner cone.
 It should not be called a carburizing flame because it does not carburize the
metal, but it does ensure the absence of the oxidizing condition.
 Application: It is used in Low alloy steel, non-ferrous metals that do not
18
tend to absorb carbon. It is very well used for high carbon steel.
456
Oxy-Acetylene Flames (cont.)
 i. Neutral flame (Max. temperature is about 3260 °C):
 The second equation shows that in the
first stage, when equal amounts of
oxygen and acetylene are burning,
neither excess acetylene nor excess
oxygen is present at the hightemperature tip of the inner cone.
 For this reason, this flame is called
neutral flame and the gas mixture is
often described as an acetylene-tooxygen ratio of 1 to 1.
Fig. Flame temperature as a
function of relative distance from
the torch tip (for a neutral
oxyacetylene flame)
 So, when the presence of carbon must be strictly avoided. When the
oxidizing condition is unacceptable, as in the case of stainless steel
welding, the use of a neutral flame is essential for good results. 19
457
Oxy-Acetylene Flames (cont.)
 Basic Features and Application of Neutral Flame:
 It has a a light blue inner cone with a darker blue outer
envelope.
 A neutral flame is named so because it effects no chemical
changes in the molten mental and therefore will not oxidize or
carburize the metal.
 Neutral flames are commonly used to weld: Mild steel, Stainless
steel, Cast iron, Aluminum, Copper.
458
20
Oxy-Acetylene Flames (cont.)
 ii. Oxidizing Flame (Max. temperature is about 3315°C) : It is
produced when more than one volume of oxygen is mixed with one
volume of acetylene.
 Basic Features and Application:
 It has a small white cone which is much shorter, much bluer in colour
and more pointed than that of neutral flame.
 The flame should be sufficiently rich in oxygen to ensure that a film
of oxide slag forms over the weld to provide shielding for the weld
pool.
 Here the oxygen-to-acetylene ratio is about 1.5/1.
 An oxidizing flame should never be used in welding steel.
 It is used only in welding copper, certain copper-base alloys and
zinc-base material.
459
21
Fluxes Requirement
 Except for lead, zinc and some precious metals, OFW of nonferrous
metals, cast irons and stainless steels generally requires a flux.
 In welding carbon steel, the gas flame shields the weld adequately,
and no flux is required.
460
22
Combustion of other gases
461
23
Oxy-hydrogen Combustion
 Complete combustion of hydrogen requires an oxygen-to-hydrogen
ratio of 1 to 2, as can be seen from the following equation:
2H2 + O2 2H2O.........................................(4)
 This gas mixture produces a strongly oxidizing flame having a
temperature of about 2760 °C (5000 °F).
 It is impossible to obtain a neutral oxy-hydrogen flame by the visual
methods of flame adjustment described for the oxyacetylene flame.
 The oxy-hydrogen flame itself is scarcely visible, and no
combustion zones.
462
24
Oxy-hydrogen Combustion (cont.)
 Basic Features of Oxy-hydrogen Combustion:
 To avoid an oxidizing flame, the pressure regulators must be set to
ensure an excess of hydrogen.
 The flame is then reducing, but not carburizing. It has no
carbon, and the temperature is several hundred degrees lower than
that of the neutral flame.
 Metering flow regulators permit establishing the desired ratio of
hydrogen to oxygen, usually 4 to 1.
 The oxy-hydrogen flame is useful for welding and brazing aluminum
alloys and lead.
463
25
Combustion of Natural Gas and Propane
 Complete combustion of natural gas (methane) and propane is
shown, respectively, by the following equations:
CH4 + 2O2 CO2 + 2H2O...........................(5)
C3H8 + 5O2 3CO2 + 4H2O........................(6)
 Note: When the flame temperature is high enough to weld steel, the
flame atmosphere is excessively oxidizing, but when the ratio of
oxygen to fuel gas is decreased to produce a carburizing condition,
flame temperature is too low for welding steel. Here the temperature
is around 2500 °C.
464
26
Classification of OFW Technique
OFW Technique is classified in following two categories:
Leftward or Forehand OFW Technique.
Rightward or Backhand OFW Technique.
 Most OFW is done with the one-pass Leftward or forehand
technique, particularly on thinner materials.
465
27
Leftward or Forehand Technique
 The welder holds welding torch in his right hand and the filler rod in the left hand.
 The welding flame directed away from the finished weld i.e. towards the
unwelded part of the joint.
 Filler rod, when used, is directed towards the welded part of the joint.
 Since the flame is pointed in the direction of the welding, it preheat the edges of
the joint.
 Good control and neat appearance are characteristics of leftward method.
 It is usually used on relatively thin metals i.e., having thickness less than 5 mm.
 For workpiece thickness over 3 mm, it is
necessary to bevel the plate edge (i.e.
included angle is 80-90deg.) so that good
root fusion may be achieved.
 When the materials over 6.5 mm thick, it is
difficult to obtain even penetration at the
bottom of the V and therefore the quality
decreases as plate thickness increases.
466
28
Rightward or Backhand Technique
 Here also the welder holds welding torch in his right hand and the filler rod in the
left hand.
 Welding begins at the left-hand end and proceeds towards the right, hence the
name rightward technique.
 As the flame is constantly directed on the edges of the V ahead of the weld
puddle, no sideward motion of weld puddle is necessary. As a results narrower Vgroove (30 deg. bevel or 60 deg. included angle) can be utilized than in leftward
welding.
 This is used on heavier or thicker (above 5 mm) base metals, because in this
technique the heat is concentrated into the metal.
 Welds with penetrations of approximately 12
mm can be achieved in a single pass.
 Upto 8.2 mm plate thickness no bevel is
necessary. This save the cost of preparation
and reduces the consumption of filler rod.
 So this technique involves lower cost of
welding than the leftward technique.
467
29
Fig.: Backhand oxyacetylene welding.
Applications
 It can be used for preheating, post heating, welding, braze welding,
and torch brazing, and it is readily converted into oxygen cutting.
 The process can be adapted to short production runs, field work and
repairs.
 Metals that can be oxy-fuel gas welded: Most ferrous and nonferrous
metals can be oxy fuel gas welded.
 Oxyfuel gas welding can be used to join thin carbon steel sheet and
carbon steel tube and pipe.
 Oxyfuel gas welding is frequently used for repairs and alterations
because the equipment is portable, welding can be done in all
positions, and acetylene and oxygen are readily available.
468
30
Advantages
Advantages of OFW:
 The equipment is versatile, low-cost, self-sufficient, and usually
portable
 It includes the ability to control heat input, bridge large gaps, avoid
melt-through, and clearly view the weld pool.
 Carbon steel sheet, formed in a variety of shapes, can often be
welded more economically by OFW than by other processes.
 Oxyfuel gas welding is capable of joining small-diameter carbon
steel pipe (up to about 75 mm diameter) with resulting weld quality
equal to competitive processes and often with greater economy.
 Pipe with wall thickness up to 4.8 mm ( 3/16 in.) can be welded in a
31
single pass.
469
Limitations
Limitations:
 Metals unsuited to OFW are the refractory metals, such as niobium,
molybdenum, tungsten and tantalum.
 As well as the reactive metals, such as titanium and zirconium.
 The disadvantage in using oxy-fuel gouging is that the heat input
may cause the crack to propagate through differential expansion in
the workpiece.
470
32
Accessories for OFW
Accessories essential to OFW include
 A friction lighter for igniting the torch;
 Welder's goggles,
 Gloves and protective clothing; and
 Related safety devices.
 Welder's goggles are covered by ANSI standard, which suggests
the following lens shade numbers for use in OFW of steel:
Steel thickness(mm)
Shade Number
≤3.2
4 or 5
3.2-13
5 or 6
>13
471
6-8
33
Proprietary Gases and Mixtures
Gas
Cylinder Colour
Acetylene
Oxygen
Argon
Hydrogen
Air (Not Breathing Quality)
Maroon
Black
Blue
Red
Grey
Carbon di Oxide, Commercial
Liquid Withdrawal
Black With White Strip Down
Length Of Cylinder
Nitrogen
Grey With Black Shoulder
Propane
Red, Wider And Shorter Cylinder
Argon/ Carbon Di Oxide
Blue Green Band On Cylinder
Argon/ Helium
Blue Green Band On Shoulder
Argon / Hydrogen
Blue Red Band On Shoulder
Argon/ Oxygen
Blue Black Band On Shoulder
472
34
Base Metal, Filler Metal and Flame Type
Base Metal
Aluminium’s
Filer Metal Type
Match Base Metal
Flame Type
Slightly Reducing
Flux Type
Aluminium
Brasses
Bronzes
Copper
Copper Nickel
Inconel
Iron, Cast
Iron, Wrought
Lead
Monel
Nickel
Nickel Silver
Low Alloy Steel
High Carbon Steel
Navy Brass
Copper Tin
Copper
Copper Nickel
Match Base Plate
Cast Iron
Steel
Lead
Match Base Plate
Nickel
Nickel Silver
Steel
Steel
Slightly Oxidising
Slightly Oxidising
Neutral
Reducing
Slightly Reducing
Neutral
Neutral
Slightly Reducing
Slightly Reducing
Slightly Reducing
Reducing
Slightly Reducing
Reducing
Borax
Borax
No Flux Required
No Flux Required
Fluoride
Borax
No Flux Required
No Flux Required
Monel
No Flux Required
No Flux Required
No Flux Required
No Flux Required
Low Carbon Steel
Medium Carbon
Steel
Stainless Steel
Steel
Steel
Neutral
Slightly Reducing
No Flux Required
No Flux Required
Match Base Plate
Slightly Reducing
Stainless Steel
473
35
END
474
36
Fundamental of Welding Science and Technology
Lecture 19: Oxy-Fuel Gas Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
475
1
Combustion of other gases
476
2
Oxy-hydrogen Combustion
 Complete combustion of hydrogen requires an oxygen-to-hydrogen
ratio of 1 to 2, as can be seen from the following equation:
2H2 + O2  2H2O.........................................(4)
 This gas mixture produces a strongly oxidizing flame having a
temperature of about 2760 °C (5000 °F).
 It is impossible to obtain a neutral oxy-hydrogen flame by the visual
methods of flame adjustment described for the oxyacetylene flame.
 The oxy-hydrogen flame itself is scarcely visible, and no
combustion zones.
477
3
Oxy-hydrogen Combustion (cont.)
 Basic Features of Oxy-hydrogen Combustion:
 To avoid an oxidizing flame, the pressure regulators must be set to
ensure an excess of hydrogen.
 The flame is then reducing, but not carburizing. It has no
carbon, and the temperature is several hundred degrees lower than
that of the neutral flame.
 Metering flow regulators permit establishing the desired ratio of
hydrogen to oxygen, usually 4 to 1.
 The oxy-hydrogen flame is useful for welding and brazing of
aluminum alloys and lead.
478
4
Combustion of Natural Gas and Propane
 Complete combustion of natural gas (methane) and propane is
shown, respectively, by the following equations:
CH4 + 2O2  CO2 + 2H2O...........................(5)
C3H8 + 5O2  3CO2 + 4H2O........................(6)
 Note: When the flame temperature is high enough to weld steel, the
flame atmosphere is excessively oxidizing, but when the ratio of
oxygen to fuel gas is decreased to produce a carburizing condition,
flame temperature is too low for welding steel. Here the temperature
is around 2500 °C.
479
5
Classification of OFW Technique
OFW Technique is classified in following two categories:
Leftward or Forehand Welding Method
Rightward or Backhand Welding Method.
480
6
Leftward or Forehand Welding Method
 The welder holds welding torch in his right hand and the filler rod in the left hand.
 The welding flame directed away from the finished weld i.e. towards the
unwelded part of the joint.
 Filler rod, when used, is directed towards the welded part of the joint.
 Since the flame is pointed in the direction of the welding, it preheat the edges of
the joint.
 Good control and neat appearance are characteristics of leftward method.
 It is usually used on relatively thin metals i.e., having thickness less than 5 mm.
 For workpiece thickness over 3 mm, it is
necessary to bevel the plate edge (i.e.
included angle is 80-90deg.) so that good
root fusion may be achieved.
 When the materials over 6.5 mm thick, it is
difficult to obtain even penetration at the
bottom of the V and therefore the quality
decreases as plate thickness increases.
481
7
Rightward or Backhand Welding Method
 Here also the welder holds welding torch in his right hand and the filler rod in the
left hand.
 Welding begins at the left-hand end and proceeds towards the right, hence the
name rightward technique.
 As the flame is constantly directed on the edges of the V ahead of the weld
puddle, no sideward motion of weld puddle is necessary. As a results narrower Vgroove (30 deg. bevel or 60 deg. included angle) can be utilized than in leftward
welding.
 This is used on heavier or thicker (above 5 mm) base metals, because in this
technique the heat is concentrated into the metal.
 Welds with penetrations of approximately 12
mm can be achieved in a single pass.
 Upto 8.2 mm plate thickness no bevel is
necessary. This save the cost of preparation
and reduces the consumption of filler rod.
 So this technique involves lower cost of
welding than the leftward technique.
482
8
Fig.: Backhand oxyacetylene welding.
Advantages
Advantages of OFW:
 The equipment is versatile, low-cost, self-sufficient, and usually
portable
 It includes the ability to control heat input, bridge large gaps, avoid
melt-through, and clearly view the weld pool.
 Carbon steel sheet, formed in a variety of shapes, can often be
welded more economically by OFW than by other processes.
 Oxyfuel gas welding is capable of joining small-diameter carbon
steel pipe (up to about 75 mm diameter) with resulting weld quality
equal to competitive processes and often with greater economy.
 Pipe with wall thickness up to 4.8 mm ( 3/16 inch) can be welded in
9
a single pass.
483
Limitations
Limitations:
 Metals unsuited to OFW are the refractory metals, such as niobium,
molybdenum, tungsten and tantalum.
 As well as the reactive metals, such as titanium and zirconium.
 The disadvantage in using oxy-fuel gouging is that the heat input
may cause the crack to propagate through differential expansion in
the workpiece.
484
10
Applications OFW
 It can be used for preheating, post heating, welding, braze welding,
and torch brazing, and it is readily converted into oxygen cutting.
 The process can be adapted to short production runs, field work and
repairs.
 Metals that can be oxy-fuel gas welded: Most ferrous and nonferrous
metals can be oxy fuel gas welded.
 Oxyfuel gas welding can be used to join thin carbon steel sheet and
carbon steel tube and pipe.
 Oxyfuel gas welding is frequently used for repairs and alterations
because the equipment is portable and welding can be done in all
positions.
485
11
Accessories for OFW
Accessories essential to OFW include
 A friction lighter for igniting the torch
 Welder's goggles
 Gloves and protective clothing and
 Related safety devices.
 Welder's goggles are covered by ANSI standard, which suggests
the following lens shade numbers for use in OFW of steel:
Steel thickness(mm)
Shade Number
≤3.2
4 or 5
3.2-13
5 or 6
>13
486
6-8
12
Base Metal, Filler Metal and Flame Type
Base Metal
Aluminium’s
Filer Metal Type
Match Base Metal
Flame Type
Slightly Reducing
Flux Type
Aluminium
Brasses
Bronzes
Copper
Copper Nickel
Inconel
Iron, Cast
Iron, Wrought
Lead
Monel
Nickel
Nickel Silver
Low Alloy Steel
High Carbon Steel
Navy Brass
Copper Tin
Copper
Copper Nickel
Match Base Plate
Cast Iron
Steel
Lead
Match Base Plate
Nickel
Nickel Silver
Steel
Steel
Slightly Oxidising
Slightly Oxidising
Neutral
Reducing
Slightly Reducing
Neutral
Neutral
Slightly Reducing
Slightly Reducing
Slightly Reducing
Reducing
Slightly Reducing
Reducing
Borax
Borax
No Flux Required
No Flux Required
Fluoride
Borax
No Flux Required
No Flux Required
Monel
No Flux Required
No Flux Required
No Flux Required
No Flux Required
Low Carbon Steel
Medium Carbon
Steel
Stainless Steel
Steel
Steel
Neutral
Slightly Reducing
No Flux Required
No Flux Required
Match Base Plate
Slightly Reducing
Stainless Steel
487
13
END
488
14
Shielded Metal Arc Welding
489
15
Introduction
 The American Welding Society defines
SMAW as Shielded Metal Arc Welding
‘Stick’
– Is commonly known as
welding or manual arc welding
– Is the most widely used arc welding
process in the world
– Can be used to weld most common
metals and alloys.
o Fusion welding process
o Required heat is produced from electric arc
490
16
Principle of the process
 Heat required for welding is obtained
from the arc struck between a coated
electrode and the workpiece.
 The arc temperature and thus the arc
heat can be increased or decreased by
employing higher or lower arc
currents.
 A high current arc with a smaller arc
length produces a very intense heat.
The arc reaches temperatures of around
10,000°F. The arc melt the electrode
and the job.
 Material droplets are transferred from electrode to the job, through the
arc and are deposited along the joint to be welded.
 The flux coating melts, produces a gaseous shield and slag to prevent
17
atmospheric contamination of the molten weld metal.
491
Shielded Metal Arc Welding Set up
 SMAW Set up








Arc welding Power source
Electrode
Workpiece
Electrode holder
Electrode lead cable
Welding table
Workpiece lead (cable)
Input power lead (cable)
 Current flows through the electrode cable, to the electrode holder,
through the electrode, and across the arc.
 On the work side of the arc, the current flows through the base
material to the work clamp and back to the welding machine.
492
18
Shielded Metal Arc Welding Set up
 Power Source
 Can be operated with AC and DC power supplies.
 A constant-current power source is preferred
493
19
Details of SMAW Process
 Different constituents of a SAMW process:
1.
2.
3.
4.
5.
6.
Electrode
Weld puddle
Arc
Shielding gas
Solidified weld metal
Slag
Electrode
Travel direction
1
4 Shielding Gas
6
Slag
2 Weld Puddle
3
Arc
5
Solidified Weld Metal
494
20
Electrodes
 SMAW electrodes are basically composed of a metal core and a flux
cover. The metal core acts as a the electrode as well as filler rod.
SMAW electrode specification (AWS Classification):
495
21
Electrodes cont.
 Primary function of flux cover:
 Shielding weld pool and metal transfer from the electrode tip to
the weld pool from atmosphere.
 Gases generates as the coating decomposes under the arc heat.
 The gas is not enough for proper shielding
• The flux coating burns and produces a protective slag
 Keeps the molten weld metal shielded from atmospheric
contamination.
• The molten slag has a lesser density,
 Floats above the molten metal
 Note: The layer of slag thus forms not only prevents the deposited
metal from atmospheric contamination but also slows down the
22
cooling rate and produces a more ductile weld deposit.
496
Electrodes cont.
 The flux composition serves the following purpose:




Induce easier arc starting
Arc stability
Improve weld bead appearance and penetration
Reduce spatter
497
23
Electrodes
 Types of electrodes based on the type of flux covering:
 There are three distinct types of electrodes are used in SMAW, usually
termed:
1. Cellulosic Electrodes
2. Rutile Electrodes
3. Basic Electrodes
498
24
Electrodes (cont.)
1. Cellulosic Electrodes
 Cellulosic electrodes are coated with flux rich in cellulose (C6H10O5)n.
 This burns to produce hydrogen and carbon monoxide, provides
shielding to the arc.
 Suitable with DC power and electrode-positive polarity.
 Presence of these gases in the arc with high ionization potentials
results in a high arc energy.
 Results in a deeply penetrating arc and a rapid burn-off rate calling for
high welding speeds.
 Makes the electrode suitable for all position welding
499
25
Electrodes (cont.)
1.Cellulosic Electrodes
Basic features of Cellulosic Electrodes:
 Deep penetration in all positions.
 Suitable for vertical down welding.
 Reasonably good mechanical properties.
 High level of hydrogen generated-risk of cracking in the heat
affected zone.
Application:
 Pipelines, tanks, pressure vessels, structural and field work where
deep penetration is necessary. Specially suited for pressure pipelines
which cannot be welded from inside.
500
26
Electrodes (cont.)
2. Rutile Electrodes
 Rutile electrodes contains high proportion of titanium oxide (rutile)
in its coating.
 Titanium oxide promotes easy arc ignition, smooth arc operation, low
spatter. This is classified as general purpose electrodes.
 Because of rutile and the ionizers in the coating, these electrodes can
be used with either polarity and all positions.
 Rutile electrodes are specially suitable for fillet welding in horizontal
and vertical position.
501
27
Electrodes (cont.)
2. Rutile Electrodes
 Basic features of Rutile Electrodes:
 Moderate weld metal mechanical properties.
 Good bead shape produced because of viscous slag.
 Positional welding possible with a fluid slag.
 Easy slag removal.
Application:
 Storage tanks, gear blanks, machinery, steel furniture, truck bodies,
foundry equipment, shaft build-up, etc.
502
28
Electrodes (cont.)
3. Basic Electrodes
 In basic electrodes the coating contains a high proportion of
calcium carbonate and calcium fluoride.
 Referred to as low hydrogen electrodes.
 Makes the slag more fluid than that at the rutile coatings.
 Slag is of fast-freezing type.
 Suitable for vertical and overhead position.
503
29
Electrodes (cont.)
3. Basic Electrodes
Basic features of Basic Electrodes:
 Weld deposit with good mechanical properties.
 Low hydrogen content in weld deposit.
 Relatively fluid slag.
 Poor bead profile.
 Slag removal difficult.
 Suitable for welding of thicker steels and steels with higher carbon
content, weld metal has excellent mechanical properties,
particularly impact property.
504
30
Electrodes (cont.)
3. Basic Electrodes
 These electrodes are used for high quality applications which call
for a low hydrogen content in weld deposit, the moisture content
of the electrode coating should be kept to a minimum.
 To prevent the electrode coating from moisture absorption, they
should be carefully stored and dried.
• Welding of HSLA steels,
 Additional baking immediately before welding
 Electrodes stored in portable driers
 Directly used from the drier
 Further eliminates possibility of moisture absorption.
Application:
 Used for welding pressure pipelines, oil storage tanks, ships,
boilers, railway wagons, etc. at high welding speeds. Also well
suited for repairing steel castings.
31
505
506
32
Fundamental of Welding Science and Technology
Lecture 20: Shielded Metal Arc Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
507
Electrodes
 Types of electrodes based on the type of flux covering:
 There are three distinct types of electrodes are used in SMAW, usually
termed:
1. Cellulosic Electrodes
2. Rutile Electrodes
3. Basic Electrodes
508
2
Electrodes (contd.)
3. Basic Electrodes
 In basic electrodes the coating contains a high proportion of
calcium carbonate and calcium fluoride.
 Referred to as low hydrogen electrodes.
 Makes the slag more fluid than that at the rutile coatings.
 Slag is of fast-freezing type.
 Suitable for vertical and overhead position.
509
3
Electrodes (cont.)
3. Basic Electrodes
Features of Basic Electrodes:
 Weld deposit with good mechanical properties
 Low hydrogen content in weld deposit
 Relatively fluid slag
 Poor bead profile
 Slag removal difficult.
 Suitable for welding of thicker steels and steels with higher carbon
content
 Weld metal has excellent mechanical properties, particularly
impact property.
510
4
Electrodes (cont.)
3. Basic Electrodes
 These electrodes are used for high quality applications which call
for a low hydrogen content in weld deposit, the moisture content
of the electrode coating should be kept to a minimum.
 To prevent the electrode coating from moisture absorption, they
should be carefully stored and dried.
• Welding of HSLA steels,
 Additional baking immediately before welding
 Electrodes stored in portable driers
 Directly used from the drier
 Further eliminates possibility of moisture absorption.
Application:
 Used for welding pressure pipelines, oil storage tanks, ships,
boilers, railway wagons, etc. at high welding speeds. Also well
suited for repairing steel castings.
5
511
Electrodes (cont.)
Metal Powder Electrodes
 Contain an addition of iron powder in the flux coating.
 Considerably improves performance.
 Amount of iron powder may range from 5 to 50%.
 Iron powder is added
 To increase deposition rate,
 To improve arc behavior.
 In conventional electrodes, current is carried wholly by the core
wire, whereas with iron powder addition in the flux the coating
becomes conductive near the arc
 Providing an additional path to the current.
512
6
Electrodes (cont.)
Metal Powder Electrodes
 Arc tends to spread out and metal deposition takes place over a
wider area.
 Reduces the current density at tip
 Reducing the penetrating force of the arc
 Causing less penetration.
 Additional conducting area in the arc limits the current surge when
a short-circuit takes place between electrode wire and the job.
 Reduces the occurrence of spatter
 Provides a smoother, more stable arc,
 Improved sidewall fusion,
 Flatter welds
513
7
Electrodes (cont.)
Metal Powder Electrodes
 Higher deposition rates are achieved by increasing the iron
powder content in the flux coating.
 Iron powder content beyond 50% causes deterioration in the
behavior of the electrode as the coating fuses unevenly.
 Higher deposition rate is actually achieved not only because
of additional metal in the flux but also because of the ability
to carry more current for the same core wire diameter.
514
8
Weld Puddle
 As the core rod, flux coating,
and work pieces heat up and
melt, they form a pool of
molten material called a weld
puddle.
4
Weld Puddle
3
 A welder generally watches
this
weld
puddle
and
manipulates the welding
operation during welding.
2
Workpiece
515
9
Shielding Gas
 Shielding gas is formed when
the flux coating burns and
melts.
Shielding Gas
4
 This protects the weld puddle
from
the
atmospheric
contamination during the
molten state.
3
2
Workpiece
516
10
Solidified Weld Metal
 As the molten weld puddle
solidifies, it forms a joint or
connection between two
pieces of base material.
Solidified
weld metal
 If the welding done properly,
it may have a strength more
than the surrounding base
metal.
3
2
Workpiece
517
11
Slag
 Slag is a combination of the flux
coating and impurities from the
base metal that float to the
surface of the weld.
 Slag quickly solidifies to form a
solid coating.
 The slag also slows the cooling
rate of the weld.
4
Slag
3
2
Workpiece
 The slag can be chipped away
and cleaned with a wire brush
when hard.
518
12
Operator Controlled Variables
o
o
o
o
Work Angle
Travel Angle
Arc Length
Travel Speed
 Work Angle: It is the angle between the electrode and the work as
depicted on the left.
 Work angles can vary depending
on the position the weld is being
made in.
 For flat welding, work angle is
90° .
519
Fig. Work angle
Operator Controlled Variables (cont.)
 Travel Angle: The travel angle is the
angle between the electrode and the
plane perpendicular to the weld axis.
 Also commonly called Lead Angle.
Fig. Travel Angle
 Arc length: After striking the arc, maintain
a gap (i.e. around 1/8”) between the
electrode and the workpiece
• If the arc length becomes too short, the
electrode will
get stuck to the
workpiece or ‘short out’
• If the arc length becomes too long;
spatter, undercut, and porosity can occur
Arc Length = 1/8”
Fig. Arc length
520
14 14
Crater
 It is a cavity which is developed at the end of weld due to shrinkage
of weld metal during solidification.
 At the end of the weld, the operator breaks the arc which creates a
‘crater. Large craters can cause weld cracking.
521
15
Filling the crater at the end of weld
 Use a short pause or slight back step at the end of the weld to fill
the crater
 Note: Back stepping is a short move in the opposite direction of
weld travel.
 Another way to get rid of crater is to attach a scrap piece at the end
of structure and to continue welding on the same. The Crater is now
left on the scrap piece which can be detached from the structure.
 Crater is also filed by holding electrode (10-15 degrees with the
vertical) at the crater for an instant, and then normal welding
proceeds.
522
1616
Filling the crater
 While welding longer workpieces a number of stick electrodes are
employed; where one finishes, welding is carried out with next
electrode.
 A Crater forms at a place where previous electrode completes and
welding is to be started with a new electrode. The generally adopted
method i.e. restarting a weld is given below.
 First of all slag is removed from weld bead adjoining crater, and, the
weld bead and crater are thoroughly cleaned of slag etc., using a wire
brush or a grinder.
1. Strike Arc Here
2. Move Electrode to Crown
of Crater
3. Resume Forward Travel
523
Fig. Restarting an Arc
17
Weaving Technique in SMAW
 In SMAW, weaving implies giving a side to side motion to the
welding arc during transferring material to the joint to be welded.
Here electrode is moved or oscillated from side to side in a set
pattern.
 Weaving becomes particularly necessary in multi-pass weld beads
where welder has to deposit wider beads and thus more weld metal
per unit pass.
 Weaving helps to give better fusion on the sides of weld.
 Note: It should be limited to weaves not exceeding 2&1/2 times the
diameter of the electrode.
524
18
Weaving Technique in SMAW
 In order to be sure of uniform deposits, it is necessary to use a
definite pattern such as those illustrated in Figure below:
 Convex weave:
 Concave weave:
 Circular weave:
 Ladder weave:
 Jagged ladder weave:
 Triangle weave:
welding direction
525
19
Advantages
Advantages of SMAW process are:
 Simple, portable and inexpensive welding equipment
 Both filler metal, arc and molten metal shielding are provided by
the electrode.
 Can be used in areas of limited access.
 Low initial cost.
 All position capabilities.
 Suitable for most of the commonly used metals and alloys.
526
20
Limitations
Limitations of SMAW process are:
 Lower consumable efficiency
 Difficult to weld very thin materials
 Frequent restarts
 Higher operator skill required for SMAW than some other
processes
 Deposition rates are generally lower than other welding process
such as GMAW.
 Maximum current that can be used is limited by the electrical
resistance of the core wire.
 Excessive current may overheat the electrode breaking down the flux
coating.
 Deteriorates arc behavior and shielding.
527
21
Applications
 Applications of SMAW:
 Suitable for most of the commonly used metals and alloys
 SMAW is used both as fabrication process and maintenances. It is
also used in repair jobs. The process finds applications in
 ship building
 pipes joining
 automotive and aircraft industry
 building and bridge construction
 air receiver, tank , boiler
 pressure vessel fabrications.
528
22
SMAW Safety
o Fumes and Gases can be dangerous
 Keep your head out of the fumes
 Use enough ventilation, exhaust at the arc.
o Electric Shock can kill – to receive a shock your body must touch the electrode
and work or ground at the same time
 Do not touch the electrode or metal parts of the electrode holder with skin
or wet clothing.
 Keep dry insulation between your body and the metal being welded or
ground.
o Arc Rays can injure eyes and skin - Choose correct filter shade.
529
23
END
530
Operator Controlled Variables (cont.)
 Travel speed: It is the speed at which the electrode moves along
the base material while welding. The travel speed impacts the
shape of the bead.
• Too fast of a travel speed results in a ropey or convex weld
• Too slow of a travel speed results in a wide weld with an
excessive metal deposit.
531
25
25
• Presence of these gases in the arc with high ionization
potentials results in a high arc voltage and therefore
a high arc energy.
 Results in a deeply penetrating arc and a rapid
burn-off rate calling for high welding speeds.
 Makes the electrode suitable for all position welding
Note: Argon 15.6, nitrogen 15.8, carbon monoxide 15.0, hydrogen 15.1, helium 20.5, mercury
vapor 10.1, iodine vapor 8.5. H atom 13.6 electron volts
532
26
533
27
Fundamental of Welding Science and Technology
Lecture 21: GTAG Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
534
Introduction of GTAW or TIG
 The GTAW process was initially called "Heliarc" as it used an
electric arc to melt the base material and helium to shield the
molten puddle. (invented in 1939-1941)
 Heat for fusion is generated from an arc between a nonconsumable
tungsten electrode and the base metal.
 An inert gas is used as a shielding medium for the arc and the
molten weld pool.
 The shielding gas also protects the electrode at the prevailing high
temperature.
 The tungsten gets readily oxidized if it comes in contact with
oxygen.
 Argon or helium is used for shielding in GTAW.
535
TIG Welding Setup
 The equipment used for TIG welding consists of:
 Power source
 Welding torch
 Tungsten electrode
Gas supply system
536
TIG Welding Setup (cont.)
 Electrode material could be pure tungsten for DCSP.
 Thoriated tungsten or ziconated tungsten can be work with AC as well
as DC. In AC welding heat input to the electrode is higher as
compared to DCSP.
 Electrode coating thoria or zirconia give steadier arc due to their
higher thermionic emissivity compared to pure tungsten electrode.
537
Principle of Operation
 Welding current, water and inert gas supply are turned on.
 The arc is struck either by touching the electrode with a scarp
metal and tungsten piece or using a high frequency unit.
 In the first method arc is initially struck on a metal piece (or a
tungsten piece) and then broken by increasing the arc length. This
procedure repeated twice or thrice to warm up the electrode. The
arc is then struck between pre-cleaned job to be welded.
 This method avoids breaking electrode tip, job contamination and
tungsten loss.
 In the second method, a high frequency circuit is superimposed
on the welding current. When electrode tip reaches within 3 to 2
mm from the job/workpiece, a spark jumps across the air gap
between the electrode and the job. This air path gets ionozed and
arc is established.
538
Principle of Operation
i.
Welding
puddle
is
developed is developed
due to arc action on the
job.
ii. Welding torch is moved
back.
iii. Filler rod is moved ahead
and filler metal is added
to the weld puddle.
Fig. Manual GTAW technique sequences iv. Filler rod is withdrawn.
 Note: The shielding gas is allowed to impinge the on the solidifying
weld pool for a few seconds even after the arc is extinguished.
539
Operating Variables
• Welding current
• Arc voltage (arc length)
• Welding speed
• Shielding gas
• Electrode type
540
Operating Variables (cont.)
 Welding Current:
 Deep penetration and fast welding speeds with DCEN
especially with helium.
• For welding aluminum, AC is used.
 Provides a cathodic cleaning (sputtering) of the weld pool,
removes the refractory oxides
 With AC to have sputtering effect, argon has to be used as
shielding gas.
 Sputtering does not take place with helium.
 In case of manual GTAW always argon is used.
541
Operating Variables (cont.)
 Polarity:
• Almost always DCEN is used.
 With DCEN, approx 70% of the heat is generated
at the plate (anode) and 30% at the electrode.
 For a given current, a deeper penetration is achieved
with DCEN as compared to that of DCEP.
• DCEP is generally limited to welding sheet metal.
 With DCEP, a cathodic cleaning action takes place
at the surface of the work-piece.
542
Operating Variables (cont.)
 Cathode cleaning
 Important for welding aluminum and magnesium because it
removes the refectory oxides
 With AC power source, the cleaning action of DCEP and deep
penetration of DCEN both are achieved.
 With AC, generally argon shielding gas is used for welding
aluminum
 Better arc starting,
 Better cleaning action,
 Superior weld quality than that with helium as shielding gas.
543
Operating Variables (cont.)
 Cathode cleaning
 Joining is made difficult by the surface formation of tenacious
refractory aluminum oxides of melting point much higher than that
of aluminum metal.
 The oxides are broken up by the cathode cleaning action of the
Electrode Positive part of the alternating current cycle.
 Once broken they float upon the molten metal and they no longer
interfere with the welding process.
544
Categories of TIG welding
 This welding can be performed manually or automatically. So,
based on this mechanism it can be categories as:
i)
Manual TIG welding process
ii) Mechanised TIG welding process
Fig. Manual GTAW process
Fig. Fully Mechanised GTAW process
545
Categories of TIG welding (cont.)
 Automatic TIG Welding:
 Here, the welding torch is automatically guided, and if a filler is
used, it is fed automatically from a reel.
 This process is attractive for large production runs.
 Here, direct current (DC) with straight polarity (i.e. DCEN) is
used instead of alternative current.
 Thickness from 0.2 up to 10 mm can be welded.
546
Process Variants
 There are three main variations of the TIG process which are
designed to improve productivity:
Cold-wire TIG
Hot-wire TIG
Orbital TIG
547
Cold-wire TIG
 With conventional GTAW the filler wire is introduced into the
leading edge of the weld puddle in the cold state (ambient
temperature).
 Energy from the arc is required to melt the wire reducing the
efficiency of the process.
548
Hot-wire TIG
 Hot-wire TIG is used predominantly for steel and nickel alloys
where the electrical resistance of the wire can be used to increase
productivity.
Fig. Hot wire GTAW (Katsuyoshi et al., 2003)
549
Hot-wire TIG (cont.)
 In this welding process, filler wire is resistance heated until close to
the melting point and added to the weld puddle behind the tungsten.
 Since nearly all of the full energy of the welding arc is available for
penetration or to generate the weld pool and fusion, a two to three
times faster travel speed is realized.
 More wire can be deposited and
deposition rates are increased as
compared
with cold wire
GTAW.
550
Fig. Metal Deposition rate vs. arc power
for cold wire TIG and hot wire TIG.
Hot-wire TIG (cont.)
 Applications of Hot wire welding
 High quality fabrications in stainless steel
 Aluminium, copper and nickel alloys
 Welding reactive and refractory metals such as titanium, tantalum
and zirconium.
 The process is used extensively in the nuclear and aerospace
industries and in the construction and maintenance of chemical and
cryogenic process plant and pipework.
551
Orbital TIG
 Orbital TIG welding is used in the nuclear, pharmaceutical,
semiconductor and food industries for the installation of pipe work
– especially where high quality standards are required.
 Specially this equipment use for tube and tube-plate welding.
 These systems may operate from the outside or inside, depending on
tube diameter and the size of the welding head.
552
Advantages of GTAW
 Welds more metals and metal alloys than any other process
 High quality and precision
 Pin point control
 No sparks or spatter
 No flux or slag
 No smoke or fumes
553
Limitations
 Lower filler metal deposition rates
 Good hand-eye coordination a required skill
 Brighter UV rays than other processes
 Slower travel speeds than other processes
 Equipment costs tend to be higher than other
processes
554
Utility
 Specially useful for welding reactive & refractory metals.
 It is highly used in carbon and alloy steels, stainless steel, heat
resisting alloy, Al alloys, Mg-alloys, Cu-alloys, Nickel alloys etc.
 Welding stainless steels, argon is recommended for manual
welding of thickness upto 12mm
 For thick sections, argon-helium mixtures or pure helium can be
used to obtain increased weld penetration.
 With AC, generally argon shielding gas is used for welding
aluminum because it provides better arc starting, better cleaning
action, and superior weld quality than that with helium as
shielding gas.
555
Welding parameter for GTAW
Table: Welding parameter of different materials (approximate value for
Butt joint of 6mm thick plate)
Material
Current
(amp)
Tungsten
Filler
electrode dia. rod dia.
(mm)
(mm)
Aluminium
200-350
(AC)
4.5
3.0-5.0
9
Magnesium
100-150
(AC)
2.5
4.0
10
Copper
250-375
(DCSP)
3.0
3.0
7
Mild, low
allow &
Stainless steel
250-350
(DCSP)
3.0
3.0-4.0
7
Gray cast iron
160-200
(AC/DCSP)
3.0
5.0
8
556
Argon gas flow
rate per min.
(lpm)
End
557
Fundamental of Welding Science and Technology
Lecture xx: Oxy-Fuel Gas Welding
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
558
Introduction
 Oxyfuel Gas Welding (OFW) is a manual welding process in which
the metal surfaces to be joined are melted progressively by heat from
a gas flame, with or without filler metal, and solidify without the
application of pressure to the parts being joined.
Oxygen + Fuel Gas
Oxy Fuel Gas Welding.
 Gas is used to produced arc.
 Commonly used fuel gas for OFW are H2, CH4, C3H8, C2H2.
 Oxy Acetylene welding (OAW) is one of the popular Oxyfuel Gas
Welding process in which acetylene is used as a gas to produce arc.
559
Oxy Acetylene Welding
 The simplest and most frequently used OFW system consists of:
 Compressed gas cylinders
 Gas pressure regulators
 Hoses, and
 A welding torch
 The most important source of heat for OFW is the oxyacetylene
welding (OAW) torch.
 The gas regulator attached to each cylinder, whether fuel gas or
oxygen, controls the pressure at which the gas flows to the welding
torch.
560
Oxy Fuel Welding Setup (Cont.)
 Oxygen and fuel gases are
stored in separate cylinders.
 At the torch, the gas passes
through an inlet control valve,
through tubes within the
handle, and into the mixing
chamber of the welding nozzle
attached to the welding torch.
 The mixed gases then pass
through the welding tip and
produce the flame at the exit
end of the tip.
 Filler metal, when needed, is provided by a welding filler rod that is melted
progressively along with the surfaces to be joined.
561
Gas Torch & Regulator
562
Gases
 Oxygen and acetylene are the principal gases used in OFW.
 Oxygen supports combustion of the fuel gases.
 Acetylene supplies both the heat intensity and the atmosphere
needed to weld steel.
 Hydrogen, natural gas (methane), propane and proprietary gases
(i.e. LPG) are used only to a limited extent in oxy-fuel gas welding
or brazing of metals with a low melting temperature.
563
Gases (cont.)
 Oxygen is supplied for oxy-fuel gas welding and cutting at a purity
of 99.5% and higher, because small percentages of contaminants
have a noticeable effect on combustion efficiency.
 When the consumption requirement is relatively small, the oxygen is
supplied and stored as a compressed gas in a standard steel cylinder
under an initial pressure of up to 180 MPa.
 The most frequently used cylinder has
a capacity of 6.91 m3.
 When oxygen consumption exceeds approximately 6.91 m3 cylinders
per week, it may be more economical to obtain and store oxygen in
liquid form.
564
Gases (cont.)
Acetylene (C2H2): It is a hydrocarbon gas.
 This gas is unstable when it is under a pressure of 203 kPa and
above, and a slight shock can cause it to explode, even in the absence
of oxygen or air.
 Safety rules for the use of acetylene and the handling of acetylene
equipment are extremely important. This gas should not be used at
pressure greater than 105 kPa.
 Acetylene cylinders must not be subjected to sudden shock and
should be stored well away from any source of heat or sparks.
565
Gases (cont.)
 Methane, propane and proprietary gases may be used with
oxygen to weld some lower- melting- temperature metals.
 Usually these gas mixtures cannot be applied to the welding of
steel because when they are burned at temperatures high enough for
welding then their flame atmospheres become excessively
oxidizing.
 If the ratios of oxygen to the fuel gas are reduced to a carburizing
condition then the flame temperatures become too low.
 So, these gases are usually limited to heating, brazing and braze
welding.
566
Gases (cont.)
 Hydrogen is used mainly for welding lower-melting-temperature
metals, such as aluminum, magnesium and lead.
 It cannot be used to weld common thicknesses of steel sheet, because
it results in a flame temperature that is too low which is not suitable to
produce good fusion.
 However it can be used in welding thin sheet, where its lower
combustion intensity (about 60% of that of acetylene) can be an
advantage.
 It is generally used for brazing and to some extent for braze welding.
 This gas is available in compressed gas cylinders of various sizes.
567
Oxyacetylene Combustion
 As the oxygen-acetylene mixture burns from the tip of the welding
torch, it displays several clearly recognizable zones of combustion.
The overall chemical equation for the complete combustion of
acetylene is:
2C2H2 + 5O2 4CO2 + 2H2O ............................................(1)
 Combustion takes place in two stages:
 The first stage: In the first stage the oxygen uses for combustion is
supplied from the oxygen cylinder. The reaction can be seen as the
small inner cone of the flame. The highest temperature is at the point
of this cone.
2C2H2 + 2O2 4CO + 2H2 .................................................(2)
568
Oxyacetylene Combustion (cont.)
 The second stage:
4CO + 2H2 + 3O2 4CO2 + 2H2O.....................................(3)
uses the oxygen supplied from the air surrounding the flame. This
combustion zone constitutes the outer envelope of the flame.
 Note: About two-fifths of the oxygen necessary for the complete
combustion of acetylene comes from the oxygen cylinder; the
remainder comes from the air.
 Because of the need for supplemental oxygen from the atmosphere,
the oxygen/acetylene flame cannot be used inside tubes of structures
subject to oxygen depletion.
569
 Oxy-Acetylene Flame Adjustment
and
 Types of Flames
570
Flame Adjustment
 The sequence for setting up a positive-pressure welding outfit is:
 Check all parts of the apparatus, making sure they are free of dirt, oil,
or grease and in proper working condition.
 Open the cylinder valve slowly and carefully. The operator should
never stand in front of the regulator when opening the cylinder valve.
 Wash out the oxygen line while the acetylene line is closed and the
acetylene line while the oxygen line is closed.
 Set the oxygen and fuel gas regulators to the recommended working
pressure with appropriate torch valve open.
 First open the acetylene (or fuel gas) inlet valve and light the
welding torch, using a spark lighter.
 Then open the oxygen inlet valve and adjust the flame, using
both inlet valves.
 Note: Different welding atmospheres and flame temperatures can be produced by
varying the relative amounts of oxygen and fuel gas in the gas flowing to the tip of
the torch.
571
Type of Oxy-Acetylene Flames
 The type of flame produced depends upon the ratio of oxygen to
acetylene in the gas mixture which leaves the torch tip.
 There are three distinct types of oxy-acetylene flames, usually termed:
i. Neutral flame
ii. Oxidizing flame (Excess Oxygen flame)
iii. Carburizing flame (Excess Acetylene flame)
572
Oxy-Acetylene Flames (cont.)
 i. Neutral flame (Max. temperature is about 3260 °C):
 The second equation shows that in the
first stage, when equal amounts of
oxygen and acetylene are burning,
neither excess acetylene nor excess
oxygen is present at the hightemperature tip of the inner cone.
 For this reason, this flame is called
neutral flame and the gas mixture is
often described as an acetylene-tooxygen ratio of 1 to 1.
Fig. Flame temperature as a
function of relative distance from
the torch tip (for a neutral
oxyacetylene flame)
 So, when the presence of carbon must be strictly avoided. When the
oxidizing condition is unacceptable, as in the case of stainless steel
welding, the use of a neutral flame is essential for good results.
573
Oxy-Acetylene Flames (Cont.)
 Basic Features and Application of Neutral Flame:
 It has a a light blue inner cone with a darker blue outer
envelope.
 A neutral flame is named so because it effects no chemical
changes in the molten mental and therefore will not oxidize or
carburize the metal.
 The neutral flame has a clear, well-defined, or luminous cone
indicating that combustion is complete.
 Neutral flames are commonly used to weld: Mild steel, Stainless
steel, Cast iron, Aluminum, Copper.
574
Oxy-Acetylene Flames (cont.)
 ii. Oxidizing Flame (Max. temperature is about 3315°C) : It is
produced when more than one volume of oxygen is mixed with one
volume of acetylene.
 Basic Features and Application:
 It has a small white cone which is much shorter, much bluer in colour
and more pointed than that of neutral flame.
 The flame should be sufficiently rich in oxygen to ensure that a film
of oxide slag forms over the weld to provide shielding for the weld
pool.
 Here the oxygen-to-acetylene ratio is about 1.5/1.
 An oxidizing flame should never be used in welding steel.
 It is used only in welding copper, certain copper-base alloys and
zinc-base material.
575
Oxy-Acetylene Flames (cont.)
 Acetylene Flame: When acetylene alone is burned in air, it produces
a flame that varies in color from yellow near the torch tip to orangered at the outer extremity.
 Depending upon the presence of excess acetylene in oxy-acetylene flame it
can be categories as:
(i) Carburizing Flame and (ii) Reducing Flame
 Carburizing Flame: As the oxygen valve in the torch is progressively
opened and the ratio of oxygen to acetylene increases, the flame becomes
generally bright. Then, the bright portion contracts toward the welding tip,
forming a distinct bright zone within a blue outer envelope.
 This is a carburizing flame because it has a large excess of acetylene; it is
sometimes described as a soft flame because it has very little force.
 Application: It has a relatively low temperature and is used in silver brazing
and soldering, as well as in the welding of lead. It is generally used for
carburizing (surface hardening) purposes.
576
Fig. Acetylene Flame
Fig. Carburizing Flame
Oxy-Acetylene Flames (cont.)
 Reducing Flame (Max. temperature is about 3040°C): The flame is
as a slightly excess acetylene or reducing flame but less than the
carburizing flame.
 As more oxygen is introduced, the bright zone of the flame contracts
farther and is seen to consist of two parts:
• A bright inner cone and
• A pale-green feather,
 The feather is caused by a slight excess of acetylene. It disappears as the
oxygen-to- acetylene ratio approaches 1 to 1.
 For welding steel, the length of the feather should be about one-eighth to
one-quarter, but never more than one-half, the length of the inner cone.
 It should not be called a carburizing flame because it does not carburize the
metal, but it does ensure the absence of the oxidizing condition.
 Application: It is used in Low alloy steel, non-ferrous metals that do not
tend to absorb carbon. It is very well used for high carbon steel.
577
Fluxes Requirement
 Except for lead, zinc and some precious metals, OFW of nonferrous
metals, cast irons and stainless steels generally requires a flux.
 In welding carbon steel, the gas flame shields the weld adequately,
and no flux is required.
578
Combustion of other gases
579
Oxy-hydrogen Combustion
 Complete combustion of hydrogen requires an oxygen-to-hydrogen
ratio of 1 to 2, as can be seen from the following equation:
2H2 + O2 2H2O.........................................(4)
 This gas mixture produces a strongly oxidizing flame having a
temperature of about 2760 °C (5000 °F).
 It is impossible to obtain a neutral oxy-hydrogen flame by the visual
methods of flame adjustment described for the oxyacetylene flame.
 The oxy-hydrogen flame itself is scarcely visible, and no
combustion zones.
580
Oxy-hydrogen Combustion(cont.)
 Basic Features Oxy-hydrogen Combustion:
 To avoid an oxidizing flame, the pressure regulators must be set to
ensure an excess of hydrogen.
 The flame is then reducing, but not carburizing. It has no
carbon, and the temperature is several hundred degrees lower than
that of the neutral flame.
 Metering flow regulators permit establishing the desired ratio of
hydrogen to oxygen, usually 4 to 1.
 The oxy-hydrogen flame is useful for welding and brazing aluminum
alloys and lead.
581
Combustion of Natural Gas and Propane
Complete combustion of natural gas (methane) and propane is shown,
respectively, by the following equations:
CH4 + 2O2 CO2 + 2H2O...........................(5)
C3H8 + 5O2 3CO2 + 4H2O........................(6)
Note:
When the flame temperature is high enough to weld steel, the flame
atmosphere is excessively oxidizing, but when the ratio of oxygen to
fuel gas is decreased to produce a carburizing condition, flame
temperature is too low for welding steel. Here the temperature is around
2500 °C.
582
Classification of OFW Technique
OFW Technique is classified in following two categories:
Leftward or Forehand OFW Technique.
Rightward or Backhand OFW Technique.
 Most OFW is done with the one-pass Leftward or forehand technique,
particularly on thinner materials.
 A two-pass weld, using the Rightward or backhand technique for the
first pass and the forehand technique for the second pass, is preferred
for thicker material (maximum thickness of 4.8 mm)
583
Leftward or Forehand technique (Contd.)
The welder holds welding torch in his right hand and the filler rod in the left hand.
The welding flame directed away from the finished weld i.e. towards the unwelded part
of the joint.
Filler rod, when used, is directed towards the welded part of the joint.
Since the flame is pointed in the direction of the welding, it preheat the edges of the
joint.
Good control and neat appearance are characteristics of leftward method.
It is usually used on relatively thin metals i.e., having thickness less than 5mm.
For workpiece thickness over 3mm, it is necessary to bevel the plate edge (i.e.
included angle is 80-90deg.) so that good root fusion may be achieved.
When the materials over 6.5mm thick, it is difficult to obtain even penetration at the
bottom of the V and therefore the quality decreases as plate thickness increases.
584
Rightward or Backhand technique
Here also the welder holds welding torch in his right hand and the filler rod in the left
hand.
Welding begins at the left-hand end and proceeds towards the right, hence the name
rightward technique (here the filler rod is back of weld deposition)
Fig.: Orientation and motion of torch relative to work piece in one-pass backhand
oxyacetylene welding.
Note: The cone of flame should be 585
kept close to, but should never touch,
the weld pool or pipe groove face.
Accessories for OFW
Accessories essential to OFW include
A friction lighter for igniting the torch;
 Welder's goggles,
Gloves and protective clothing; and
Related safety devices.
Welder's goggles are covered by ANSI standard, which suggests
the following lens shade numbers for use in OFW of steel:
Steel Thickness(mm)
Steel Thickness(in)
Shoulder Number
≤3.2
≤ 0.125
4 or 5
3.2-13
0.125-0.5
5 or 6
>13
>0.5
6-8
586
Proprietary gases and Mixtures
Gas
Acetylene
Air (Not Breathing Quality)
Cylinder Colour
Maroon
Grey
Carbon Di Oxide, Commercial
Liquid Withdrawal
Black With White Strip Down
Length Of Cylinder
Argon
Hydrogen
Nitrogen
Blue
Red
Grey With Black Shoulder
Oxygen
Propane
Black
Red, Wider And Shorter Cylinder
Argon/ Carbon Di Oxide
Blue Green Band On Cylinder
Argon/ Helium
Blue Green Band On Shoulder
Argon / Hydrogen
Blue Red Band On Shoulder
Argon/ Oxygen
Blue Black Band On Shoulder
587
Base metal, Filler metal and Flame type
Base Metal
Aluminium’s
Filer Metal Type
Match Base Metal
Flame Type
Slightly Reducing
Flux Type
Aluminium
Brasses
Bronzes
Copper
Copper Nickel
Inconel
Iron, Cast
Iron, Wrought
Lead
Monel
Nickel
Nickel Silver
Low Alloy Steel
High Carbon Steel
Navy Brass
Copper Tin
Copper
Copper Nickel
Match Base Plate
Cast Iron
Steel
Lead
Match Base Plate
Nickel
Nickel Silver
Steel
Steel
Slightly Oxidising
Slightly Oxidising
Neutral
Reducing
Slightly Reducing
Neutral
Neutral
Slightly Reducing
Slightly Reducing
Slightly Reducing
Reducing
Slightly Reducing
Reducing
Borax
Borax
No Flux Required
No Flux Required
Fluoride
Borax
No Flux Required
No Flux Required
Monel
No Flux Required
No Flux Required
No Flux Required
No Flux Required
Low Carbon Steel
Medium Carbon
Steel
Stainless Steel
Steel
Steel
Neutral
Slightly Reducing
No Flux Required
No Flux Required
Match Base Plate
Slightly Reducing
Stainless Steel
588
Applications
It can be used for preheating, post heating, welding, braze welding, and
torch brazing, and it is readily converted into oxygen cutting.
The process can be adapted to short production runs, field work and
repairs.
Metals That Can Be Oxy fuel Gas Welded: Most ferrous and
nonferrous metals can be oxy fuel gas welded.
Oxyfuel gas welding can be used to join thin carbon steel sheet and
carbon steel tube and pipe.
Oxyfuel gas welding is frequently used for repairs and alterations
because the equipment is portable, welding can be done in all positions,
and acetylene and oxygen are readily available
589
Advantages
Advantages of OFW:
The equipment is versatile, low-cost, self-sufficient, and usually
portable
It includes the ability to control heat input, bridge large gaps, avoid
melt-through, and clearly view the weld pool.
Carbon steel sheet, formed in a variety of shapes, can often be
welded more economically by OFW than by other processes.
Oxyfuel gas welding is capable of joining small-diameter carbon
steel pipe (up to about 75 mm diameter) with resulting weld quality
equal to competitive processes and often with greater economy.
Pipe with wall thickness up to 4.8 mm ( 3/16 in.) can be welded in a
single pass.
590
Limitations
Limitations:
Metals unsuited to OFW are the refractory metals, such as niobium,
molybdenum, tungsten and tantalum.
As well as the reactive metals, such as titanium and zirconium.
The disadvantage in using oxyfuel gouging is that the heat input may
cause the crack to propagate through differential expansion in the
workpiece.
591
Thank you
592
Fundamental of Welding Science and Technology
Lecture 23: Submerged Arc Welding (SAW)
Pankaj Biswas (PhD)
Department of Mechanical Engineering
IIT Guwahati
593
Introduction of SAW
 Heat for fusion is generated from an arc between a continuous
consumable electrode and the base metal.
 The filler metal is a bare consumable electrode wire, fed through a
wire feeder.
 The arc, end of the electrode and molten pool remain completely
hidden and are invisible being submerged under a blanket of
granular flux.
 Basic Features
 Arc fully submerged in flux.
 Heat loss is minimum. Thermal efficiency as high as 80-90%.
 Produces no visible arc light
 welding is spatter free
 High deposition
1
 Downhand welding process
594
Principle of SAW
 In SAW process, instead of a flux covered electrode, granular flux
and bare (or copper coated) electrode is used.
 Arc between the electrode and job is the heat source and remains
buried/ suppressed under the flux.
 The flux serves as shield and protects the molten weld pool from
atmospheric contamination.
 The process may be semi-automatic or fully-automatic.
595
2
GMAW Setup
596
wire electrode
wire feed
granulated flux
current
slides
welding current
unused flux
slag
.........
.
......
.........
....
.....
....
....
...
...
..
........
.......
....
.......
......
....
...... ...
.........
....
........
...
...
...
.......
.......
.........
..........
deposited
metal
direction of welding
Schematic representation of submerged arc welding process
SAW
597
Principle
598
Operating Characteristics
• Fully mechanized process
• Electrode acts as the filler wire
 Fed continuously by a wire-feeding mechanism
• Flux is fed directly on the arc from a hopper
• Arc heat burns some of the flux, electrode tip and the
adjacent edges of the base metal, creating a pool of
molten metal below a layer of liquid slag (burnt flux)
599
SAW
• Slag floats on the molten metal and thus completely
shields the molten zone from the atmosphere.
 It also dissolves impurities in the base metal and
electrode and floats them up to the surface.
• Slag shield results in a slower cooling rate for the
deposited weld metal and thus provides an annealing
effect to the weld deposit.
600
SAW
Power Source
 Constant-voltage power supply, being self-regulating,
is used with a constant speed wire feeder normally
in a DCEP mode.
601
SAW
Operating Variables
• Welding current
• Polarity
• Welding voltage
• Welding speed
• Electrode diameter
• Electrode extension (length of stick out)
• Type of flux
• Width and depth of flux layer
• Wire feed rate
602
SAW
Electrode Extension
• An important operating variable for current densities
above 125 A/mm2.
• Electrode melting rate increases because of resistance
heating of the electrode
 Increases deposition rates by 25% to 50% with no
change in welding current.
• Increase in deposition rate is accompanied by a decrease
in penetration.
• About 8 times the electrode diameter
603
SAW
The relation between electrode melting rate and electrode
Extension is given by
mr 
I
60
[ 0.35 
d2
645
 2.0810
Where
d = electrode diameter in mm,
Le = length of stickout in mm.
604
SAW
7


I  Le  1.22
d2
]
gm/s
Flux
• Apart from shielding it also provides
 Stability of the arc
 Chemical composition of the weld metal
 Mechanical properties of weld deposit
• Granular fusible minerals containing oxides of manganese,
silicon, titanium, aluminum, calcium, zirconium,
magnesium and other compounds such as calcium fluoride.
(short form MSC MA TZ)
• Wire flux combination yields desired mechanical properties
605
SAW
Fluxes are primarily of two types:
Bonded fluxes
• Bonded with a low melting compound such as sodium
silicate.
• Contain metallic deoxidisers,
 Help to prevent weld porosity.
 Commonly used deoxidizers in metallurgy
 Ferrosilicon, ferromanganese, calcium silicide - used in steelmaking in production of
carbon steels, stainless steels, and other ferrous alloys
 Manganese - used in steelmaking
 Silicon carbide, calcium carbide - used as ladle deoxidizer in steel production
 Aluminium dross - used to deoxidize slag in secondary steelmaking
 Calcium - used as a deoxidizer, desulfurizer, or decarbonizer for ferrous and
nonferrous alloys
 Titanium - used as a deoxidizer for steels 606
SAW
Fused fluxes
• Produced by mixing the ingredients, then melting,
cooling and grinding.
• Smooth stable arcs, with welding currents up to 2000A.
• The flux prior to use should be backed (around 900°C )to
remove moisture.
• Moisture, if present in the flux, will cause porosity in the
weld deposit.
 Containing oxides of manganese, silicon, titanium, aluminum, calcium, zirconium,
magnesium and other compounds such as calcium fluoride.
607
SAW
Width & Depth of Flux
• Bead appearance and soundness of the finished weld
depend on the width and depth of the granular flux layer.
• Layers are too deep
 Gases generated during welding can not readily escape
and results in a distorted weld surface.
• Layer is too shallow
 Arc may not get fully submerged causing flashing and
spattering of molten metal.
 Will result in a poor bead appearance and may cause
porosity.
608
SAW
Cracks in submerged arc welds
The factors controlling solidification cracking are:
 weld metal composition,
 weld solidification pattern (depends on shape of the weld),
 Strain on the solidifying weld.
• A parameter has been developed to calculate cracking
susceptibility using the weld metal composition:
UCS (units of cracking susceptibility)
= 230C + 190S + 75P + 45Nb - 12.3Si - 5.4Mn - 1.
609
SAW
Cracks in submerged arc welds (cont’d)
• In case of butt welds, trouble should not be expected
for UCS less than 25, provided that the weld has an
acceptable shape.
• The shape of the weld influences the solidification pattern
• To minimize cracking the columnar grains of the
solidifying metal should appear in an upward
pattern rather than inwards.
610
SAW
Cracks in submerged arc welds (cont’d)
• The tendency of the columnar grains to grow inwards
rather than upwards give a more pronounced centreline
segregation of impurity elements and also concentrate
the contraction strain in the same region.
• To avoid cracking, consumables should be selected
with low carbon and sulphur, and high manganese
and silicon contents.
 UCS (units of cracking susceptibility)
= 230C + 190S + 75P + 45Nb - 12.3Si - 5.4Mn - 1.
611
SAW
Advantages
 This gives consistently high quality welds
with minimum operator skills.
Molten flux provides very suitable conditions for high current to
flow.
 Minimum of welding fume and of arc visibility (radiation).
 Well suited to welding thick sections. Practically, no edge
preparation is necessary for materials under 12 mm in thickness.
 Suitable for welding carbon, low alloy and alloy steels.
This process can be used for welding in exposed areas with
relatively high winds.
 Because of high heat concentration, considerably higher welding
speed can be used.
 Relatively high metal deposition rates. The ability to produce
high quality, defect free welds.
612
SAW
Disadvantages of SAW

Weld may contain slag inclusions.
 Limited applications of the process - mostly for welding
horizontally located plates.
In small thickness (i.e. less than 4.8mm) burn through is
likely to occur.
Weld metal chemistry is difficult to control. A change in
welding variables especially when using alloyed fluxes may
affect weld metal composition adversely.
Cast iron, Al-alloy, Mg-alloy, Pb and Zn cannot be welded.
613
SAW
Application
 SAW is widely used for welding carbon, carbon manganese, alloy
and stainless steels.
 It is also used for joining some nickel based alloys.
 Fabrication of pipes, pressure vessels, boilers, structural shapes, rail
road, crane, bridge, girders, under structures of railway coaches,
locomotives etc.
It is widely used in automotive, aviation (aero-plane), ship-building
and nuclear power industries.
 High deposition rates and with deep weld penetration makes the
SAW process highly suitable for all mechanized and automatic
welding and surfacing applications.
 It is widely used for cladding carbon and alloy steels with stainless
steel and nickel alloy deposits.
 It is also used in hardfacing tractor rollers & idlers and crane
pulleys
SAW
614
End
615
SAW
616
A deoxidizer is a chemical compound used in a reaction or process to remove oxygen. In
comparison with antioxidants, deoxidizers are not used for stabilization during storage but
for oxygen removal during manufacture. Deoxidizers are mainly used in metallurgy, to
decrease the content of oxygen in metals.
Commonly used deoxidizers in metallurgy
Ferrosilicon, ferromanganese, calcium silicide - used in steelmaking in production of
carbon steels, stainless steels, and other ferrous alloys
Manganese - used in steelmaking
Silicon carbide, calcium carbide - used as ladle deoxidizer in steel production
Aluminium dross - used to deoxidize slag in secondary steelmaking
Calcium - used as a deoxidizer, desulfurizer, or decarbonizer for ferrous and nonferrous
alloys
Titanium - used as a deoxidizer for steels
Phosphorus, copper(I) phosphide - used in production of oxygen-free copper
Calcium hexaboride - used in production of oxygen-free copper, yields higher conductivity
copper than phosphorus-deoxidized
Yttrium - used to deoxidize vanadium and other non-ferrous metals
Zirconium
Magnesium
Carbon
Tungsten
Oct.2008
617
SAW
25
Welding Defect & Inspection
INDIAN INSTITUTE OF TECHNOLOGY GUWAHATI
DEPARTMENT OF MECHANICAL ENGINEERING
Guwahati -781039, Assam, India
618
Weld Joint Defect/Discontinuities
•
Misalignment
Undercut
Underfill
Concavity or Convexity •
Excessive reinforcement •
Improper reinforcement •
Overlap
Burn-through
Incomplete or Insufficient
Penetration
• Incomplete Fusion
• Surface irregularity
•
•
•
•
•
•
•
•
•
– Overlap
• Arc Strikes
Inclusions
– Slag
– Tungsten
Spatter
Arc Craters
Cracks
–
–
–
–
–
–
–
Longitudinal
Transverse
Crater
Throat
Toe
Root
Underbead and
Heat-affected zone
– Hot
– Cold or
delayed
619
• Base Metal
Discontinuities
– Lamellar tearing
– Laminations and
Delaminations
– Laps and Seams
• Porosity
–
–
–
–
Uniformly Scattered
Cluster
Linear
Piping
• Heat-affected zone
microstructure alteration
• Base Plate laminations
Misalignment
 A joint is out of alignment at the root
• Cause: Due to Carelessness. Also due to joining of different
thicknesses (transition thickness)
• Repair: Grinding. It is difficult for Inside of Pipe /Tube.
620
Undercut
• Definition: A groove cut at the toe of the weld and
left unfilled.
• Cause: Due to High amperage, long arc length,
rust.
• Repair: Weld with smaller electrode ,
sometimes must be low hydrogen with
preheat.
Note: Undercut typically has an allowable limit. Different codes
and standards vary greatly in the allowable amount.
 Plate - the lesser of 1/32” or 5%.
621
Insufficient Fill
 Here the weld surface is below the adjacent surfaces of the base metal
• Cause: Improper welding techniques
• Prevention: Apply proper welding techniques for the weld type and
position.
• Repair: Simply weld to fill. May require preparation by grinding.
Insufficient Fill on the Root Side (Suck back)
Repair: Backweld to fill.
622
Excessive Concavity or Convexity
• Definition: Concavity or convexity of a fillet weld which
exceeds the specified allowable limits.
• Cause: Amperage and travel speed
• Prevention: Observe proper parameters and techniques.
• Repair: Grind off or weld on.
623
Concavity
624
Convexity
625
Reinforcement
 The amount of a groove weld which extends beyond the surface of
the plate
• Excessive
• Insufficient
• Improper contour
Face Reinforcement
626
Root Reinforcement
Excessive Reinforcement
• Definition: Typically, Reinforcement should be flush to 1/16”(pipe)
or flush to 1/8” (plate).
• Cause: Travel speed too slow, amperage too low
• Prevention: Set amperage and travel speed on scrap plate.
• Repair: Remove excessive reinforcement.
627
Insufficient Reinforcement
• Definition: Typically, Under-fill may be up to 5% of metal thickness
or not to exceed 1/32”. Sometime it is called Root Concavity.
• Cause: On root reinforcement - Too little filler metal will
cause thinning of the filler metal.
• Prevention: Use proper welding technique. Use backing or
consumable inserts. Use back weld or backing.
628
Improper Weld Contour
• Definition: When the weld exhibits less than a 1350
transition angle at the weld toe.
1350
• Cause: Poor welding technique.
• Prevention: Use proper techniques. A weave motion can
often eliminate the problem.
629
Overlap
 It can be defined: When the face of the weld extends beyond the toe
of the weld. It is a contour problem.
• Cause: Improper welding technique. Typically travel speed.
• Prevention: Proper welding technique will prevent this
problem.
• Repair: Overlap must be removed to blend smoothly into
the base metal.
630
Overlap
No amount of overlap is typically allowed.
631
Burn-through
• Definition: When an undesirable open hole has been completely
melted through the base metal. The hole may or may not be left open.
• Cause: Excessive heat input.
• Prevention: Reduce heat input by increasing travel speed, use of a heat
sink.
632
Incomplete or Insufficient Penetration
• Definition: When the weld metal does not extend to the required
depth into the joint root.
• Cause: Low amperage, low preheat, tight root opening, fast travel
speed, short arc length.
• Prevention: Correct the contributing factor(s).
• Repair: Back gouge and back weld.
633
Incomplete Fusion
• Definition: Where weld metal does not form a cohesive bond with
the base metal.
• Cause: Low amperage, fast travel speed, short arc gap, lack of
preheat, unclean base metal.
• Prevention: Eliminate the potential causes.
Fig. Lack of side-wall fusion
Fig. Lack of root fusion
634
Fig. Lack of inter-run fusion
Arc Strike
• Definition: A localized coalescence outside the weld zone. Which
may contain cracks and are thus to be avoided.
• Cause: Carelessness
• Prevention: In difficult areas,
adjacent areas can be protected
using fire blankets.
635
Inclusions
 Slag &Tungsten
Slag Inclusion
• Definition: Slag entrapped within the weld
• Cause: Low amperage. Normally by the presence of mill scale and/or rust on
prepared surfaces, or electrodes with cracked or damaged coverings
• Prevention: Increase amperage or preheat, grind out mill scale .
• Repair: Remove by grinding. Re-weld.
636
Tungsten Inclusion
• Definition: A tungsten particle embedded in a weld. (Typically
GTAW only)
• Cause: Tungsten electrode too small, amperage too high, electrode
dipped into the weld pool or touched with the fill rod, electrode split.
• Prevention: Eliminate the cause
• Repair: Grind out and reweld
637
Spatter
• Definition: Small particles of weld metal expelled from the welding
operation which adhere to the base metal surface.
• Cause: Long arc length, high
amperages, globules of molten metal.
• Prevention: Correct the cause. Base
metal can be protected with
coverings or hi-temp paints.
• Repair: Remove by grinding or sanding.
638
Arc Craters
• Definition: A depression left at the termination of the weld where the
weld pool is left unfilled.
• Cause: Improper weld termination
techniques
• Repair: If no cracks exist, simply
fill in the crater.
639
Cracks
•
•
•
•
•
•
•
•
•
Longitudinal
Transverse
Crater
Throat
Toe
Root
Underbead and Heat-affected zone
Hot
Cold or delayed
640
Longitudinal Crack
• Definition: A crack running in the direction of the weld axis. May be
found in the weld or base metal.
• Cause: Fast cooling problem. Also caused by
shrinkage stresses in high constraint areas.
• Prevention: Weld toward areas of less constraint. Also preheat.
• Repair: Remove and reweld.
Transverse Crack
• Definition: A crack running into or inside a weld, transverse to the
weld axis direction.
• Cause: Weld metal hardness problem
641
Crater Crack
• Definition: A crack, generally in the shape of an “X” which is found
in a crater. Crater cracks are hot cracks.
• Cause: The center of the weld pool becomes solid before the outside
of the weld pool, pulling the center apart during cooling
• Prevention: Use crater fill, fill the crater at weld termination.
642
Throat Crack
• Definition: A longitudinal crack located in the weld throat area.
• Cause: Transverse Stresses, probably from shrinkage. Indicates
inadequate filler metal selection or welding procedure.
• Prevention: Increasing preheat may prevent it. Use a more ductile
filler material.
• Repair: Remove and reweld using appropriate procedure.
643
Toe Crack
• Definition: A crack in the base metal beginning at the toe
of the weld
• Cause: Transverse shrinkage stresses. Indicates a HAZ
brittleness problem.
• Prevention: Increase preheat if possible, or use a more
ductile filler material.
• Root Crack: Same as a throat crack.
644
Underbead Crack
• Definition: A crack in the unmelted parent metal of the HAZ.
• Cause: Hydrogen embrittlement
• Prevention: Use Low Hydrogen
electrodes and/or preheat
• Repair: Remove and reweld.
645
Hot / Solidification Cracking
• Definition: A crack in the weld that occurs during solidification.
• Cause:
• Micro stresses from weld metal shrinkage pulling apart weld metal as it cools from
liquid to solid temp.
• Large depth/width ratio of weld bead
• Prevention: Preheat or use a low tensile filler material.
646
Cold Crack
• Definition: A crack that occurs after the metal has completely
solidified
• Cause: Shrinkage, Highly restrained welds, Discontinuities
• Prevention: Preheat, use a more ductile weld metal
• Repair: Remove and reweld, preheat may be necessary.
647
Laminations
 Base Metal Discontinuity
 May require repair prior to welding
 Formed during the milling process
 Lamination effects can be reduced by joint design:
648
Uniformly Distributed Porosity
 Resulting from the entrapment of gas in
solidified weld metal
• Causes:
 Gas may originate from dampness or
grease on consumables or workpiece, or
by nitrogen contamination from the
atmosphere
 If the weld wire used contains
insufficient deoxidant it is also possible
for carbon monoxide to cause porosity
649
Surface Porosity
• Reasons:
 Excessive contamination from grease,
dampness, or atmosphere entrainment
 Occasionally caused by excessive
sulphur in consumables or parent metal
650
END
651
Welding Testing
INDIAN INSTITUTE OF TECHNOLOGY GUWAHATI
DEPARTMENT OF MECHANICAL ENGINEERING
Guwahati -781039, Assam, India
652
DESTRUCTIVE
AND
NON-DESTRUCTIVE
653
DESTRUCTIVE TESTING
• These can be divided into two parts:
 Tests capable of being performed in the workshop.
 Laboratory tests i.e.:
microscopic-macroscopic, chemical and corrosive.
• Why this testing is required?
 Defects occur during welding which affect the quality of the
plate.
654
TYPES OF WORKSHOP TESTS
TENSILE
IMPACT
BENDING
HARDNES
FATIGUE
CRACKING
655
TENSILE
Material is sectioned and edges
rounded of to prevent cracking.
Punch marks are made to see
elongation.
656
BEND TESTING
• It provides the following:
Physical condition of the weld
Also determine welds efficiency
• Tensile strength
• Ductility
• Fusion and penetration
657
BEND TEST
O
 Bend through 180
 The specimen should be a minimum of 30mm wide
 The fulcrums diameter is 3 times thickness of the plate
 The bottom rollers have a distance of the diameter of
the former + 2.2 times the thickness of the plate
 Upper and lower surfaces ground or filed flat and
edges rounded off.
 The tests should be 2 different types i.e.: one against
the root and another against the face and in some
cases a side bend.
658
Root bend
659
FACE BEND
660
IMPACT
 CHARPY AND IZOD
• Gives the toughness and shock loading of the
material and weld at varying temperatures with
a notch such as under cut
• The measurement is the energy required to
break a specimen with a given notch
o
• 2mm depth at a 45 bevel or a “U” notch.
661
Note: 300 div for Charpy Test & 168 div for Izod Test (1div =2 J Energy)
662
CHARPY IMPACT TEST
663
IZOD IMPACT TEST
664
HARDNESS TESTS
• This gives the metals ability to show resistance
to indentation which show it’s resistance to
wear and abrasion.
• The tests are as follows:
 Brinell
 Rockwell
 Vickers
diamond pyramid
665
VICKERS HARDNES
666
FATIGUE
• The testing of Material that is subject to fluctuating
loads
• HAIGH Electro magnetic tester.
• Rotating chuck with weight
667
CRACKING
• REEVES Test: It study the hardening and cracking
of welds.
• The compatibility of electrodes for the metal being
joined.
668
CRACKING
• 3-Sides Are Welded with
Known Compatible Electrodes.
• The front edge is welded with
the test electrode.
• If incompatible it will crack.
669
MICROSCOPIC
 Used to
determine the actual
structure of the weld and parent
metal
 A transmission election microscope
(TEM) can magnify up to
1,000,000 times. A scanning
electron microscope (SEM) can
magnify up to 200,000.
 The simple light microscope,
which only magnifies by 500
times.
 Polishing must be of a very high
standard
670
MACROSCOPIC
 It is examined using a
magnifying glass.
 It has magnification from 2
to 20 time.
 It will show up slag
entrapment or cracks.
 Polishing not as high as
microscopic test specimen.
671
ETCHING REAGENT
• These are acids compositions used to show up different structures in
metals
• For steels the most common is “1-2 % nitric acid in distilled water
or alcohol (Nital Solution).
• Aluminum uses a solution of 10-20% caustic soda in water etc.
or
• Aluminum uses a solution of Keller’s reagent (i.e. a mixture of nitric
acid, hydrochloric acid and hydrofluoric acid).
672
NON-DESTRUCTIVE
TESTING
673
Common methods used in NDE
• Visual Inspection (VT)
• Magnetic Particle Inspection (MT)
• Liquid (Dye) Penetrant Inspection (PT)
• X-Ray inspection (RT)
• Ultrasonic testing (UT)
• Acoustic
• Air or water pressure testing (LT)
674
Visual Inspection (VT)
• Visual is the most common inspection method
• It reveals spatter, excessive buildup, incomplete slag removal,
cracks, distortion, undercutting & poor penetration.
• Typical tools for VT consist of Fillet gauges, Magnifying
glasses, Flashlights & Tape measures or calipers.
• While welding
– The rate the electrode
melts
– The way the weld metal
flows
– Sound of the arc
675
• After welding
– Under cut
– Lack of root fusion
– Amount of spatter
– Any pin holes from gas
– Dimensions of weld
Visual Inspection (VT)
• Fillet gauges measure
 The “Legs” of the weld
 Convexity
• (weld rounded outward)
 Concavity
• (weld rounded inward)
 Flatness
676
Magnetic Particle Inspection (MT)
• Magnetic Particle Inspection (commonly referred to as Magna-flux
testing) is effective only at checking for flaws located at or near the
surface.
• It uses a metallic power or liquid along with strong magnetic field
probes to locate flaws. (Generally, particles will align along voids)
• It can only be used on materials that can be magnetized.
• The presence of voids or cracks in the section results in an leakage in
the magnetic field.
677
Liquid (Dye) Penetrant Inspection (PT)
• It uses colored or fluorescent dye to check for surface flaws
which is not visible.
•
The welded part is sprayed with or dipped into a dye containing
a fluorescent material.
• The surface to be inspected is then wiped, dried and viewed in
darkness.
 It does not show sub-surface flaws.
 It can be used on both metallic and non metallic surfaces such
as glass, ceramic, plastic and metal.
 It dose not require the part to be Magnetized.
678
RADIOGRAPHIC TEST
Two types of methods used
 X-RAY
 GAMMA RAY
 It is electro magnetic radiation
of short duration
 Both of these methods are
harmful to health.
679
X-Ray inspection (RT)
• Welds may be checked for internal discontinuities by
means of X- Rays.
• An X-Ray is a wave of energy that will pass through most
materials and develop the negative image of what it
passes through on film.
• A Radiograph ( X-Ray picture) is a permanent record of
a weld used for quality inspection purposes
680
Ultrasonic testing (UT)
• It (UT) is a method of determining the size and location of
discontinuities within a component using high frequency
sound waves.
• Sound waves are sent through a transducer into the material
and the shift in time require for their return or echo is plotted.
• Ultrasonic waves will not travel through air therefore flaws
will alter the echo pattern.
• So, on encountering a discontinuity, the signal is reflected
back.
• The time interval between sending and reflecting signal
determine the location of discontinuity.
681
682
Air or water pressure testing (LT)
• Pressure testing (or leak testing) can be performed with
either by using gasses or liquids.
• Voids that allow gasses or liquids to escape from the
component can be classified as gross (i.e. large) or fine leaks.
• Extremely small gas leaks measured in PPM (parts per
million) require a “Mass Spectrometer” .
683
ACOUSTICS TEST
 Striking with a rounded object
 If no defect then Ringing tone
 Tone changes when object is
cracked
684
THE END
685
Worm Holes
• Resulting from the entrapment of gas
between the solidifying dendrites of weld
metal.
• Causes:
– The gas may arise from contamination
of surfaces to be welded,
686
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