We use Non Destructive Testing (NDT) when we wish to
assess the integrity of a structure without destroying it
The 4 of the common types of NDT used when assessing
weldments are:
Procedure
First the work must be cleaned thoroughly, then a
penetrant is applied for a specified time*
Once the contact time has elapsed, the penetrant is
removed and a developer is then applied*
Any penetrant that has been drawn into a crack by
capillary action will be drawn out into the developer*
Two types of penetrants are:
1) Colour contrast 2) Fluorescent Penetrant*
Method
Apply Penetrant
Clean then apply Developer
Result*
Advantages
Disadvantages
1) Low operator skill level
1) Highly clean metal
2) All materials (Non Porous)
2) Surface flaws only
3) Low cost method
3) Extremely messy
4) Simple equipment
4) No permanent record*
Procedure
First the work must be cleaned and a whitener applied for
contrast. A magnetic flux is then applied by permanent magnet,
electro magnet, or straight current*
A magnetic ink is applied which will concentrate in areas of
flux leakage, as those caused by flaws*
The weld length must be crossed at 90° by the magnetic field*
The types of magnetic media used are:
1) Wet ink
2) Dry powder
3) Fluorescent ink*
Method
Contrast paint
Magnet & Ink
Result*
Advantages
Disadvantages
1) Low operator skill level
1) Fe Magnetic metal only
2) Sub surface flaws
2) De-magnetize after use
3) Relatively cheap
3) Can cause arc strikes #
4) Simple equipment
4) No permanent record*
# When using the straight current prod technique
Procedure
First the work must be cleaned thoroughly, then a couplant is
applied to increase sound transmission*
A probe is then applied with the correct angle for the weld
preparation and sound waves are transmitted*
Any imperfections will rebound the sound waves causing a
signal to occur on the cathode ray tube*
Method
Apply Couplant
Sound wave
Result*
CRT display
Signal rebounded
from Lack of fusion
Advantages
Disadvantages
1) Can find lack of fusion
1) High operator skill
2) Most materials
2) Difficult to interpret
3) No safety requirements
3) Requires calibration
4) Portable/instant results
4) No permanent record*
Procedure
A film is placed inside a cassette between lead screens.
It is then placed to the rear of the object to be radiographed
A radiographic source, is exposed to the work and film for a
pre-calculated time*
Any imperfections in line with the beam of radiation will be
shown on the film after exposure and development*
The 2 types of radiation used in industrial radiography:
1)
2)
X rays (from Cathode Ray Tube)
Gamma rays (from a Radioactive Isotope)*
Method
Load film
Exposure to Radiation
Radioactive source
IQI
Film cassette
Interpret Graph
Developed
Graph
Latent image on the film
Advantages
Disadvantages
1) A permanent record ?
1) High operator skill
2) Most materials
2) Difficult interpretation
3) Assess root pen’ in pipe
3) Lack of sidewall fusion
4) Gamma ray is portable
4) Safety requirements*
New TWI Video
“Non Destructive Testing”
30 Minutes
Weld Repairs:
Weld repairs can be divided into two specific areas:
1)
Production repairs
2)
In service repairs*
Production repairs are usually identified by the Welding
Inspector, or NDT operator during the process of inspection,
or evaluation of reports to the code, or applied standard*
A typical defect is shown below: *
Prior to repair the defect may need to undergo the following:
1) A defect analysis and report
2) An assessment of defect extremity
3) An excavation procedure
4) NDT procedures
5) A welding repair procedure
6) Welder approval to the approved repair procedure
7) Any subsequent treatments procedures i.e. PWHT *
Plan View of defect with drilled ends *
Side View of defect excavation*
Completed repair*
NDT confirmation of successful repair:
After the excavation has been filled the weldment should
then be undergo a complete retest using NDT to ensure
no further defects have been introduced by the repair.
NDT may also need to be further applied after any
additional post weld heat treatment has been carried out*
Residual welding stresses are defined as those stresses that
remain inside a material after welding has been carried out
Stresses are caused by the heat of welding, which produces
local expansion and contraction to take place
If a metal was heated & cooled uniformly no stresses would
remain, as expansion & contraction would be uniform
Welding causes only local heating and cooling conditions to
exist, hence some residual stresses to remain in the metal*
Residual stresses can have very complex patterns in
welded constructions.
In simple butt welded plates they may be indicated as
shown below:
Tensile 
Tensile 
Compressive 
*
We can say that expansion/contraction has three directions.
Transverse
Weld
Plan View
of plate
Longitudinal
End View
of plate
Short transverse
One effect of welding related stresses is distortion
Distortion is the movement of material in one area caused by
expansion and contraction, and local elastic/plastic movement
that misshapes the component*
The various types of distortion produced are caused by the
directions and amount of expansion and contractional stresses
involved, and the ability of the material to resist the stress
without the formation of elastic/plastic strain, or deformation
It is this deformation that produces distortion in a product*
Transverse
distortion
Longitudinal
distortion
The volume of weld metal in a joint will also effect the
amount of local expansion and contraction
Hence the more volume of weld metal then the overall
amount of distortion will be higher*
Many methods are used to control the effects of distortion.
Perhaps the best of these is to to pre set the materials to allow
distortion to bring it to its final shape. This method is called
offsetting, or pre-setting*
a) Practical
b) Practical
c) Impractical*
Other forms of distortion control stop the movement of material
from occurring by using such methods as clamping, jigging,
strong backs, and tacking etc*
These methods will reduce the distortion, but will also amass
the maximum amount of residual stresses to exist*
Back-step & balance welding are sequences which may also
be used to control the effects of distortion*
All heat treatments applied to metals are cycles of 3 elements.
Temp
1)
Heating
2)
Soaking
3)
Cooling
2
1
3
Time
Annealing:
Used to make metals soft and ductile
For steels, the component is heated above its UCT, or
upper critical temperature, soaked for 1 hour/25mm of
thickness and left in the furnace to cool
Produces a coarse grain structure & low toughness*
Normalising: Used to make steels tough
As for annealing, but the steel is removed from the
furnace after soaking to cool in still air
Produces a fine grain structure with good toughness*
Hardening:
Used to make some steels harder
Used to increase the hardness of some plain carbon &
alloy steels. Plain carbon > 0.3%
The cycle is the same as previously but the cooling is
rapid i.e. Quenched in water, oil, but sometimes air*
Tempering:
Used after hardening to balance the
properties of Toughness & Hardness
The temperature range is from 220 – 723 °C
The
cooling part of the cycle should not be too rapid, but
over heating will over temper the steel*
PWHT:
Used after welding to release residual stresses,
caused by welding operations*
Stress
Force/Stress required to
induce plastic strain*
Y
The effect of heat
on the position of
the yield point*
Strain
By heating the steel, the yield point is suppressed/reduced relieving
residual stresses as plastic strain at a much lower level of stress*
Pre-Heating:
Used mainly on steels to retard the cooling
rate of a hardenable steel and reduce the
hardening effect (Martensite formation)
Is also used to help diffusion of Hydrogen
from the HAZ of hardenable steels to avoid
hydrogen cracking. Typically < 350 °C
Is also used to produce a more uniform rate
of cooling, and control distortion, or effects
of high contractional strains*
*
Name
Pictorial view
Uses
Neutral
Fusion welding most
metals. Flame temp >
3,200 °C*
Oxidising
Bronze Welding*
Carburising
Hard surfacing &
fusion & Brazing
Aluminium & alloys*
A jet of pure oxygen reacts with high temperature iron
(>1100  C) to produce Fe3 O4 by exothermic reaction.
This dross is then removed by the pressure of the oxygen jet*
Different types of fuel gases may be used for the pre-heating
flame in oxy fuel gas cutting: i.e.
Acetylene. Hydrogen. Propane. Etc*
By adding Iron powder to the flame we are able to cut most
metals “Iron Powder Injection”*
The high intensity of heat and rapid cooling will cause
hardening in low alloy, and medium, or high C steels They
are thus pre-heated to avoid the hardening effect*
Slightly rounded top edge caused
by too close a nozzle gap
The “Kerf”
50mm
Flutes
MMA electrodes are specially produced for cutting and gouging.
Oxy/Arc and Arc/Air are arc cutting process that produce lots of
fume and arc air produce a high noise level. All these processes
require good extraction, and ear protection is vital for arc/air
Both oxy arc and arc air use special types of electrodes and gas
supplies, which will be described by the course lecturer*
Safety is the responsibility everyone.
As respected officers, it is the duty of all welding inspectors
to ensure that safe working practices are strictly followed.
Safety in welding can be divided into several areas, some of
which are as follows:
(Areas to be expanded briefly by the lecturer/presenter)
1)
Welding/cutting process safety.*
2)
Electrical safety.*
3)
Welding fumes & gases. Use & storage of gases.*
4)
Safe use of lifting equipment.*
5)
Safe use of hand tools and grinding machines.*
Careful consideration should be given to safety hazards
when using a particular welding process. This may include:*
1) The hazards of infra red and/or ultra violet light
2) The hazards of heat, burns and fire
3) The hazards of toxic and non toxic gases from process,
coatings, or purging, and gases stored at high pressure
4) The hazards of working with high voltages & currents
5) The hazards of working in confined spaces*
Careful consideration should be given to safety when
using gas, or arc cutting systems by:*
1) Removing any combustible materials from the area
2) Checking all containers to be cut are fume free and have
a Permit to Work
3) Providing ventilation and extraction where required
4) Ensuring good gas safety is being practised
5) Keeping oil and grease away from oxygen
6) Using ear defenders when arc air cutting
Safe working with electrical power is mostly common sense.
Ensure that insulation is used where required and that cables
and connections are in good condition. (Check the duty cycle)*
Gases should be stored separately, and cylinders should be
secured when used in the vertical position, especially oxygen*
Exposure to dangerous welding fumes and gases from electrodes,
plating, i.e.ozone, nitrous oxide, phosgene, cadmium, beryllium
are to name just a few. Always use extraction/breathing systems.
If in any doubt, stop the work*
From the points that have been covered in the
safety lecture, and/or the Video, complete the
safety check list exercise in your course text*
“Weldability” is a term used in welding engineering to describe
the ease of which a material can be welded by the common
welding processes and still retain the properties for which it
was designed*
If we say that a material has limited weldability, it means that
we need to take special measures to ensure that the properties
as required are maintained*
Most materials are weldable with certain processes.
The weldability of steel is mainly dependant on carbon content
& alloying, though most steels have a degree of weldability*
1) Hydrogen induced HAZ cracking in Low Alloy Steels
2) Hydrogen weld metal cracking in Micro Alloy Steels
3) Solidification cracking in Ferritic steels
4) Lamellar tearing in Ferritic steels
5) Inter-granular corrosion in Stainless Steels*
Steels are classified into groups as follows:*
Plain Carbon Steels:*
1) Low Carbon Steel 0.01 – 0.3% Carbon*
2) Medium Carbon Steel 0.3 – 0.6% Carbon*
3) High Carbon Steel 0.6 – 1.4% Carbon*
Plain carbon steels contain only iron & carbon as main alloying
elements, traces of Mn Si Al S & P may also be present*
An Alloy steel is one that contains more than Iron & Carbon
as a main alloying elements*
Alloy steels are divided into 2 groups:*
Low Alloy Steels< 7% extra alloying elements*
High Alloy Steels> 7% extra alloying elements*
The following basic foundation information on metallurgy
will not form any part of your CSWIP examination*
A most important function in the metallurgy of steels, is the
ability of iron to dissolve carbon in solution*
The carbon atom is very much smaller than the iron atom and
does not replace it in the atomic structure, but fits between it*
Iron atoms
Carbon atoms*
Iron is an element that can exist in 2 types of cubic structures,
depending on the temperature. This is an important feature*
At temperatures below Ac/r 1, (LCT) iron exists like this*
α Alpha iron
This structure occurs below 723 °C and is
body centred, or BCC in structure
It can only dissolve up to 0.02% Carbon
*
Also known as Ferrite or BCC iron*
Compressed representation could appear like this
At temperatures above the Ac/r 3, (UCT) iron exists like this*
γ Gamma iron
This structure occurs above the UCT in
Plain Carbon Steels and is FCC in structure.
It can dissolve up 2.06% Carbon
Also called Austenite or FCC iron*
*
Compressed representation could appear like this
If steel is heated and then cooled slowly in equilibrium, then
exact reverse atomic changes take place*
If a steel that contains more than 0.3% Carbon is cooled quickly,
then the carbon does not have time to diffuse out of solution,
hence trapping the carbon in the BCC form of iron.
This now distorts the cube to an irregular cube, or tetragon*
This supersaturated solution is called Martensite and is the
hardest structure that can be produced in steels*
If some steels are cooled quickly their structure looks like this*
Martensite can be defined as:
A supersaturated solution of carbon in
BCT iron (Body Centred Tetragonal)
It is the hardest structure we can produce
in steels*
*
Compressed representation could appear like this
Solubility of Carbon in BCC & FCC phases of steels*
Ferrite: a
Low carbon solubility. Maximum 0.02%*
Austenite: g High carbon solubility. Maximum 2.06%*
Martensite: The hardest phase in steels, which is produced
by rapid cooling from the Austenite phase
It mainly occurs below 300 °C*
The atomic structures of plain carbon steels have been briefly
identified and explained in this lecture*
To summarize the effect of increasing the hardness of steels by
thermal treatment, it can be said that the formation of Martensite
is caused by the entrapment of carbon in solution, produced
by rapid cooling from temperatures above the Upper Critical*
In plain carbon steels there must be sufficient carbon to trap.
In low alloy steels however, the alloying elements play a
significant part in the thermal hardening of steels*
Microstructure:
H² HAZ & weld metal cracking
a. HAZ (Longitudinal)
b. Weld metal (Transverse)
a. All hardenable steels including:
b. HSLA steels
c. Quench & Tempered steels
Martensite*
Occurs when:
Hydrogen
Hardness
Stress
Temperature
is above 15 ml/100 gm weld metal
is above 350 VPN
is greater than 0.5 of the yield stress
is below 300 ºC*
Crack type:
Location:
Steel types:
Cellulosic electrodes produce
hydrogen as a shielding gas
Hydrogen produced from
oil, or paint on plate
H2
Martensite forms from γ
Hydrogen absorbed in a
long, or unstable arc
Hydrogen crack
H2
H2 diffuses to γ in HAZ*
HSLA or Micro-Alloyed Steels are high strength steels that
derive their high strength from finite alloying*
Typically the level of alloying is in the region of 0.05% and
elements such as vanadium molybdenum and titanium. are
used. It would be impossible to match this micro alloying in
the electrode due to the effect of losses across an electric arc*
It is however important to match the strength of the weld to the
strength of the plate, and so a simple way of matching weld
strength must be found and utilised*
To find a simple method we would need to look at the effect of
increasing carbon content on the properties of iron*
Increasing the carbon content will increase the strength, but will
also increase greatly the formation of martensite in the weld.
This may now produce H2 Cracks across weld*
0.83 % Carbon (Eutectoid)*
Tensile Strength
Hardness
Ductility
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.4 1.6 % Carbon
H2 HAZ Cracks
in Alloy steels*
Transverse Weld
Cracks in HSLA Steels*
Low ductility weld metal
Longitudinal contractional strain
1) Maintain calculated preheats, and never allow the interpass temperature to go below the pre-heat value*
2) Use Low Hydrogen processes with short arcs & ensure
consumables are correctly baked & stored as required*
3) If using a cellulosic E 6010 for the root run, insert the
“Hot pass” as soon as possible. (Before HAZ < 300 °C)*
4) Remove any paint, oil or moisture from the plate or pipe*
5) Carry out any specified PWHT as soon as possible*
6) Avoid any restraint, and use high ductility weld metal*
Crack type:
Location:
Steel types:
Microstructure:
Solidification cracking
Weld centre (longitudinal)
High sulphur & phosphorus steels.
Columnar grains
In direction of solidification*
Occurs when:
Liquid iron sulphides are formed around solidifying
grains. High contractional strains are present
High dilution processes are being used.
There is a high carbon content in the weld metal*
1) The first steps in eliminating this problem would be to
choose a low dilution process, and change the joint design*
2) Grind and seal in any lamination and avoid further dilution*
3) Add Manganese to the electrode to form spherical Mn/S
which form between the grain and maintain grain cohesion*
4) As carbon increases the Mn/S ratio required increases
exponentially and is a major factor. Carbon content % should
be a minimised by careful control in electrode and dilution*
5) Limit the heat input, hence low contraction, & minimise
restraint*
Liquid Iron Sulphide films
Solidification crack
*
Contractional strain
Add Manganese to weld metal *
Spherical Mn Sulphide balls
form between solidified grains
Cohesion and strength
between grains remains
Contractional strain
Crack type:
Location:
Steel types:
Microstructure:
Lamellar tearing
Below weld HAZ
High sulphur & phosphorous steels
Lamination & Segregation*
Occurs when:
High contractional strains are through the short
transverse direction. There is a high sulfur content in the
base metal.
There is low through thickness ductility in the base metal.
There is high restraint on the work*
Restraint
Lamellar tear
High contractional strains
Plate to be tested*
Through thickness
tensile test*
Contractional strain*
Full fusion compound
welded cruciform joint*
Machined
test piece*
The test piece is machined from the cruciform joint and
placed under tension. If Lamellar tearing was present it
would fail at a low value*
Assessment of susceptibility to Lamellar Tearing:
•
Carry out through thickness tensile test
•
Carry out cruciform welded tensile test
•
Carry out Ultra-sonic testing
•
Carry out penetrant testing of plate edges
•
Carry out full chemical analysis (S < 0.05%)*
Methods of avoiding Lamellar Tearing:*
1)
Avoid restraint*
2)
Use controlled low sulfur plate *
3)
Grind out surface and butter *
4)
Change joint design *
5)
Use a forged T piece (Critical Applications)*
Re-design weld*
Grind and infill with
ductile weld metal*
Control restraint*
For critical work a forged
T piece may be used*
Forged T Piece
Crack type:
Inter-granular corrosion
Location:
Weld HAZ. (longitudinal)
Steel types:
Stainless steels
Microstructure:
Occurs when:
Sensitised grain boundaries*
An area in the HAZ has been sensitised by the formation
of chromium carbides. This area is in the form of a line
running parallel to and on both sides of the weld.
This depletion of chromium will leave the effected grains
low in chromium oxide which is what produces the
corrosion resisting effect of stainless steels.
If left untreated corrosion and failure will be rapid*
During the welding of stainless steels, a small grain area in the
HAZ, parallel to the weld will form chromium carbide at the
grain boundaries. This depletes this grain of the corrosion
resisting chrome oxide
We say that the steel has become “Sensitised” or has become
sensitive to corrosion*
1) Use Stabilised Stainless Steels*
2) Use Low Carbon Stainless Steels ( Below .04%)*
3) A sensitised Stainless Steel may be de-sensitised by
heating it to above 1100 °C where the Chrome carbide
will be dissolved. The steel is normally quenched from
this temperature to stop re-association*
WIS 5 E Only
Attempt the end of course Multi Choice question paper
answering 30 questions
Time allowed 30 minutes
Attempt the specific question paper answering 4 from 6
questions
Time allowed 1 hour
WIS 5 Only
Practice observing & reporting using the pipes or
plates and forms provided
Use your nominated pipe code for all your visual
inspections.
Practice this up and till the end of the day, or as
directed by your course lecturer*