Course Content Index
RESPONSIBLE WELDING CO-ORDINATOR
Course Notes
Contents:
1. CE Marking Explanation
2. Brief introduction into basic metallurgy
3. Welding principles and processes (MAG/MMA only)
4. Consumables and gas selection (MAG/MMA only)
5. Pre-heat applications and calculating the requirements for the use of
6. Procedure (WPQR / WPS / Welder Qualification in accordance with EN 15614-1 + EN
9606 – 1
7. Components of welds and weld symbols
8. NDT overview and destructive testing to meet the requirements of EN 15614-1 + EN
9606-1
9. Visual inspection of welds and defect type identification including hold time
requirements
10. Working in accordance with acceptance / rejection criteria (NSSS 6th edition - CE
Marked) and BS EN ISO 5817:2014
11. Question and answer session
12. Examination: 50 multiple choice question paper
4 brief narrative questions to be answered from a choice of 7 questions
CE MARKING EXPLANATION
ISO 3834, Quality Requirements for Fusion Welding of Metallic Materials, is a specification that was
first published as an EN specification, EN 729, almost 20 years ago, becoming an ISO specification in
2005. It spells out in Parts 2, 3 and 4 what is regarded as best practice with regard to the control of
welding and its associated activities. Not being a mandatory specification, it has, to a large extent, been
ignored by welding fabricators who have adopted the attitude that they will implement the requirement
when they have to. That point has now been reached for many companies following the publication of
the Construction Products Regulations (CPR) and a number of related specifications that reference ISO
3834 and will therefore directly affect the structural steel industry.
The CE Marking of construction products became mandatory in the summer of 2014 at which time
fabricators must be able to demonstrate compliance with BS EN 1090, Execution of Steel Structures
and Aluminium Structures, the harmonised standard for construction products.
The CPR requires that the manufacturer implements a Factory Production Control (FPC) system to
ensure that products comply with the design and service criteria by means of written procedures and
inspections and tests. BS EN 1090 Part 1, clause 6.3, which states that an FPC system conforming to
EN ISO 9001 and made specific to the requirements of BS EN 1090 is regarded as acceptable.
Welding, however, is identified in ISO 9001 as a ‘special process’ and therefore additional controls are
required to ensure that welding and its related activities are competently managed – compliance with
the relevant part of ISO 3834 satisfies this requirement and is therefore specified in BS EN 1090. The
CPR also requires that the FPC system is accredited by a Notified Body (NB), an NB being an
independent third party approved by the government in the UK via the UK Accreditation Services
(UKAS).
BS EN 1090 Part 2 – Steels – divides construction products into four Execution Classes (EXC). EXC1
includes unwelded items, welded items not subject to dynamic loading and items in steels with a
specified minimum yield strength below 355 MPa. EXC2, 3 and 4 are for increasingly onerous service
conditions and for all steels of S355 grade and above. Manufacturers working to EXC2 with ISO 3834
Part 3, standard Quality Requirements and to EXC classes 3 and 4 with ISO 3834 Part 2
Comprehensive Quality Requirements. Because of the requirement with respect to S355 steels, it is
likely that most fabricators will need to comply with ISO 3834 Part 3 as a minimum.
Brief Introduction into Basic Metallurgy and Weldability of Carbon Steels
Weldability is defined as the capacity to be welded under fabrication conditions, into a suitability
designed structure and to be acceptable in its intended service. The Weldability of a given joint type is
often regarded as ‘The ease of making a default-free weld’ but achieving acceptable properties such as
strength, toughness, hardness and in the case of certain materials – corrosion resistance – should not
be taken for granted or over-looked.
The properties of a given material are dependent upon their microstructure, which is derived mainly
from its composition, and to the degree of mechanical working the material has received i.e. hot or cold
drawn, rolling or extrusion etc. The Weldability of carbon steel is dependent upon its carbon and
manganese content, primarily the increase of the amount of these two elements gives a result in
increased mechanical properties (strength-hardness). This also increases the susceptibility to crack
mechanisms, often regarded to as ‘hardenability’. This course will only give consideration to materials
with a ‘carbon equivalent value’ of no greater than 0.45%, this figure is derived from the following
equation:
𝐶𝐸𝑉 = 𝐶 +
𝑀𝑛
6
+
𝐶𝑅+𝑀𝑜+𝑉
5
+
𝑁𝑖+𝐶𝑢
15
The first bit of good news is that for low to medium carbon content steels, only the first part of the
equation is required, i.e.
𝐶𝐸𝑉 = 𝐶 +
𝑀𝑛
6
The second bit is that all this information should be stated in your material certification within the
‘LADLE ANALYSIS’ section of the certificate. A CEV of approximately 0.4% would mean good
weldability; CEV’s above 0.52% would give poor weldability. The carbon equivalent value can be looked
upon as parameter representing the risk of ‘Hydrogen Induced Cold Cracking’.
Steel and Weld Composition
For plain carbon steels it is the carbon content that has the greatest effect on weldability and properties.
For structural applications, the most important properties of steels are their yield strength, tensile
strength and toughness (often indicated by the Charpy impact energy). Variation of these properties
with carbon content is given in the figure.
Manganese is one of the most important elements that affect weldability of plain carbon steel. It acts as
a deoxidiser and reduces harmful effect of sulphur by forming MnS, and it improves strength principally
as a solid solution alloy.
Even small amounts of nickel have a profound effect on fracture toughness, since nickel improves the
low temperature cleavage resistance of ferrite. Cryogenic steels (which may have high toughness at
temperatures down to -196°C) contain between 2.5 and 9% nickel. However, note that NACE limits
nickel to 1% for offshore applications (because levels higher than this are considered detrimental in
sour conditions).
Undesirable elements in a steel’s composition include residual elements such as sulphur and
phosphorous. When the amount of sulphur in steel rises from 0.001% up to 0.01% the Charpy impact
energy at a given temperature can drop from 250J to 50J. Residual elements can increase the risk of
solidification cracking as well as cause low toughness, and sulphur is generally limited to 0.05% in
steels (often much less in clean steels). Additions of manganese tie up the sulphur as MnS inclusions
and can help improve toughness and resistance to solidification cracking.
High nitrogen in the parent metal is undesirable as it can cause strain age embrittlement. This is
particularly a problem for root passes with high parent metal dilution. Additions of Al or Ti tie up the
nitrogen and can avoid this problem.
Grain Size
A large grain size results in low notch toughness. This is true both in the HAZ where grain growth can
occur and in the weld metal where the as-cast microstructure can give a large effective grain size. Grain
refinement in weld metal can be achieved through nucleation of small grains on fine oxide or nitride
particles to form ‘acicular ferrite’. For MMA or SAW, using basic flux promotes this microstructural
constituent in the weld metal, which has a very small effective grain size and is a high toughness
microstructure. Small additions of nickel can also promote acicular ferrite.
Multi-pass welds have a smaller grain size than equivalent single pass welds due to the low heat input
per pass, meaning there is limited grain growth. One benefit of multi-pass welding is that the reheat
thermal cycle of subsequent passes normalises and refines parts of the microstructure in the previous
weld metal. This can give improved toughness. The tempering effect of subsequent welds can result in
reduced residual stresses and gives a preheat effect for subsequent passes. This and short diffusion
distances helps to reduce hydrogen levels. Also less dilution means there is better control of weld
composition. But, there is lower productivity from larger number of passes, and sometimes a risk of
hard HAZ from the low heat input.
Heat Input
Limiting the heat input during welding helps to limit HAZ grain growth, and hence improve toughness.
This also gives some weld metal grain size control. However, if the heat input is too low it is possible to
get hard phases such as bainite or martensite in the HAZ due to a fast cooling rate. These are prone to
cracking and also have low toughness. The heat input therefore needs to be carefully controlled.
Cracking in C-Mn Steels Welds
Carbon-manganese steels are easily weldable; however, they are susceptible to a few types of cracking
including hydrogen cracking, solidification cracking and lamella tearing. However, lamellar tearing is
less important for most modern steel compositions.
Hydrogen Cracking
Hardenable (high carbon equivalent) steels are susceptible to hydrogen cracking since they form hard
and brittle martensite in the weld and HAZ. Medium and low carbon and manganese steels are less
susceptible to hydrogen cracking due to their low tendency to harden during welding. For hydrogen
cracking to be a risk, there are four contributing factors:
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A susceptible microstructure e.g. martensite in the HAZ
Presence of hydrogen, which could be from moisture, lubricants, oil, grease. This contamination
can be from the welding consumables or the parent metal
Tensile stresses which are inevitable from welding residual stresses
Temperature below 150°C
To avoid hydrogen cracking use a low hydrogen welding process and consumables, apply preheat and
maintain a minimum interpass temperature along with weld heat input control. The effect of weld bead
tempering in multi-pass welding can help avoid this cracking, and restraint and stress concentrations
should be minimised. Post heating (for hydrogen release) in beneficial, as is the use of alternative filler
materials such as nickel based or austenitic stainless steel which have a high solubility for hydrogen,
but these measures should not be necessary for C-Mn steels.
Rolling and Heat Treatment
Hot and Cold Rolling
All metals are crystalline, which means they consist of numerous small grains or crystals which can be
seen easily when a sample that has been polished and etched in weak acid is examined using an
optical microscope.
During shaping by rolling the grains are deformed (flattened) as the piece of steel is elongated. In cold
rolling, any deformation of the crystal structure is permanent, as shown in Figure 2.1 (a). This limits the
amount of deformation that can be achieved without further heat treatment of the steel. In hot rolling,
however when the temperature is high enough (i.e. when it is above the recrystallization temperature)
the deformed grains are replaced by new grains, Figure 2.1 (b). This enables much greater deformation
than can be achieved with cold rolling and the simultaneous deformation and recrystallization is a
characteristic feature of hot rolling. Rolling at higher temperatures also consumes less energy than cold
working and so hot working at temperatures above 800°C is normally used to achieve most of the
required shape change in steel sections.
Principles of Rolling
Essentially, rolling consists of passing a steel workpiece between two rolls rotating in opposite
directions as illustrated in Figure 2.1. The rolls are driven at the same peripheral speed and the gap
between them is less than the height of the workpiece entering them. The metal enters the rolls at a
speed that is lower than the peripheral roll speed and emerges at a higher speed. Most of the
deformation takes place in the thickness direction, although there is some increase in the width, but the
principal dimensional change is an increase in length of the workpiece.
CRACKING IN WELDED JOINTS
HYDROGEN (COLD) CRACKING
Hydrogen dissolved in the steel crystal lattice causes embrittlement and cracking under stress at
relatively low temperatures. There are four factors that affect the likelihood of cold cracking – critical
levels for all four factors in combination need to be reached for hydrogen cracking to occur;
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Hydrogen level
Hardness
Stress
Temperature below 150°C
Steel Supply Conditions
The different supply conditions for structural steel are, typically, as-rolled, normalised, normalised rolled
and thermomechanically rolled. The different processes and benefits of each are briefly explained
below. The aim of all these processes is to produce higher toughness and strength properties through
grain refinement and at low Carbon Equivalent Value (CEV) levels to maintain weldability. However,
most of the standard rolled sections supplied for structural steelwork as in the as-rolled condition:
i)
ii)
iii)
Normalising: The steel is reheated to approximately 900°C after rolling and then air cooled.
Properties are controlled by the composition and plate thickness and additions of niobium,
vanadium and aluminium as single or combined additions are used to refine the structure
and improve toughness.
Normalising Rolling: The rolling process itself is controlled to produce a refined structure
during the hot working operation. This is achieved by a combination of restricting the slab
reheating temperature prior to rolling and / or introducing increased amount of deformation
at lower temperatures. The final material condition is equivalent to that obtained after
normalising.
Thermomechanical Rolling: Further modification to the rolling process involving more
deformation at lower temperatures, producing increased strength and toughness. The
strength of thermomechanically rolled steel cannot be achieved or repeated by heat
treatment alone.
Welding Principles
Welding is used to connect items in many combinations i.e. materials, configuration, methods and joint
types. A high importance is placed on the integrity of the deposited weld, as if performed incorrectly,
failure may possibly occur, which in some instances could have disastrous or fatal results! For this
reason, in many industries welding is deemed ‘a specialist process’ and is heavily scrutinised.
Arc Welding
Could be described as ‘a process that creates an arc between the work piece and the consumable filler
material’, which in turns melts the parent material to create the weld pool which in the case of Carbon
Steel would have an approximate temperature of 2600°C to 3000°C, with the actual arc temperature
close to 6000°C. The arc is monitored primarily by the arc length (parent material to end of the
consumable) using the MMA process both polarities can be used (AC and DC), with MAG using DC
polarity only. Handheld process uses a current ranging from 50 – 350 amps approximate. Not all energy
from the arc is absorbed into the parent plate, and the terms ‘Arc Energy’ and ‘Heat Input’, will be later
covered in greater detail in the pre-heat and their applications section of the course notes. Atmospheric
contamination prevention is covered in the ‘welding processes’ section of the notes for the MAG and
MMA process.
Known is the USA as gas metal arc welding (GMAW). The MIG/MAG welding process is a versatile
technique suitable for both thin sheet and thick section components in most metallic materials. In the
process, an arc is struck between the end of a wire electrode and the workpiece, melting both to form a
weld pool. The wire serves as the source of heat (via the arc at the wire tip) and filler metal for the joint.
The wire is fed through a copper contact tube (also called a contact tip) which conducts welding current
into the wire. The weld pool is protected from the surrounding atmosphere by a shielding gas fed
through a nozzle application. The wire is fed from a reel by a motor drive, and the welder or machine
moves the welding gun or torch along the joint line. The process offers high productivity and is
economical because the consumable wire is continuously fed. A diagram of the process is shown in
Figure 1.
The MIG/MAG process uses semiautomatic, mechanised, or automatic equipment. In semiautomatic
welding, the wire feed rate and arc length are controlled automatically, but the travel speed and wire
position are under manual control. In mechanised welding, all parameters are under automatic control,
but can be varied manually during welding, e.g. steering of the welding head and adjustment of wire
feed speed and arc voltage. With automatic equipment, there is no manual intervention during welding.
Figure 1.1 shows equipment required for the MIG/MAG process.
Advantages of the MIG/MAG process:
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Continuous wire feed
Automatic self-regulation of the arc length
High deposition rate and minimal number of stop/start locations
High consumable efficiency
Heat inputs in the range 0.1 – 2.0kJ/mm
Low hydrogen potential process
Welder has good visibility of weld pool and joint line
Little or no post weld cleaning
Can be used in all positions (dip transfer)
Good process control possibilities
Wide range of application
Disadvantages:
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No independent control of filler addition
Difficult to set up optimum parameters to minimise spatter levels
Risk of lack of fusion when using dip transfer on thicker weldments
High level of equipment maintenance
Lower heat input can lead to high hardness values
Higher equipment cost than MMA (manual metal arc) welding
Site welding requires special precautions to exclude draughts which may disturb the gas shield
Joint and part access is not as good as MMA or TIG welding
Cleanliness of base metal slag processes can tolerate greater contamination
Process Variables
The primary variables in MIG/MAG welding are:
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Welding current/wire feed speed
Voltage
Gases
Travel speed and electrode orientation
Inductance
Contact tip to work distance
Nozzle to work distance
Shielding gas nozzle
Type of metal transfer
Welding current / wire feed speed
On MIG/MAG welding sets there is no control to set the welding current. The electrical characteristics of
the welding set (flat or constant voltage type) automatically alters the welding current with changes to
the set wire feed speed to achieve a constant arc length.
Increasing the wire feed, and therefore current, increases wire burn-off, deposition rate and penetration.
Current type is almost always DC+ve, although some cored wires require DC-ve for best results.
Voltage
This is set to achieve steady smooth welding conditions and is generally increased as the wire feed
speed is increased.
Increase in voltage increases the width of the weld and reduces penetration.
Travel speed and electrode orientation
The faster the travel speed the less penetration, narrower bead width and the higher risk of undercut.
Dip Transfer
Key characteristics:
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Metal transfer by wire dipping or short circuiting into the weld pool
Relatively low heat input process
Low weld pool fluidity
Used for thin sheet metal above 0.8 and typically less than 3.2mm, positional welding of thicker
section and root runs in open butt joints
Process stability and spatter can be a problem if poorly tuned
Lack of fusion risk if poorly set up and applied
Not used for non-ferrous metals and alloys
In dip transfer the wire short-circuits the arc between 50-200 times/sec. This type of transfer is normally
achieved with CO2 or mixtures of CO2 and argon gas + low amps and welding volts < 24V.
Spray transfer:
Key characteristics:
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Free-flight metal transfer
High heat input
High deposition rate
Smooth, stable arc
Used on steels above 6mm thickness and aluminium alloys above 3mm thickness
Spray transfer occurs at high currents and high voltages. Above the transition current, metal transfer is
in the form of a fine spray of small droplets, which are projected across the arc with low spatter levels.
The high welding current produces strong electromagnetic forces (known as ‘the pinch effect’ that
cause the molten filament supporting the droplet to neck down). The droplets detach from the tip of the
wire and accelerate across the arc gap.
With steels it can be used only in down-hand butts and HN fillet welds, but gives significantly higher
deposition rate, penetration and fusion than the dip transfer mode. With aluminium alloys, it can be
used in all positions.
MIG/MAG Filler Wires
Solid filler wires manufactured for MIG/MAG generally have chemical compositions that have been
formulated for particular base materials and the wires have compositions similar to these base
materials. Solid wires for welding steels with active shielding gases are deoxidised with manganese and
silicon to avoid porosity. There may also be titanium and aluminium additions. Mild steel filler wires are
available with different levels of deoxidants, known as double or triple de-oxidised wires. More highly
deoxidised wires are more expensive but are more tolerant of the plate surface condition, e.g. mill
scale, surface rust, oil, paint and dust. There may, therefore, be a reduction in the amount of cleaning of
the steel before welding.
These deoxidiser additions yield a small amount of glassy slag on the surface of the weld deposit,
commonly referred to as silica deposits. These small pockets of slag are easily removed with light
brushing; but when galvanising or painting after welding, it is necessary to use shot blasting. During
welding, it is common practice to weld over these small islands since they do not represent a thick slag,
and they usually spall off during the contraction of the weld bead. However, when multipass welding,
the slag level may build up to an unacceptable level causing weld defects and unreliable arc starting.
Steel wires usually have a flash coating of copper to improve current pick-up and to extend the shelf life
of the wire. However, the copper coating can sometimes flake off and be drawn into the liner and wire
feed mechanism, particularly if there is a misalignment in the wire feed system. This may cause
clogging and erratic wire feed. Uncoated wires are available as an alternative, although electrical
contact may not be as good as with copper coated wires, and contact tip operating temperatures may
be higher.
Some typical Standards for specification of steel wire consumables are:
EN 14341: Welding consumables – Wire electrodes and deposits for gas shielded metal arc welding of
non-alloy and fine grain steels – Classification.
Wire sizes are typically in the range 0.8 – 1.2mm diameter
Spools should be labelled to show the classification of the wire and its diameter
MIG/MAG gas shielding
For non-ferrous metals and their alloys (such as Al, Ni and CU) an inert shielding gas must be used.
This is usually either pure argon or an argon rich gas with a helium addition.
The use of a fully inert gas is the reason why the process is also called MIG welding (metal inert gas)
and for precise use of terminology this name should only be used when referring to the welding of nonferrous metals.
The addition of some CO2 to argon gives a more uniform heat concentration within the arc plasma and
this affects the shape of the weld bead profile.
Argon-CO2 mixtures effectively give a hotter arc and so they are beneficial for welding thicker base
materials those with higher thermal conductivity e.g. copper or aluminium.
For welding of steels – all grades, including stainless steels – there needs to be a controlled addition of
oxygen or carbon dioxide in order to generate a stable arc and give good droplet wetting. Because
these additions react with the molten metal they are referred to as active gases and hence the name
MAG welding (metal active gas) is the technical term that is used when referring to the welding of
steels.
The percentage of carbon dioxide (CO2) or oxygen depends on the type of steel being welded and the
mode of metal transfer being used.
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Argon + 5 to 25% CO
Widely used for carbon and some low alloy steels (and FCAW of stainless steels)
Manual Metal-Arc/Shielded Metal Arc Welding (MMA/SMAW)
The most versatile of the welding processes, manual metal arc (MMA) welding is suitable for welding
most ferrous and non-ferrous metals, over a wide range of thicknesses. The MMA welding process can
be used in all positions, with reasonable ease of use and relatively economically. The final weld quality
is primarily dependent on the skill of the welder.
When an arc is stuck between the coated electrode and the workpiece, both the electrode and
workpiece surface melt to form a weld pool. The average temperature of the arc is approximately
6000°C, which is sufficient to simultaneously melt the parent metal, consumable core wire and the flux
coating. The flux forms gas and slag, which protects the weld pool from oxygen and nitrogen in the
surrounding atmosphere. The molten slag solidifies and cools and must be chipped off the weld bead
once the weld run is complete (or before the next weld pass is deposited). The process allows only
short lengths of weld to be produced before a new electrode needs to be inserted in the holder.
Power Requirements
Manual metal arc welding can be carried out using either direct (DC) or alternating (AC) current. With
DC welding current either positive (+ve) or negative (-ve) polarity can be used, so current is flowing in
one direction. AC welding current flows from negative to positive and is two directional.
Power sources for MMA welding are transformers (which transforms mains AC to AC suitable for
welding), transformer-rectifiers (which rectifies AC to DC), diesel or petrol driven generators (preferred
for site work) or inverters (a more recent addition to welding power sources). For MMA welding a power
source with a constant current (drooping) output characteristic must be used.
The power source must provide:
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An open circuit voltage (OCV) to initiate the arc, between 50 and 90V.
Welding voltage to maintain the arc during welding, between 20 and 30V.
A suitable current range, typically 30 – 350A.
A stable arc. Rapid arc recovery or arc re-ignition without current surge.
A constant welding current. The arc length may change during welding, but consistent electrode
burn-off rate and weld penetration characteristics must be maintained during welding.
Welding variables
Other factors or welding variables which affect the final quality of the MMA weld are:
Current (amperage)
Voltage
Affects heat input
Travel Speed
Polarity
Type of electrode
Current (amperage)
Amperage controls burn-off rate and depth of penetration. Welding current level is determined by the
size of the electrode and the welding position manufacturers recommend the normal operating range
and current.
Incorrect amperage settings when using MMA can contribute to the following:
Amperage too low:
Poor fusion or penetration, irregular weld bead shape, slag inclusion,
unstable arc, porosity, potential arc strikes, difficult starting.
Amperage too high:
Excessive penetration, burn-through, undercut, spatter, porosity, deep
craters, electrode damage due to overheating, high deposition make
positional welding difficult.
Voltage
Open circuit voltage (OCV) is the voltage measured between the output terminals of the power source
when no current is flowing through the welding circuit.
For safety reason this should not exceed 100V, and is usually between 50-90V.
Arc voltage is the voltage required to maintain the arc during welding and is usually between 20-30V.
As arc voltage is a function of arc length the welder controls the arc length and therefore the arc
voltage.
Arc voltage controls weld pool fluidity.
The effects of having the wrong arc voltage can be:
Arc voltage too low:
Poor penetration, electrode sticking, lack of fusion defects, potential for
arc strikes, slag inclusion, unstable arc condition, irregular weld bead
shape.
Arc voltage too high:
Excessive spatter porosity, arc wander, irregular weld bead shape, slag
inclusions, fluid weld pool making positional welding difficult.
Travel Speed
Travel speed is related to whether the welding is progressed by stringer beads or by weaving. Often the
run out length (ROL) i.e. the length of the deposit from one standard electrode is quoted on procedures
rather than speed as it is easier for the welder to visualise.
Travel speed too fast:
Narrow thin weld bead, fast cooling, slag inclusions, undercut, poor
fusion/penetration.
Travel speed too slow:
Cold lap, excess weld deposition, irregular bead shape, undercut.
Covered electrode manufacture
Electrode manufacturers produce electrodes by:
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Straightening and cutting core wire to standard lengths (typically 300, 350 and 450mm
depending on electrode classification and diameter).
Making a dry mix of powdered compounds/minerals (precise levels of additions depend on
individual manufacturers’ formulations).
Making a wet mix by adding the dry powders to a liquid binder.
Extruding the covering (concentrically) on to the core wire.
Hardening the covering by drying the electrodes.
Carrying out batch tests – as required for electrode certification.
Packing the electrodes into suitable containers.
For low hydrogen electrodes this is a high temperature bake - ≥~450°C.
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Vacuum packed electrodes are packed in small quantities into packaging that is immediately
vacuum sealed – to ensure no moisture pick-up.
Electrodes that need to be re-baked are packed into standard packets as this may be some time
after baking and the packaging may not be sealed, they do not reach the end-user in a
guaranteed low hydrogen condition, they therefore require re-baking at a typical temperature of
350°C for approximately 2 hours. Note! You should always follow the manufacturer’s
recommendations.
For individual batch certification this will require the manufacture of a test pad for chemical analysis and
may require manufacture of a test weld from which a tensile test and Charpy V notch test pieces are
tested.
Electrode coverings
Core wires used for most C-Mn electrodes, and some low alloy steel electrodes, are a very low C steel*
and it is the formulation of the covering that determines the composition of the deposited weld metal
and the operating characteristics of the electrode (*typically ~0.06%C, ~0.5%Mn)
The flux covering on an electrode is formulated to aid the manufacturing process and to provide a
number of functions during welding.
The major welding functions are:
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Facilitate arc ignition/re-ignition and give arc stabilisation.
Generate gas for shielding the arc and molten metal from contamination by air.
Interact with the molten weld metal to give de-oxidation and flux impurities into the slag to
cleanse/refine the molten weld metal.
Form a slag for protection of the hot weld metal from air contamination.
Provide elements to give the weld metal the required mechanical properties.
Enable positional welding by means of slag formers that freeze at temperatures above the
solidification temperature range of the weld metal.
Type of current and polarity
Polarity will determine the distribution of heat energy at the welding arc. The preferred polarity of the
MA system depends primarily upon the electrode being used and the desired properties of the weld.
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Direct Current. Electrode positive (DCEP/DC+)
Usually produces the greatest penetration but with lesser deposition rate. Known in some
standards as reverse polarity.
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Direct Current. Electrode negative (DCEN/DC-)
Usually produces less penetration with greater deposition rate. Known in some standards as
straight polarity.
When using direct current the arc can be affected by arc blow. The deflection of the arc from its normal
path due to magnetic forces.
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Alternating current (AC)
The distribution of heat energy at the arc is equal.
Rutile Electrodes
Rutile is a mineral that consists of about 90% titanium dioxide (TiO 2) and is present in C and C-Mn steel
rutile electrodes at typically ~50%.
Characteristics of rutile electrodes are:
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They have a very smooth and stable arc and produce a relatively thin slag covering that is easy
to remove.
They give a smooth weld profile.
They are regarded as the most user-friendly of the various electrode types.
They have relatively high combined moisture content and because they contain typically up to
~10% cellulose they cannot be baked and consequently they do not give a low H weld deposit.
Because of the risk of cracking, they are not designed for welding of high strength or thick
section steel (although electrodes and manufactured in classes E60xx, E70xx, E80xx, the
E60xx grade is by far the most commonly used).
They do not give high toughness at low temperatures (typically only down to about -20°C)
The above listed characteristics mean that this type of electrode is used for general-purpose fabrication
of unalloyed, low strength steels in relatively thin sections (typically ≥ - 13mm).
Rutile electrode variants
By adding iron powder to the covering a range of thick-coated electrodes have been produced in order
to enhance productivity.
Such electrodes give weld deposits that weight between ~135 and 190% of their core wire weight and
so referred to as high recovery electrodes, or more specifically for example, a 170% recovery electrode.
The weld deposit from such electrodes can be relatively large and fluid and this restricts welding to the
flat position and for standing fillets for electrodes with the highest recovery rates.
In all other respects, these electrodes have the characteristics listed for standard rutile electrodes.
Basic Electrodes
Basic electrodes are so named because the covering is made with a high proportion of basic
minerals/compounds (alkaline compounds), such as calcium carbonate (CaCO3), magnesium
carbonate (MgCO3) and calcium fluoride (CaF2).
A fully basic electrode covering will be made up with about 60% of these basic minerals/compounds.
Characteristics of basic electrodes are:
•
•
•
•
•
•
The basic slag that forms when the covering melts reacts with impurities, such as sulphur and
phosphorus, and also reduces the oxygen content of the weld metal by de-oxidation.
The relatively clean weld metal that is deposited gives a very significant improvement in weld
metal toughness (C-Mn electrodes with Ni additions can give good toughness down to -90°C)
They can be baked at relatively high temperatures without any of the compounds present in the
covering being destroyed, thereby giving low moisture content in the covering and low hydrogen
levels in weld metal.
In order to maintain the electrodes in a low hydrogen condition, they need to be protected from
moisture pick-up.
o By means of baking use (typically at ~350°C), transferring to a holding oven (typically at
~120°C) and issued in small quantities and/or using heated quivers (‘portable ovens’) at
the work station (typically ~70°C).
o By use of vacuum packed electrodes that do not need to be re-baked before use.
Basic slag is relatively viscous and thick, which means that electrode manipulation requires
more skill and should be used with a short arc to minimise the risk of porosity.
The surface profile of the weld deposits from basic electrodes tends to be convex and slag
removal requires more effort.
Metal powder electrodes contain an addition of metal powder to the flux coating to increase the
maximum permissible welding current level. Thus, for a given electrode size, the metal deposition rate
and efficiency (percentage of the metal deposited) are increased compared with an electrode
containing no iron powder in the coating. The slag is normally easily removed. Iron powder electrodes
are mainly used in the flat and H/V positions to take advantage of higher deposition rates. Efficiencies
as high as 130 to 140% can be achieved for rutile and basic electrodes without marked deterioration of
the arcing characteristics but the arc tends to be less forceful which reduces bead penetration.
Applications of basic electrodes
Basic electrodes have to be used for all applications that require good fracture toughness at
temperatures below ~ -20°C.
To avoid the risk of hydrogen cracking basic electrodes have to be used for welding hardenable steels
(most C-Mn and all low alloy steels) and for most steels when the joint thickness is greater than about
15mm.
The Effect of Hydrogen in Steel
• Hydrogen causes general embrittlement.
• The existence of an excessive amount
of hydrogen plus other factors can lead
to cracking.
• Hydrogen is a diatomic element, i.e. it
prefers to exist in its molecular state.
• Hydrogen is the smallest atom known to
man.
• Four criteria need to exist before
hydrogen cracking will occur in steel:
o Hydrogen in sufficient amounts
o It’s A grain structure susceptible to
cracking
o Stress
o A temperature below 150°C.
The Effect of Hydrogen in Steel
• Steel in the expanded condition during.
• Hydrogen enters the weld via the arc
during welding. Hydrogen will break up
into its atomic state above
approximately 150°C.
• At temperatures below approximately
150°C the steel contracts leaving small
gaps between the atoms within the
steel. The hydrogen atoms collect in
these gaps and revert to a molecular
state.
• When the hydrogen collect in large
amounts great pressure may be
exerted.
Factors necessary for
HICC
Grain structure susceptible to cracking – e.g.
carbon content, alloy content (ceq%), speed of
quench.
H2 – e.g. from
moisture or
contamination on
the preparation
or in the flux/gas.
Speed of quench
HICC
will occur
if all are
present in
sufficient
concentration
Stress – e.g. unequal
expansion/contraction,
residual stresses, poor
assembly/fit up, speed
of quench
Temperature 200°C to -100°C
Note: The lower temperature within this range,
the higher the H2 pressure will be
The Purpose of Pre-Heat
1. Reduces the risk of hydrogen cracking
2. Reduces the hardness of the welds heat affected zone
3. Promotes reduced shrinkage stresses during cooling and improves the distribution of residual
stress
Pre-heat for the majority of your applications is determined by calculating the carbon equivalent content
of the material and welding heat input and combined material thickness.
The Above 3 Formulas Explained
Carbon Equivalent Formula
Carbon equivalent content is achieved by using the formula:
𝐶𝐸 = %𝐶 +
%𝑀𝑁 %𝐶𝑟 + %𝑀𝑜 + %𝑉 %𝑁𝑖 + %𝐶𝑢
+
+
6
5
15
The good news is that materials in the BS ENI0025:S275 and S355 range only require the first part of
the formula, as most of the alloying elements are just trace percentages and have very little effect on
the overall equations so we will use the formula:𝐶𝐸 = %𝐶 +
%𝑀𝑛
6
All this information can be found on the mill certificate for the material, which should state the ladyle
analysis percentages of carbon and manganese.
Heat Input
Heat input is the energy supplied by the welding arc to the work piece and is expressed in terms of arc
energy X thermal efficiency factor.
Arc Energy (kj/mm) = Arc Voltage X Welding Current
Welding Speed (mm/sec) x 1000
Now you have determined the arc energy value. To gain the heat input you must multiply the arc energy
by the given K factor (thermal efficiency). For the MAGS/FCAW process this is 0.8, for MMA 0.8 should
also be used.
You now have established your carbon equivalent value and heat input. By using the chart find your Ce
in the left column and your combined thickness on the top column, where the two lines meet, that us
your pre heat temperature.
Combined Thickness Definition
For a butt weld = material thickness x 2
For a fillet weld = material thickness x 3
The temperature required should be a minimum of 75mm either side of the area to be welded as well as
the prep. Temperature readings should be taken from the other side of the material where possible for
accurate readings to take place. Allow the affected area a minimum 3 minutes to stabilize prior to
commencing welding.
Hydrogen Scales
The hydrogen scale to be used for any arc welding process depends principally on the weld diffusible
hydrogen content and should be as given in Table C.2. The value used should be stated by the
consumable manufacturer in accordance with the relevant standard where it exists (or as independently
determined) in conjunction with a specified condition of supply and treatment.
Table C.2 Hydrogen Scale
Diffusable
hydrogen content
Ml/100g of deposited
metal
>
15
10
≤
15
5
≤
10
3
≤
5
≤
3
Hydrogen Scale
A
B
C
D
E
MMA (Depending on
coating/storage)
Solid Wire MAG
Qualification of Procedure Records (WPQR)
When a requirement has arisen to qualify a WPQR, careful consideration should be given to several
important points! Rather than falling into the ‘qualifying a joint procedure’, which may not offer as much
range of approval as could have been achieved. This can lead to additional cost being incurred! Below
is a list of points to be considered.
1. Many welds, joint types and material thickness can be approved by a limited amount of carefully
chosen WPQRs.
2. As a generalisation, a harder to achieve joint type will approve other less difficult to achieve joints
i.e. a simple V Butt Unbacked would approve a Double V Butt for instance.
3. In the case of carbon steels (ISO15608 – Group 1) careful material selection will also approve
other sub-groups in the same material main group i.e. the need to establish WPQRs in a minimum
amount of material groups i.e. S355 grade material will approve S275 but not vice-versa.
4. Many supporting WPS’ can be written ‘in house’ at no additional cost, as long as they are
supported by a WPQR! Creating a WPS will be covered in greater depth later in this chapter, as will
material group and thickness approval.
5. Correct approvals for positions that welds may be deposited, any site welding using the MMA
process for example.
Welder Approval Qualification in Accordance with ISO 9606-1:2017
Both BS EN 1090-2, Technical Requirements for the Execution of Steel Structures and NSSS (6 th
edition / CE marked edition) make welder qualifications to be in accordance with ISO 9606-1:2017
We will study the enclosed welder approval qualification and go into greater detail of each part of the
document one at a time. Particular attention will be given to weld type, material approval (group and
thickness etc.), consumable and shielding gas composition, welding positions and approval validity for
prolongation purposes.
NON DESTRUCTIVE TESTING
This chapter looks at the four mainly used methods of NDT within the structural steel industry. The first
two methods are regarded to as ‘surface flaw detection’ and the other two are ‘volumetric flaw
detection’ methods. For some applications, one method can complement another, on completion of the
course notes, briefly describing each method a practical demonstration of three of the processes and
radiographs will be available to be viewed.
DESTRUCTIVE TESTING
Mechanical testing is a range of destructive test methods performed to establish qualitative and
quantitative values that can be used for designing specific joint types or for qualifying welding
procedure tests. A qualitative test could be described as an ‘accept-reject’ quality type test, where a
quantitative type test can be assessed and measured. The methods that will be covered will be to meet
the requirements of BSEN ISO 15614-1 ‘Welding Procedure Test’ and ISO 9606-1.
Tensile Test
Bend Test (root/cap/side)
Vickers Hardness Survey
Charpy V Notch Impact Test
Macro Section
DYE PENETRANT INSPECTION (DPI)
This type of surface flaw detection method uses the forces of capillary action to detect surface breaking
defects. It is impossible to detect defects which do not break the surface with this method, but it can be
used on both magnetic and non-magnetic materials providing they are non-porous.
There are several types of penetrant systems, this includes the following which are shown in a
descending order of flaw detection sensitivity within the structural steel industry – solvent based / colour
contrast is most widely used.
•
•
•
•
•
Post-emulsifiable – fluorescent
Solvent based – fluorescent
Water based – fluorescent
Solvent based – colour contrast
Water based – colour contrast
Fluorescent penetrants require the use of an ultraviolet (UV-A) light to view indications, whilst colour
contrast penetrants are viewed with the naked eye.
A typical sequence of operations on a steel test item is as follows:
1. Clean area using wire brush, cloth and solvent. On aluminiums, other soft alloys and plastics,
wire brushing should not be used, as there is a danger that surface breaking defects may be
closed.
2. Apply penetrant – leave for 15 minutes. Colour contrast penetrants are normally red in colour
and should remain on the part long enough to be drawn into any surface discontinuities. This
time can vary from about ten minutes to several hours, depending on the type of material and
size/type of defect sought.
3. Remove surface penetrant using cloth and solvent. Apply solvent to the cloth and not directly on
to the workpiece. Clean thoroughly.
4. Apply developer – leave for 15 minutes. The develop draws any penetrant remaining in any
surface breaking discontinuities with a blotting action.
5. Interpret area. Any discontinuities are indicated by a red mark, e.g. line or dot against a white
background. Fluorescent penetrants would show green-yellow when viewed with an ultra-violet
(UV-A) light.
MAGNETIC PARTICLE INSPECTION (MPI)
This method of NDT may detect surface, and in certain cases, slight sub-surface discontinuities up to 23mm below the surface. MPI can be used on ferromagnetic materials only!
A magnetic field is introduced into a specimen to be tested, fine particles of ferromagnetic powder, or
ferromagnetic particles in a liquid suspension, are then applied to the test area. Any discontinuity which
interrupts the magnetic lines of force will create a leakage field, which has a north and south pole on
either side of it. This attracts the ferromagnetic particles in great numbers. The discontinuity may show
as a black indication against the contrasting background – usually white contrast paint or as a
fluorescent indication which is usually green/yellow against a dark violet background.
When MPI is carried out using fluorescent inks the use of an ultraviolet (UV-A) light is necessary to
cause fluorescence of the particles, although there is no need to apply a contrast paint.
Fluorescent ink methods are more sensitive than black ink methods.
There are many ways to apply a magnetic field, e.g. a permanent magnet, coils, prods, cables and
threading bar.
Listed below is a sequence of operations to insect a weld, using a permanent magnet with black ink:
1.
2.
3.
4.
5.
6.
7.
Clean area using wire brush and cloth plus solvent if necessary.
Apply a thin layer of white contrast paint.
When paint is dry, straddle the magnet over the weld.
Apply ink (1.25 to 3.5% particles to a paraffin base).
Interpret area.
To look for transverse defects, turn magnet approximately 90° and re-apply ink.
Interpret area.
RADIOGRAPHIC TESTING
Principles
Radiography is carried out using X-Ray machines or artificial gamma sources (radio-isotopes).
X-rays or gamma rays pass through the object to be radiographed and record an image on a
radiographic film on the opposite side. The radiation reaching the film will be determined by the object’s
thickness and density, e.g. lack of root penetration in a weld will increase the amount of radiation falling
on the film in that area due to a reduction in thickness.
It is the wavelength of the radiation which governs its penetrating power; this is governed by the
kilovoltage (kV) when using x-rays and isotope type with gamma rays. The intensity of the radiation is
governed by the milli-amperage (mA) when using x-rays and by the activity of the specific isotope with
gamma. Activity is measured in Curies (Ci) or gigabecquerels (GBq).
A negative is produced when the film in processed. The thin areas of an object will be darker than the
thicker areas, therefore most weld defects show up dark in relation to the surrounding areas; exceptions
are excess weld metal, spatter, tungsten and copper inclusions.
RADIOGRAPHIC QUALITY
An overall assessment of radiographic quality is made by the use of image quality indicators (IQIs);
these usually consist of seven thin wires decreasing in thickness. IQIs are pre-placed on the weld being
examined and therefore show on the radiographic image. The more wires visible, the better the flaw
detection sensitivity is likely to be.
The density – degree of blackness – of a radiograph is also measured by using a densitometer to
ensure it lies within a specified range for optimum quality.
ADVANTAGES AND DISADVANTAGES
X-radiography requires bulky and expensive machinery in comparison with gamma radiography, but xradiography generally produces better quality radiographs and is safer.
X-ray machines can be switched on and off, unlike gamma sources which permanently produce
radiation and therefore require shielding when not in use.
A major disadvantage with radiography is that it will only detect defects which have significant depth in
relation to the axis of the x-ray beam – roughly over 2% of the wall thickness in the same axis as the xray beam, i.e. radiography will not usually detect plate laminations, lack of inter-run fusion or cracks
perpendicular to the x-ray beam.
A major advantage of radiographic testing is that a permanent record is produced, i.e. the radiograph.
ULTRASONIC TESTING
This method uses the ability of high frequency sound waves, typically above 2 MHz (2,000,000 c.p.s) to
pass through materials.
A probe is used which contains a piezo electric crystal to transmit and receive ultrasonic pulses.
Ultrasound hitting any air interface or an interface with a different material density, which is
perpendicular to the ultrasonic beam is reflected back and displayed on a cathode ray tube (c.r.t.). The
actual display relates to the time taken for the ultrasonic pulses to travel the distance to an interface
and back, i.e. the longer the time, the further away the interface.
An interface could be the opposite side of the plate; therefore, wall thickness measurements can easily
be made.
Lamination checks are easily carried out using ultrasonic methods (opposite to radiography). Welds can
be tested using angle type probes, although this requires more operator skill to apply and interpret
results. Defects in welds usually can be located but the type of defect is sometimes difficult to identify.
To detect a linear defect with radiography, the defects must have depth in line with the radiation beam;
the opposite is true for ultrasonic flaw detections, i.e. when using ultrasonic testing the defects should
ideally have their major face at 90° to the axis of the ultrasonic beam.
For the ultrasonic to enter a material a couplant must be used as a medium to introduce the probe and
the specimen, e.g. grease, oil, glycerine or water, because ultrasound does not travel very well through
air.
Ultrasonic equipment is quite portable, but one major disadvantage with most of the equipment used is
that no permanent record of results is produced. Equipment that is able to record results is currently
expensive.
NON DESTRUCTIVE TESTING
PENETRANT TESTING
This type of testing uses the forces of capillary action to detect surface breaking defects. It is impossible
to detect defects which do not break the surface with this method, but it can be used on both magnetic
and non-magnetic materials providing they are non-porous.
There are several types of penetrant systems; this includes the following which are shown in a
descending order of flaw detection sensitivity:
•
•
•
•
•
Post-emulsifable – fluorescent
Solvent based – fluorescent
Water based – fluorescent
Solvent based – colour contrast
Water based – colour contrast
Fluorescent penetrants require the use of an ultraviolet (UV-A) light to view indications, whilst colour
contrast penetrants are viewed with the naked eye.
One of the most common ‘site’ used penetrant systems uses solvent based colour contrast penetrants
in aerosols. A typical sequence of operations on a steel test item is as follows:
1. Clean area using wire brush, cloth and solvent. On aluminiums, other softs alloys and plastics,
wire brushing should not be used, as there is a danger that surface breaking defects may be
closed.
2. Apply penetrant – leave for 15 minutes. Colour contrast penetrants are normally red in colour
and should remain on the part long enough to be drawn into any surface discontinuities. This
time can vary from about ten minutes to several hours depending on the type of material and
size/type of defect sought.
3. Remove surface penetrant using cloth and solvent. Apply solvent to the cloth and not directly on
to the work piece. Clean thoroughly.
4. Apply developer – leave for 15 minutes. The developer draws any penetrant remaining in any
surface breaking discontinuities with a blotting action.
5. Interpret area. Any discontinuities are indicated by a red mark, e.g. line or dot against a white
background. Fluorescent penetrants wold show green-yellow when viewed with an ultraviolet
(UV-A) light.
MAGNECTIC PARTICLE INSPECTION
This method of NDT may detect surface, and in certain cases, slight sub-surface discontinuities up to 23 mm below the surface. M.P.I can be used on ferromagnetic materials only.
A magnetic field is introduced into a specimen to be tested, fine particles of ferromagnetic powder, or
ferromagnetic particles in a liquid suspension, are then applied to the test area. Any discontinuity which
interprets the magnetic lines of force will create a leakage field, which has a north and south pole on
either side of it. This attracts the ferromagnetic particles in great numbers. The discontinuity may show
as a black indication against the contrasting background – usually white contrast paint – or as a
fluorescent indication which is usually green/yellow against a dark violet background.
When M.P.I is carried out using fluorescent inks, the use of an ultraviolet (UV-A) light is necessary to
cause fluorescence of the particles, although there is no need to apply a contrast paint. Fluorescent ink
methods are more sensitive than black ink methods. There are many ways to apply a magnetic field,
e.g. a permanent magnet, coils, prods, cables and a threading bar.
Listed below is a sequence of operations to inspect a weld using a permanent magnet with black ink:
1.
2.
3.
4.
5.
6.
7.
Clean area using wire brush and cloth plus solvent if necessary.
Apply a thin layer of white contrast paint.
When paint is dry, straddle the magnet over the weld.
Apply ink (1.25 to 3.5% particles to a paraffin base).
Interpret area.
To look for transverse defects, turn magnet approximately 90° and re-apply ink.
Interpret area.
RADIOGRAPHIC TESTING
Principles
Radiography is carried out using x-ray machines or artificial gamma sources (radioisotopes).
X-rays or gamma rays pass through the object to be radiographed and record an image on a
radiographic film on the opposite side. The radiation reaching the film will be determined by the object’s
thickness and density, e.g. lack of root penetration in a weld will increase the amount of radiation falling
on the film in that area to a reduction in thickness.
It is the wavelength of the radiation which governs its penetrating power; this is governed by the
kilovoltage (kV) when using x-rays, and isotope type with gamma rays.
The intensity of the radiation is governed by the milli-amperage (mA) when using x-rays, and by the
‘activity’ of the specific isotope with gamma. Activity is measured in Curries (Ci) or gigabecquerels
(GBq).
A negative is produced when the film in processed. The thin areas of an object show up dark in relation
to the surrounding areas; exceptions are excess weld metal, spatter, tungsten and copper inclusions.
Radiographic quality
An overall assessment of radiographic quality is made by the use of image quality indicators (IQIs);
these usually consist of seven thin wires decreasing in thickness. IQI(s) are pre-placed on the weld
being examined and therefore show on the radiographic image. The more wires visible, the better the
flaw detection sensitivity is likely to be.
The density – degree of blackness – of a radiograph is also measured by using a densitometer to
ensure it lies within a specified range for optimum quality.
Advantages and disadvantages
X-radiography requires bulky and expensive machinery in comparison with gamma radiography, but xradiography generally produces better quality radiographs and is safer. X-ray machines can be
switched on and off, unlike gamma sources which permanently produce radiation and therefore require
shielding when not in use.
A major disadvantage with radiography is that it will only detect defects which have significant depth in
relation to the axis of the x-ray beam – roughly over 2% of the wall thickness in the same axis as the xray beam, i.e. radiography will not usually detect plate laminations, lack of inter-run fusions or cracks
perpendicular to the x-ray beam.
A major advantage of radiographic testing is that a permanent record is produced, i.e. the radiography.
ULTRASONIC TESTING
This method uses the ability of high frequency sounds waves, typically above 2 MHz (2,000,000 c.p.s)
to pass through materials.
A probe is used which contains a piezo electric crystal to transmit and receive ultrasonic pulses.
Ultrasound hitting any air interface or an interface with a different material density, which is
perpendicular to the ultrasonic beam, is reflected back, i.e. the longer the time, the further away the
interface.
An interface could be the opposite side of the plate; therefore, wall thickness measurements can easily
be made.
Lamination checks are easily carried out using ultrasonic methods (opposite to radiography). Welds can
be tested using angle type probes, although this requires more operator skill to apply and interpret
results. Defects in welds usually can be located but the type of defect is sometimes difficult to identify.
To detect a linear defect with radiography, the defects must have depth in line with the radiation beam;
the opposite is true for ultrasonic flaw detection, i.e. when using ultrasonic testing the defects should
ideally have their major face at 90° to the axis of the ultrasonic beam.
For the ultrasound to enter a material a couplant must be introduced between the probe and the
specimen, e.g. grease, oil, glycerine or water, because ultrasound does not travel very well through air.
Ultrasonic equipment is quite portable, but one major disadvantage with most of the equipment used is
that no permanent record of results is produced. Equipment that is able to record results is currently
expensive.
Advantages and limitations of NDT test methods for weld inspection
Visual Testing (VT)
Equipment required:
Welding gauge; steel ruler; inspection light
Enables detection of:
Surface flaws – cracks, porosity, unfilled craters, misalignments, improper fit-up and mechanical
damage.
Advantages:
Low cost; can be applied whilst work is in process, permitting correction of faults; rapid results; gives
indications of incorrect procedures; no post cleaning; safe
Limitations:
Applicable to surface defects only, some defects can be missed; provides no permanent record; no
indication of depth
Remarks:
Should always be the primary method of inspection, no matter what other techniques are required; is
the only productive type of inspection; is the necessary function of everyone who in any way contributes
to the making of the weld.
Magnetic Particle Testing (MPI)
Equipment required:
Electromagnetic yoke, magnetic ink; background paint; fluorescent ink for high sensitivity, if required
Enables detection of:
Excellent for detecting surface cracks; slightly sub-surface defects detectable with d.c.
Advantages:
Simple to use; permits controlled sensitivity; rapid results; relatively low cost method; less surface
preparation than penetrant testing
Limitations:
Applicable to ferromagnetic materials only; skilled interpretation of indications and recognition of nonrelevant indications; surface or near surface defects only; accessible surfaces only; no indication of
depth, best suited to linear defects
Remarks:
Linear defects parallel to the magnetic field may not give an indication, for this reason, the field should
be applied from two directions at or near right angles to each other.
Liquid Penetrant Testing (DPI)
Equipment required:
Dye penetrants, developers and removers; a source of ultraviolet light – if fluorescent method is used
Enables detection of:
Surface cracks not readily visible to the unaided eye; can be used for locating leaks in weldments
Advantages:
Applicable to magnetic and non-magnetic materials; easy to use; low cost; simple to use; amount of
bleed out can indicate size/depth of defects
Limitations:
Only surface defects are detectable; no real indication of depth; cannot be used effectively on hot
assemblies; surface penetration is critical; post cleaning often required as penetrants can contaminate
component; interpretation can be difficult when component geometry makes cleaning difficult.
Radiographic Testing (RT)
Equipment required:
Commercial x-ray or gamma units; film and processing facilities; viewing equipment
Enables detection of:
Internal and external defects, cracks, porosity, blow holes, non-metallic inclusions, incomplete root
penetration, excessive root penetration; burn through and undercut
Advantages:
Permanent record; can be carried out on most materials; little surface preparation; less reliant on
operator skill
Limitations:
Protection against harmful radiation; skilled interpretation; orientation of planar defects; no indication of
depth; limited thickness; expensive; large; heavy equipment; set up and processing time; access to
both sides
Remarks:
X-ray inspection is required by many codes and specifications; useful in qualification of welders and
welding processes; because of health risk, it should be limited to those areas where other methods will
not provide the quality assurance required
Ultrasonic Testing (UT)
Equipment required:
Analogue and digital flaw detectors; various angled probes; calibration blocks; fully automated systems
Enables detection of:
Surface and sub-surface flaws, including those too small to be detected by other methods; laminations;
corrosion
Advantages:
Very sensitive; can measure defect’s depth; permits probing of joint inaccessible to radiography or too
thick for radiography; only needs access from one side; lightweight and portable equipment which can
be used in difficult access situations
Limitations:
Requires high degree of skill in interpreting pulse echo patterns; no permanent record with manual
scanning; good surface finish required; expensive
DESTRUCTIVE TESTING OF WELDS
MECHANICAL PROPERTIES
Definitions
•
•
•
•
•
Brittle – The tendency of a material to fail suddenly by breaking without plastic deformation of
the material before failure.
Ductility – The ability of a material to be drawn into wire.
Elasticity –The ability of a material to return back to its original size.
Toughness – The ability of a material to resist cracking.
Hardness – The ability of a material to resist indentation, abrasion and scratching.
Mechanical testing of welded joints may be carried out for the following reasons:
•
•
•
Welding procedure approval
Welder approval
Production quality approval
A welding consumable manufacturer will carry out all-weld metal tests for each consumable type they
manufacture. The parent material is normally subjected to extensive testing prior to its acceptance by a
client and subsequent use on a contract. However, separate testing is still required for a welded joint,
because it usually consists of three metallurgically different areas which interrelate: the weld, h.a.z. and
parent material.
Mechanical testing is a destructive procedure and is not carried out on any component required for use,
therefore, representative test samples produced under similar conditions to the in-service components
are normally used and comparisons made.
The tests most frequently used to assess the properties of welded joints are:
•
•
•
•
•
Tensile
Bend
Impact
Hardness
Macroscopic / Microscopic
TENSILE TEST
Purpose of test
A tensile test can be used to assess the following:
•
•
•
The yield point of a specimen – the point at which the specimen undergoes plastic deformation
The ultimate tensile strength (U.T.S) of the specimen – the maximum load a specimen can
withstand
The ductility of the specimen – expressed as % elongation
There are different types of tensile tests for welds; these include:
•
•
The transverse tensile test – used on joints containing butt welds
The all-weld tensile test – used to test either the filler material properties, or the quality of the
deposited weld metal as a whole
Strain (extension) is the result of stress (load applied), therefore as the load is applied the test piece will
begin to extend.
If the load is removed at any point up to the elastic limit (E), the specimen will behave elastically and
return to its original length. Beyond the elastic limit, the upper yield point (u.y.p.) is reached; this is
where the material yields and plastic deformation occurs. At the point there is a noticeable drop in the
applied load until the lower yield point (l.y.p.) is reached. Beyond the lower yield point, the load begins
to increase again.
Yielding is the slipping of the atomic structure; it commences at the upper yield point and stops at the
lower yield point. Yielding stops because the slipping action cold works the steel and increases the
tensile strength.
After yielding, the load increases to the point of ultimate tensile strength (u.t.s.), this is the maximum
load the specimen can withstand, it is not the point of fracture. Beyond the u.t.s., the specimen necks
locally; this leaves a high load acting on the reduced cross-sectional area, therefore fracture at X rapidly
follows.
Preparation of specimen
Test specimens are cut from designated areas of the welded assembly, the length and width, method of
cutting (thermal or machine), and requirements for the removal or leaving of the weld reinforcement
would be stated in the appropriate specification. The edges should be made smooth – normally filed –
and any comers in the test area radiused slightly to reduce stress raisers.
On an all-weld tensile test, it is a common requirement to test for the elongation percentage. In this
case, two punch marks would be applied in line with the applied stress. The distance between two
punch marks, which would be specified, is called the gauge length.
Test procedure
Two sets of vice jaws are used to clamp the test specimen at the top and bottom, hydraulic power is
then applied to force the specimen apart.
For transverse tensile test made on C or C-Mn steel welds, fracture usually occurs in the parent
material. A specimen which snaps in the weld region is usually acceptable providing the specification
requirements are met.
BEND TEST
Purpose of test
Bend tests are carried out on butt welds to determine the soundness of the weld zone. The commonly
applied bend test is the transverse bend test when the specimen is cut so that the weld in transverse to
the specimen length. There are three ways to perform this type of bend test on a single sided weld
1. The root bend test puts the root in tensions. Main aim is to open up and enlarge surface
breaking discontinuities.
2. The face bend test puts the face in tension. Main aim is to open up and enlarge surface
breaking discontinuities.
3. The side bend test puts the side of the weld in tension. Main aim is to open up and enlarge subsurface discontinuities.
IMPACT TEST
General
The Charpy V-notch test uses a swinging pendulum which break accurately prepared notched test
specimens. Each specimen is broken with a single blow. Impact testing assesses notch toughness by
measuring the energy absorbed by each test specimen during impact.
Naturally occurring notches, e.g. tools marks, undercut and lack of root penetration, reduce a
component’s endurance, especially at low temperature. An impact test carried out on a weld determines
the notch toughness of the weld at a given temperature. Toughness is the ability of a material to
withstand cracking or tearing.
VICKERS HARDNESS SURVEY
General
Hardness testing may be used to determine the hardness gradient across a weld zone and parent
material for comparison purposes. The maximum hardness of the weld zone may also be determined.
The hardness of a weld and h.a.z. will give an indication of the weldability of the Material.
The test uses a small pyramid shaped diamond indentor with an angle of 136° between the opposite
faces. A force is applied to press the indentor into the surface for a period of 10 – 15 seconds. The
force applied in 10Kg.
The diamond indentor leaves a pin tip sized square indentation on the surface of the test specimen
which is measured by way of a built-in microscope. The diagonal dimension of the indentation is
measured using two adjustable shutters; there are integrated with a digital readout, or similar, to give a
value which corresponds to indentation depth and therefore hardness.
However, this displayed value is not the final hardness value. Firstly, both the diagonal lengths need to
be measured and averaged. Secondly, the average value needs to be converted to a Vickers pyramid
hardness value (H) by reference to special tables, or a formula, which also takes into account the
applied load. The final hardness value reported should include the load applied, e.g. 200 H at 10 kg.
MACROSCOPIC (MACRO) EXAMINATION
Macro examination of welds is carried out on full thickness specimens which include cap and root. The
width of the cross-section should include the heat affected zone plus some parent material. The same
test piece is sometimes used for hardness testing after macro examination – typically under x5
magnification.
Test procedure
Test specimens are usually cut transversely from a weld. Each test specimen is then ground, polished
and etched to the degree required by specification, e.g. for ferritic steels – P400 grade finish with an
acid etch using 10-15% nital (nitric acid + alcohol).
After etching, the test specimen is examined visually. The intent is to disclose any cracks, lack of fusion
porosity, slag, etc.
Visual Inspection of the Welds
This method is the most commonly used NDT method and should be conducted before other methods
are used, which may be applicable to meet a contracts requirements, i.e. why would you waste time
and money conducting supplementary tests when the item concerned would not meet the acceptance /
rejection criteria? Although visual inspection requires no set up costs, it is portable and reasonably easy
to establish a basic understanding. It does have limitations that have been discussed in an earlier
chapter. Guidance and basic requirements for visual inspection are documented in BS EN ISO
17637:2011, and if in doubt should be referred to. Visual inspection is an invaluable aid in identifying
imperfections and defects that are present at the surface of the weld or adjacent to it.
All imperfections/defects that are commonly encountered when using the MAGS process on carbon
steel, will be discussed individually along with causes, prevention etc. the method of learning will have a
‘hands on approach’ on actual defective welds as visual aids to complement the course notes.
BS EN ISO 6520-1 gives the following definitions: Imperfection: Any deviation from the ideal weld
Defect: An unacceptable imperfection
Surface Detectable
Cracks
Gas pores and porosity
Dimensional
Under cut
Inclusions
Cold laps
Spatter
Sub-Surface Detectable Only
Volumetric gas pores and porosity
Lack of side wall and interun fusion
Root defects (access from one side only i.e.
LHS or RHS)
Incomplete penetration (double V butts)
Definitions according to BS EN ISO 6520-1
Imperfection: Any deviation from the ideal weld
Defect: An unacceptable imperfection
It would be impossible to cover this subject in depth in just one section of a 2 day course! Due to the
many defect types, some of which are only encountered when using a specific welding process. For
this reason, we will only reference to defect types that may be encountered when using the MAGS
process to weld carbon steels.
Defect Types
Cracks
Undercut
Overlap / Cold Laps
Crater Cracks
Spatter
Contour Type Defects
Unequal Leg Lengths
Incomplete / Excess Root Penetration
Lack of Side Wall / Root Fusion
Root Concavity (Suck Back)
Burn Through – See Excess Root Penetration
Porosity
Cracks
Cracks can occur in several directions i.e. transverse, longitudinal, and can be found in parent plate,
weld metal and the heat affected zone, due to their straight (PLANAR) orientation, this can be
considered as a major weld defect as propagation can occur, which may lead to failure of a
component. All acceptance / rejection criteria’s have no tolerance whatsoever due to this risk. The
main crack that is encountered in the structured steel industry is ‘Hydrogen Induced Cold Cracking’
HICC, this subject is covered in greater detail in the ‘pre-heat requirements’ section of the course
notes. Other possible causes of cracking can be insufficient preheat, depth to width ratios of butt
welds being too great, high sulphur content of parent material, joint restraint, poor stop / start
techniques and premature cooling of thicker sections.
Spatter
Arc excess filler metal globules that are lying on the surface if the weld and around it, usually nnot of
any detriment to the weld unless welding exotic materials, so receives a wide latitude of allowance,
however this defect type looks very unsightly and most things that are not pleasing to the eye draw
attention to that area. Excessive amperage is the usual cause of spatter, but please note, the
greater CO2 content of your shielding gas, the more the arc becomes active and hotter, which can
lead to excess spatter! 5% CO2 content will always give fewer spatters than a 20% CO2 mixture.
Contour Type Defects
These defect types are usually associated with poor torch manipulation and incorrect torch angles
when depositing welds. In the case of butt welds, incomplete fillet welds, excess cap height, etc., in
the case of fillet welds, unequal leg lengths are the most common defect, which is again down to
incorrect torch angle. This may not appear to be a big problem, but if one leg length is greater than
the other or both leg lengths are too small, how can you achieve your design throat thickness?
Incomplete / Excess Root Penetration
If access can only be gained from one side, .i.e. joining CHS or box section, this defect can be very
problematic as the full strength potential can’t be achieved if deposited weld metal isn’t at least the
material wall thickness! Again, these defects are deemed as PLANAR and give little or no
allowance. Excess root penetration has a greater tolerance as it is of less detriment usually. Once
again, amperage settings and speed of travel are usually major factors in this defect type.
Undercut
Usually found at the toes of fillet welds, but can be observed on butt weld caps and occasionally
running parallel to the root run bend. Undercut can be intermittent or continuous, again a PLANAR
type defect, so very little or any depth or duration is allowed in the majority of acceptance/rejection
criteria’s. Causes are mainly incorrect torch angle and too high amperage, in certain circumstances
excessive weaving and travel speed can contribute also.
Cold Laps
The majority of overlap/cold lap type defects are observed at the toes of a fillet weld and the cap of
the butt welds. This occurs when solidifying weld metal doesn’t fuse with the parent plate and lays
on the weld toes rather than bonding to the parent plate, this defect cause is nearly always
encountered if travel speed is too slow, with too much amperage, poor technique and working
positionally can be a factor.
Lack of Side Wall / Root Fusion
Only usually detectable when access to the root of a butt weld is possible, side wall fusion defects
can’t be observed with the usual inspection method and would require radiography, ultrasonics or
macro inspection. Again, PLANAR type defects with very little tolerance for acceptability as
propagation can occur working in strict accordance with your WPS prevent the majority of this
defect type.
Root Concavity (Suck Back)
Only deemed a defect on un-backed butt welds as double V butts and background butts. The defect
can be repaired by excavating into the concavity, however a single V un-backed butt weld i.e.
joining box section or tubes. The wall depth will not be equal to or greater than the wall thickness,
again creating a PLANAR defect and possibly not meeting the design requirements for that
particular product type application. Usually found when root gap is too wide, speed of travel too fast
and amperage too high. Again in certain access situation can only be observed when using
volumetric NDT methods.
Burn Through
The next stage on after ‘excessive root penetration’ where the root has totally collapsed! Not such
as issue on double V butts or background single V butts. Causes are as per excessive root
penetration, but a too wide root gap or too small root face can be factors.
Porosity
A singular surface breaking ‘gas pore’ is defined as just that. But groups, clusters and patches
collectively, are defined as ‘porosity’ which can be linear, intermittent, gross or scattered. Primary
cause when using the MAGS process is lack of shielding gas around the solidifying weld pool,
however in extreme cases, too much shielding gas flow can create a vortex which pulls in the
oxygen rich atmosphere causing this defect. Dirty and damp material and welding consumables that
have not been stored in accordance with the manufacturer’s instructions and recommendations can
also be a factor. Avoidance of porosity can be achieved by monitoring a ’10 to 15mm torch to work
piece policy’ and approximately 15 to 18 ‘litres per minute’ of shielding gas around the solidifying
weld. Clean material at a minimum of 5°C temperature also promotes high integrity deposited welds
free from possible porosity attack.
From the eleven weld example photo’s, identify each defect type from
the list given below and state the applicable answer by the side of each
photo.
Root concavity (suck back)
Porosity
Lack of root penetration
Fillet weld toe crack
Lack of side wall fusion
Unequal fillet weld leg length
Excess spatter
Undercut
Joint restraint crack
Inclusions
Crater crack
PROCEDURE FOR THE VISUAL EXAMINATION OF FILLET &
BUTT WELDS IN FERRITIC & STAINLESS STEEL &
ALUMINIUM IN ACCORDANCE WITH BSEN ISO 17637:2016
SCOPE
The object of this procedure is to provide guidance to enable welding
inspectors to carry out their duties in ensuring that all welding and associated
activities are carried out in accordance with the relevant specifications and
welding procedures. These specifications usually include National/ International
standards, client specifications and welding procedures.
PERSONNEL
All welding inspections shall be conducted by personnel who have met the
requirements of the 2-day Responsible Welding Coordinator course, unless
noted otherwise (to be agreed by concerned parties before inspection activities
commence).
EQUIPMENT
The following is a list of equipment that may be used to assist visual inspection
and help in the measurement of weld dimensions and weld imperfections.
1.
2.
3.
4.
5.
Ruler / tape
Optical aids
Welding gauge (claw or Cam Type)
Contour Gauge
Fillet profile gauge
INSPECTION PRE, DURING & POST WELDING
{All to be conducted in a minimum of 500 LUX white light environment}
1. ROOT EXAMINATION
When and where possible examine the root area for penetration,
concavity, burn through etc and ensure that it all conforms to the
relevant acceptance / rejection criteria.
2. CONTOUR
Check that the contour of the weld and the height of the excess
weld reinforcement are in accordance with the agreed acceptance /
rejection criteria. Check that the weld surface is of an acceptable
regular profile with satisfactory appearance.
3. CAP WIDTH
Check that the weld preparation has been completely filled, check that
the weld width is consistent over the complete joint and that it complies
with specification & drawing requirements. In the case of fillet welds
check that the required leg lengths or throat thickness have been
achieved.
4. UNDERCUT
Measure any detected undercut {cap or root} and refer against the
acceptance / rejection criteria.
5. WELD EFFECTS
Examine the weld and heat affected zone areas for defects, if any defects
are found, document the size, dimensions, depth etc. and refer to the
acceptance / rejection criteria. In the event of being unable to determine
the full extent of the defect, a request for additional NDT should be
carried out to assist in making an assessment.
6. INSPECTION OF COMPLETED FABRICATION
When all of the relevant checks in the previous sections have been
completed and are satisfactory, check any damaged areas after the
removal of temporary attachments etc. if any are found, ensure that they
are dressed / blended. Where post-weld heat treatment is required
ensure it is conducted in strict accordance with the applicable approved
procedure, ensuring all heating rates, soak times and cooling rates are
achieved. When complete, re-inspect the weld and heat affected zones
to ensure no defects have occurred.
INSPECTION REPORTS
Records should be kept at each stage of inspection, where specified
by the relevant specification.