Telechargé par Suleiman Al-Rashdi

Defects imperfections in welds -porosity

Defects/imperfections in
welds - porosity
The characteristic features and principal causes of
porosity imperfections are described. Best practice
guidelines are given so welders can minimise
porosity risk during fabrication.
Porosity is the presence of cavities in the weld metal
caused by the freezing in of gas released from the weld pool as it solidifies. The porosity can
take several forms:
Surface breaking pores
Crater pipes
Cause and prevention
Distributed porosity and surface pores
Distributed porosity (Fig. 1) is normally found as fine pores throughout the weld bead.
Surface breaking pores (Fig. 2) usually indicate a large amount of distributed porosity
Fig. 1. Uniformly distributed porosity
Fig. 2. Surface breaking pores (T fillet weld in
primed plate)
Porosity is caused by the absorption of nitrogen, oxygen and hydrogen in the molten weld
pool, which is then released on solidification to become trapped in the weld metal.
Nitrogen and oxygen absorption in the weld pool usually originates from poor gas shielding.
As little as 1% air entrainment in the shielding gas will cause distributed porosity and greater
than 1.5% results in gross surface breaking pores. Leaks in the gas line, too high a gas flow
rate, draughts and excessive turbulence in the weld pool are frequent causes of porosity.
Hydrogen can originate from a number of sources including moisture from inadequately dried
electrodes, fluxes or the workpiece surface. Grease and oil on the surface of the workpiece or
filler wire are also common sources of hydrogen.
Surface coatings like primer paints and surface treatments such as zinc coatings, may generate
copious amounts of fume during welding. The risk of trapping the evolved gas will be greater
in T joints than butt joints especially when fillet welding on both sides (see Fig 2). Special
mention should be made of the so-called weldable (low zinc) primers. It should not be
necessary to remove the primers but if the primer thickness exceeds the manufacturer's
recommendation, porosity is likely to result especially when using welding processes other
than MMA.
The gas source should be identified and removed as follows:
Air entrainment
- Seal any air leak
- Avoid weld pool turbulence
- Use filler with adequate level of deoxidants
- Reduce excessively high gas flow
- Avoid draughts
- Dry the electrode and flux
- Clean and degrease the workpiece surface
Surface coatings
- Clean the joint edges immediately before welding
- Check that the weldable primer is below the recommended maximum thickness
Characteristically, wormholes are elongated pores (Fig.
3), which produce a herring bone appearance on the
Elongated pores or wormholes
Wormholes are indicative of a large amount of gas being
formed, which is then trapped in the solidifying weld
metal. Excessive gas will be formed from gross surface
contamination or very thick paint or primer coatings.
Entrapment is more likely in crevices such as the gap
beneath the vertical member of a horizontal-vertical, T joint which is fillet welded on both
When welding T joints in primed plates it is essential that the coating thickness on the edge of
the vertical member is not above the manufacturer's recommended maximum, typically 20µ,
through over-spraying.
Eliminating the gas and cavities prevents wormholes.
Gas generation
- Clean the workpiece surfaces
- Remove any coatings from the joint area
- Check the primer thickness is below the manufacturer's maximum
Joint geometry
- Avoid a joint geometry, which creates a cavity
Crater pipe
A crater pipe forms during the final solidified weld pool and is often associated with some gas
This imperfection results from shrinkage on weld pool solidification. Consequently,
conditions, which exaggerate the liquid to solid volume change, will promote its formation.
Switching off the welding current will result in the rapid solidification of a large weld pool.
In TIG welding, autogenous techniques, or stopping the wire before switching off the welding
current, will cause crater formation and the pipe imperfection.
Crater pipe imperfection can be prevented by removing the stop or by welder technique.
Removal of stop
- Use run-off tag in butt joints
- Grind out the stop before continuing with the next electrode or depositing the
subsequent weld run
Welder technique
- Progressively reduce the welding current to reduce the weld pool size
- Add filler (TIG) to compensate for the weld pool shrinkage
Porosity susceptibility of materials
Gases likely to cause porosity in the commonly used range of materials are listed in the Table.
Principal gases causing porosity and recommended cleaning methods
C Mn steel
Hydrogen, Nitrogen and
Grind to remove scale coatings
Stainless steel
Degrease + wire brush + degrease
Aluminium and
Chemical clean + wire brush + degrease
+ scrape
Copper and alloys
Hydrogen, Nitrogen
Degrease + wire brush + degrease
Nickel and alloys
Degrease + wire brush + degrease
Detection and remedial action
If the imperfections are surface breaking, they can be detected using a penetrant or magnetic
particle inspection technique. For sub surface imperfections, detection is by radiography or
ultrasonic inspection. Radiography is normally more effective in detecting and characterising
porosity imperfections. However, detection of small pores is difficult especially in thick
Remedial action normally needs removal by localised gouging or grinding but if the porosity
is widespread, the entire weld should be removed. The joint should be re-prepared and rewelded as specified in the agreed procedure.
Weld defects / imperfections - incomplete
root fusion or
The SS Schenectady, an all welded
tanker, broke in two whilst lying in
The characteristic features and principal causes of dock in 1943. Principal causes of this
incomplete root fusion are described. General
failure were poor design and bad
guidelines on 'best practice' are given so welders
can minimise the risk of introducing imperfections
during fabrication.
Fabrication and service defects
and imperfections
As the presence of imperfections in a welded joint
may not render the component defective in the
sense of being unsuitable for the intended
application, the preferred term is imperfection rather than defect. For this reason, production
quality for a component is defined in terms of a quality level in which the limits for the
imperfections are clearly defined, for example Level B, C or D in accordance with the
requirements of EN 25817. For the American standards ASME X1 and AWS D1.1, the
acceptance levels are contained in the standards.
The application code will specify the quality levels, which must be achieved for the various
Imperfections can be broadly classified into those produced on fabrication of the component
or structure and those formed as result of adverse conditions during service. The principal
types of imperfections are:
Lack of fusion
Incorrect weld shape and size
Brittle fracture
Stress corrosion cracking
Fatigue failure
Welding procedure and welder technique will have a direct effect on fabrication
imperfections. Incorrect procedure or poor technique may produce imperfections leading to
premature failure in service.
Incomplete root fusion or penetration
Incomplete root fusion is when the weld fails to fuse one side of the joint in the root.
Incomplete root penetration occurs when both sides of the joint are unfused. Typical
imperfections can arise in the following situations:
An excessively thick root face in a butt weld (Fig. 1a)
Too small a root gap (Fig. 1b)
Misplaced welds (Fig. 1c)
Failure to remove sufficient metal in cutting back to sound metal in a double sided
weld (Fig. 1d)
Incomplete root fusion when using too low an arc energy (heat) input (Fig. 1e)
Too small a bevel angle,
Too large an electrode in MMA welding (Fig 2)
Fig. 1 Causes of incomplete root
a) Excessively thick root face
b) Too small a root gap
c) Misplaced welds
d) Power input too low
e) Arc (heat) input too low
Fig. 2 Effect of electrode size on root
a) Large diameter electrode
b) Small diameter electrode
These types of imperfection are more likely in consumable electrode processes (MIG, MMA
and submerged arc welding) where the weld metal is 'automatically' deposited as the arc
consumes the electrode wire or rod. The welder has limited control of weld pool penetration
independent of depositing weld metal. Thus, the non-consumable electrode TIG process in
which the welder controls the amount of filler material independent of penetration is less
prone to this type of defect.
In MMA welding, the risk of incomplete root fusion can be reduced by using the correct
welding parameters and electrode size to give adequate arc energy input and deep penetration.
Electrode size is also important in that it should be small enough to give adequate access to
the root, especially when using a small bevel angle (Fig 2). It is common practice to use a
4mm diameter electrode for the root so the welder can manipulate the electrode for
penetration and control of the weld pool. However, for the fill passes where penetration
requirements are less critical, a 5mm diameter electrode is used to achieve higher deposition
In MIG welding, the correct welding parameters for the material thickness, and a short arc
length, should give adequate weld bead penetration. Too low a current level for the size of
root face will give inadequate weld penetration. Too high a level, causing the welder to move
too quickly, will result in the weld pool bridging the root without achieving adequate
It is also essential that the correct root face size and bevel angles are used and that the joint
gap is set accurately. To prevent the gap from closing, adequate tacking will be required.
Best practice in prevention
The following techniques can be used to prevent lack of root fusion:
In TIG welding, do not use too large a root face and ensure the welding current is
sufficient for the weld pool to penetrate fully the root
In MMA welding, use the correct current level and not too large an electrode size for
the root
In MIG welding, use a sufficiently high welding current level but adjust the arc
voltage to keep a short arc length
When using a joint configuration with a joint gap, make sure it is of adequate size and
does not close up during welding
Do not use too high a current level causing the weld pool to bridge the gap without
fully penetrating the root.
Acceptance standards
The limits for lack of penetration are specified in BS EN 25817 (ISO 5817) for the three
quality levels.
Lack of root penetration is not permitted for Quality Level B (stringent). For Quality Levels C
(intermediate) and D (moderate) long lack of penetration imperfections are not permitted but
short imperfections are permitted.
Incomplete root penetration is not permitted in the manufacture of pressure vessels but is
allowable in the manufacture of pipework depending on material and wall thickness.
Remedial actions
If the root cannot be directly inspected, for example using a penetrant or magnetic particle
inspection technique, detection is by radiography or ultrasonic inspection. Remedial action
will normally require removal by gouging or grinding to sound metal, followed by re-welding
in conformity with the original procedure.
Relevant standards
EN 25817:1992 (ISO 5817) Arc welded joints in steel - Guidance on quality levels for
EN 30042: 1994 Arc welded joints in aluminium and its weldable alloys - Guidance on
quality levels for imperfections.
Defects/imperfections in welds - slag
Prevention of slag inclusions by
grinding between runs
The characteristic features and principal causes of slag imperfections are described.
Slag is normally seen
as elongated lines
either continuous or
discontinuous along
the length of the weld.
This is readily
identified in a
radiograph, Fig 1. Slag
inclusions are usually
associated with the flux
processes, i.e. MMA, FCA and submerged arc, but they can also occur in MIG welding.
Fig. 1. Radiograph of a butt weld showing two slag lines in the
weld root
As slag is the residue of the flux coating, it is principally a deoxidation product from the
reaction between the flux, air and surface oxide. The slag becomes trapped in the weld when
two adjacent weld beads are deposited with inadequate overlap and a void is formed. When
the next layer is deposited, the entrapped slag is not melted out. Slag may also become
entrapped in cavities in multi-pass welds through excessive undercut in the weld toe or the
uneven surface profile of the preceding weld runs, Fig 2.
As they both have an effect on the ease of slag removal, the risk of slag imperfections is
influenced by
Type of flux
Welder technique
The type and configuration of the joint, welding position and access restrictions all have an
influence on the risk of slag imperfections.
Fig. 2. The influence of welder technique on the risk of slag inclusions when welding
with a basic MMA (7018) electrode
a) Poor (convex) weld bead profile
resulted in pockets of slag being
trapped between the weld runs
b) Smooth weld bead profile allows
the slag to be readily removed
between runs
Type of flux
One of the main functions of the flux coating in welding is to produce a slag, which will flow
freely over the surface of the weld pool to protect it from oxidation. As the slag affects the
handling characteristics of the MMA electrode, its surface tension and freezing rate can be
equally important properties. For welding in the flat and horizontal/vertical positions, a
relatively viscous slag is preferred, as it will produce a smooth weld bead profile, is less likely
to be trapped and, on solidifying, is normally more easily removed. For vertical welding, the
slag must be more fluid to flow out to the weld pool surface but have a higher surface tension
to provide support to the weld pool and be fast freezing.
The composition of the flux coating also plays an important role in the risk of slag inclusions
through its effect on the weld bead shape and the ease with which the slag can be removed. A
weld pool with low oxygen content will have a high surface tension producing a convex weld
bead with poor parent metal wetting. Thus, an oxidising flux, containing for example iron
oxide, produces a low surface tension weld pool with a more concave weld bead profile, and
promotes wetting into the parent metal. High silicate flux produces a glass-like slag, often
self-detaching. Fluxes with lime content produce an adherent slag, which is difficult to
The ease of slag removal for the principal flux types are:
Rutile or acid fluxes - large amounts of titanium oxide (rutile) with some silicates.
The oxygen level of the weld pool is high enough to give flat or slightly convex weld
bead. The fluidity of the slag is determined by the calcium fluoride content. Fluoridefree coatings designed for welding in the flat position produce smooth bead profiles
and an easily removed slag. The more fluid fluoride slag designed for positional
welding is less easily removed.
Basic fluxes - the high proportion of calcium carbonate (limestone) and calcium
fluoride (fluorspar) in the flux reduces the oxygen content of the weld pool and
therefore its surface tension. The slag is more fluid than that produced with the rutile
coating. Fast freezing also assists welding in the vertical and overhead positions but
the slag coating is more difficult to remove.
Consequently, the risk of slag inclusions is significantly greater with basic fluxes due to the
inherent convex weld bead profile and the difficulty in removing the slag from the weld toes
especially in multi-pass welds.
Welder technique
Welding technique has an important role to play in preventing slag inclusions. Electrode
manipulation should ensure adequate shape and degree of overlap of the weld beads to avoid
forming pockets, which can trap the slag. Thus, the correct size of electrode for the joint
preparation, the correct angles to the workpiece for good penetration and a smooth weld bead
profile are all essential to prevent slag entrainment.
In multi-pass vertical welding, especially with basic electrodes, care must be taken to fuse out
any remaining minor slag pockets and minimise undercut. When using a weave, a slight dwell
at the extreme edges of the weave will assist sidewall fusion and produce a flatter weld bead
Too high a current together with a high welding speed will also cause sidewall undercutting
which makes slag removal difficult.
It is crucial to remove all slag before depositing the next run. This can be done between runs
by grinding, light chipping or wire brushing. Cleaning tools must be identified for different
materials e.g. steels or stainless steels, and segregated.
When welding with difficult electrodes, in narrow vee butt joints or when the slag is trapped
through undercutting, it may be necessary to grind the surface of the weld between layers to
ensure complete slag removal.
Best practice
The following techniques can be used to prevent slag inclusions:
Use welding techniques to produce smooth weld beads and adequate inter-run fusion
to avoid forming pockets to trap the slag
Use the correct current and travel speed to avoid undercutting the sidewall which will
make the slag difficult to remove
Remove slag between runs paying particular attention to removing any slag trapped
in crevices
Use grinding when welding difficult butt joints otherwise wire brushing or light
chipping may be sufficient to remove the slag.
Acceptance standards
Slag and flux inclusions are linear defects but because they do not have sharp edges compared
with cracks, they may be permitted by specific standards and codes. The limits in steel are
specified in BE EN 25817 (ISO 5817) for the three quality levels. Long slag imperfections are
not permitted in both butt and fillet welds for Quality Level B (stringent) and C (moderate).
For Quality Level D, butt welds can have imperfections providing their size is less than half
the nominal weld thickness. Short slag related imperfections are permitted in all three-quality
levels with limits placed on their size relative to the butt weld thickness or nominal fillet weld
throat thickness.
Job knowledge for welders
Standards - application
standards, codes of practice
and quality levels
Production at Dennis vehicle manufacturers
Application standards and codes of practice ensure that a structure or component will have an
acceptable level of quality and be fit for the intended purpose.
In this document, the requirements for standards on welding procedure and welder approval
are explained together with the quality levels for imperfections. It should be noted that the
term approval is used in European standards in the context of both testing and documentation.
The equivalent term in the ASME standard is qualification.
Application standards and codes
There are essentially three types of standards, which can be referenced in fabrication:
Application and design
Specification and approval of welding procedures
Approval of welders
There are also specific standards covering material specifications, consumables, welding
equipment and health and safety. British Standards are used to specify the requirements, for
example, in approving a welding procedure, they are not a legal requirement but may be cited
by the Regulatory Authority as a means of satisfying the law. Health and Safety guidance
documents and codes of practice may also recommend standards.
Codes of practice differ from standards in that they are intended to give recommendations and
guidance, for example, on the validation of power sources for welding. It is not intended that
should be used as a mandatory, or contractual, document.
Most fabricators will be working to one of the following:
Company or industry specific standards
National BS (British Standard)
European BS EN (British Standard European Standard)
US AWS (American Welding Society) and ASME (American Society of Mechanical
International ISO (International Standards Organisation)
Examples of application codes and standards and related welding procedure and welder
approval standards are listed in Table 1.
Table 1 Examples of application codes and standards and related welding procedure
and welder approval standards
Welding standard
Procedure approval
Welder approval
Pressure Vessels
BS 5500
BS EN 288
BS EN 287
Process Pipework
BS 2633
BS 4677
BS 2971
BS EN 288 (Part 3)
BS EN 288 (Part 4)
BS EN 288 (Part 3)
(if required)
BS EN 287 (Part 1)
BS EN 287 (Part 2)
BS 4872/BS EN
AWS D1.1
AWS D1.2
BS 5135
BS 8118
AWS D1.1
AWS D1.2
BS EN 288 (Part 3)
BS EN 288 (Part 4)
AWS D1.1
AWS D1.2
BS EN 287
BS EN 287
BS 4872
Storage Tanks
BS 2654
BS 2594
API 620/650
BS EN 288 (Parts 3 & BS EN 287
BS EN 287
BS EN 288 (Parts 3 & ASME IX
Note 1: Reference should be made to the application codes/standards for any additional
requirements to those specified in BS EN 287, BS EN 288 and ASME IX.
Note 2: Some BS Standards have not been revised to include the new BS EN standards: BS
EN 287 and BS EN 288 should be substituted, as appropriate, for BS 4871 and BS 4870,
respectively, which have been with drawn.
In European countries, national standards are being replaced by EN standards. However,
when there is no equivalent EN standard, the National standard can be used. For example, BS
EN 287 replaces BS 4871 but BS 4872 remains as a valid standard.
Approval of welding procedures and welders
An application standard or code of practice will include requirements or guidelines on
material, design of joint, welding process, welding procedure, welder qualification and
inspection or may invoke other standards for example for welding procedure and welder
approval tests. The manufacturer will normally be required to approve the welding procedure
and welder qualification. The difference between a welding procedure approval and a welder
qualification test is as follows:
The welding procedure approval test is carried out by a competent welder and the
quality of the weld is assessed using non-destructive and mechanical testing
techniques. The intention is to demonstrate that the proposed welding procedure will
produce a welded joint, which will satisfy the specified requirements of weld quality
and mechanical properties.
The welder approval test examines a welder's skill and ability in producing a
satisfactory test weld. The test may be performed with or without a qualified welding
procedure (note, without an approved welding procedure the welding parameters
must be recorded).
The requirements for approvals are determined by the relevant application standard or as a
condition of contract (Table 1).
EN 287 and ASME IX would be appropriate for welders on high quality work such as
pressure vessels, pressure vessel piping and off-shore structures and other products where the
consequences of failure, stress levels and complexity mean that a high level of welded joint
integrity is essential. In less demanding situations, such as small to medium building frames
and general light structural and non- structural work, an approved welding procedure may not
be necessary. However, to ensure an adequate level of skill, it is recommended that the welder
be approved to a less stringent standard e.g. BS 4872.
'Coded welder' is often used to denote an approved welder but the term is not recognised in
any of the standards. However, it is used in the workplace to describe those welders whose
skill and technical competence have been approved to the requirements of an appropriate
Quality Acceptance Levels for Welding Procedure and Welder Approval
When welding to application standards and codes, consideration must be given to the
imperfection acceptance criteria, which must be satisfied. Some standards contain an
appropriate section relating to the acceptance levels while others make use of a separate
standard. For example, in welding procedure and welder approval tests to EN 288 Pt3 and EN
287 Pt1, respectively, reference is made to EN 25817 (ISO 5817). It is important to note that
the application standard may specify more stringent imperfection acceptance levels and/or
require additional tests to be carried out as part of the welding procedure approval test. For
example, for joints, which must operate at high temperatures, elevated temperature tensile test
may be required whereas for low temperature applications, impact or CTOD tests may be
Guidance on permissible levels of imperfections in arc-welded joints in steel (thickness range,
3 to 63mm) is given in EN 25817. Production quality, but not fitness-for-purpose, is defined
in terms of three levels of quality for imperfections:
Moderate - Level D
Intermediate- Level C
Stringent - Level B
The standard applies to most arc welding processes and covers imperfections such as cracks,
porosity, inclusions, poor bead geometry, lack of penetration and misalignment.
As the quality levels are related to the types of welded joint and not to a particular
component, they can be applied to most applications for procedure and welder approval. The
quality levels which are the most appropriate for production joints will be determined by the
relevant application standard which may cover design considerations, mode of stressing (e.g.
static, dynamic), service conditions (e.g. temperature, environment) and consequences of
When working to the European Standards, the welding procedure, or the welder, will be
qualified if the imperfections in the test piece are within the specified limits of Level B except
for excess weld metal, excess convexity, excess throat thickness and excess penetration type
imperfections when Level C will apply.
Guidance levels for aluminium joints are given in EN 30042.
For the American standards ASME IX and AWS D1.1, the acceptance levels are contained in
the standard. Application codes may specify more stringent imperfection acceptance levels
and/or additional tests.
Relevant Standards
American Welding Society, Structural Welding Code, AWS D1.1
ASME Boiler and Pressure Vessel Code, Section IX: Welding Qualifications
BS 4872 Approval Testing of Welders when Welding Procedure Approval is not
EN 287:1997 Approval Testing of welders for fusion welding
EN 288: Specification and approval of welding procedures for metallic materials
EN 25817:1992 (ISO 5817) Arc welded joints in steel - Guidance on quality levels
for imperfections.
EN 26520 Classification of imperfections in metallic fusion welds, with explanations.
EN 30042:1994 Arc-welded joints in aluminium and its weldable alloys. Guidance on
quality levels for imperfections.
Weld defects/imperfections in welds lack of sidewall and inter-run fusion
Demagnetising a pipe
This article describes the characteristic features and principal causes of lack of
sidewall and inter-run fusion. General guidelines on best practice are given so that
welders can minimise the risk of imperfections during fabrication.
Lack of fusion imperfections can occur when the weld metal fails
To fuse completely with the sidewall of the joint (Fig. 1)
To penetrate adequately the previous weld bead (Fig. 2).
Fig. 1. Lack of side wall fusion
Fig. 2. Lack of inter-run fusion
The principal causes are too narrow a joint preparation, incorrect welding parameter
settings, poor welder technique and magnetic arc blow. Insufficient cleaning of oily or
scaled surfaces can also contribute to lack of fusion. These types of imperfection are
more likely to happen when welding in the vertical position.
Joint preparation
Too narrow a joint preparation often causes the arc to be attracted to one of the side
walls causing lack of side wall fusion on the other side of the joint or inadequate
penetration into the previously deposited weld bead. Too great an arc length may also
increase the risk of preferential melting along one side of the joint and cause shallow
penetration. In addition, a narrow joint preparation may prevent adequate access into
the joint. For example, this happens in MMA welding when using a large diameter
electrode, or in MIG welding where an allowance should be made for the size of the
Welding parameters
It is important to use a sufficiently high current for the arc to penetrate into the joint
sidewall. Consequently, too high a welding speed for the welding current will increase
the risk of these imperfections. However, too high a current or too low a welding
speed will cause weld pool flooding ahead of the arc resulting in poor or non-uniform
Welder technique
Poor welder technique such as incorrect angle or manipulation of the
electrode/welding gun, will prevent adequate fusion of the joint sidewall. Weaving,
especially dwelling at the joint sidewall, will enable the weld pool to wash into the
parent metal, greatly improving sidewall fusion. It should be noted that the amount of
weaving might be restricted by the welding procedure specification limiting the arc
energy input, particularly when welding alloy or high notch toughness steels.
Magnetic arc blow
When welding ferromagnetic steels lack of fusion imperfections can be caused
through uncontrolled deflection of the arc, usually termed arc blow. Arc deflection
can be caused by distortion of the magnetic field produced by the arc current (Fig. 3),
Residual magnetism in the material through using magnets for handling
Earth’s magnetic field, for example in pipeline welding
Position of the current return
The effect of welding past the current return cable, which is bolted to the centre of the
place, is shown in Fig. 4. The interaction of the magnetic field surrounding the arc and
that generated by the current flow in the plate to the current return cable is sufficient
to deflect the weld bead. Distortion of the arc current magnetic field can be minimised
by positioning the current return so that welding is always towards or away from the
clamp and, in MMA welding, by using AC instead of DC. Often the only effective
means is to demagnetise the steel before welding.
Fig. 3. Interaction of magnetic forces
causing arc deflection
Fig. 4. Weld bead deflection in DC
MMA welding caused by welding
past the current return connection
Best practice in prevention
The following fabrication techniques can be used to prevent formation of lack of
sidewall fusion imperfections:
Use a sufficiently wide joint preparation
Select welding parameters (high current level, short arc length, not too high a
welding speed) to promote penetration into the joint side wall without causing
Ensure the electrode/gun angle and manipulation technique will give adequate
side wall fusion
Use weaving and dwell to improve side wall fusion providing there are no heat
input restrictions
If arc blow occurs, reposition the current return, use AC (in MMA welding) or
demagnetise the steel
Acceptance standards
The limits for incomplete fusion imperfections in arc-welded joints in steel are
specified in BS EN 25817 (ISO 5817) for the three quality levels (see Table). These
types of imperfection are not permitted for Quality Level B (stringent) and C
(intermediate). For Quality level D (moderate) they are only permitted providing they
are intermittent and not surface breaking.
For arc-welded joints in aluminium, long imperfections are not permitted for all threequality levels. However, for quality levels C and D, short imperfections are permitted
but the total length of the imperfections is limited depending on the butt weld or the
fillet weld throat thickness.
Acceptance limits for specific codes and application standards
Acceptance limit
Level B and C not permitted.
ISO 5817:1992
Level D intermittent and not surface breaking.
Levels B, C, D.
Long imperfections not permitted.
ISO 10042:1992
Levels C and D.
Short imperfections permitted.
Pressure vessels BS5500: 1997
Not permitted
Storage tanks
BS2654: 1989
Not permitted
'L' not greater than 15mm
Pipe work
BS2633: 1987
(depending on wall thickness)
'L' not greater than 25mm
Line pipe
API 1104:1983
(less when weld length <300mm)
Detection and remedial action
If the imperfections are surface breaking, they can be detected using a penetrant or
magnetic particle inspection technique. For sub-surface imperfections, detection is by
radiography or ultrasonic inspection. Ultrasonic inspection is normally more effective
than radiography in detecting lack of inter-run fusion imperfections.
Remedial action will normally require their removal by localised gouging, or
grinding, followed by re-welding as specified in the agreed procedure.
If lack of fusion is a persistent problem, and is not caused by magnetic arc blow, the
welding procedures should be amended or the welders retrained.
This information was prepared by Bill Lucas with help from Gene Mathers.
Copies of other articles in the 'Job knowledge for welders' series can be found under
Practical Joining Knowledge or by using the search engine.
Defects - solidification cracking
Weld repair on a cast iron exhaust manifold
A crack may be defined as a local discontinuity produced by a fracture, which can
arise from the stresses, generated on cooling or acting on the structure. It is the most
serious type of imperfection found in a weld and should be removed. Cracks not only
reduce the strength of the weld through the reduction in the cross section thickness but
also can readily propagate through stress concentration at the tip, especially under
impact loading or during service at low temperature.
Visual appearance
Solidification cracks are normally readily distinguished from other types of cracks
due to the following characteristic factors:
They occur only in the weld metal
They normally appear as straight lines along the centreline of the weld bead,
as shown in Fig. 1, but may occasionally appear as transverse cracking
depending on the solidification structure
Solidification cracks in the final crater may have a branching appearance
As the cracks are 'open', they are easily visible with the naked eye
Fig. 1 Solidification crack
along the centre line of the
On breaking open the weld, the crack surface in steel and nickel alloys may have a
blue oxidised appearance, showing that they were formed while the weld metal was
still hot.
The cracks form at the solidification boundaries and are characteristically inter
dendritic. The morphology reflects the weld solidification structure and there may be
evidence of segregation associated with the solidification boundary.
The overriding cause of solidification cracking is that the weld bead in the final stage
of solidification has insufficient strength to withstand the contraction stresses
generated as the weld pool solidifies. Factors that increase the risk include:
Insufficient weld bead size or shape
Welding under high restraint
Material properties such as high impurity content or a relatively large amount
of shrinkage on solidification.
Joint design can have a significant influence on the level of residual stresses. Large
gaps between component parts will increase the strain on the solidifying weld metal,
especially if the depth of penetration is small. Therefore, weld beads with a small
depth-to-width ratio, such as formed in bridging a large gap with a wide, thin bead,
will be more susceptible to solidification cracking, as shown in Fig. 2. In this case, the
centre of the weld, which is the last part to solidify, is a narrow zone with negligible
cracking resistance.
Fig. 2 Weld bead penetration too small
Segregation of impurities to the centre of the weld also encourages cracking.
Concentration of impurities ahead of the solidifying front weld forms a liquid film of
low freezing point that, on solidification, produces a weak zone. As solidification
proceeds, the zone is likely to crack as the stresses through normal thermal
contraction build up. An elliptically shaped weld pool is preferable to a teardrop
shape. Welding with contaminants such as cutting oils on the surface of the parent
metal will also increase the build up of impurities in the weld pool and the risk of
As the compositions of the plate and the filler determine the weld metal composition
they will, therefore, have a substantial influence on the susceptibility of the material
to cracking.
Cracking is associated with impurities, particularly sulphur and phosphorus, and is
promoted by carbon whereas manganese and silicon can help to reduce the risk. To
minimise the risk of cracking, fillers with low carbon and impurity levels and
relatively high manganese content are preferred. As a general rule, for carbonmanganese steels, the total sulphur and phosphorus content should be no greater than
Weld metal composition is dominated by the consumable and as the filler is normally
cleaner than the metal being welded, cracking is less likely with low dilution
processes such as MMA and MIG. Plate composition assumes greater importance in
high dilution situations such as when welding the root in butt welds, using an
autogenous welding technique like TIG, or a high dilution process such as submerged
arc welding.
In submerged arc welds, as described in BS 5135 (Appendix F), the cracking risk may
be assessed by calculating the Units of Crack Susceptibility (UCS) from the weld
metal chemical composition (weight %):
UCS = 230C* + 190S + 75P + 45Nb - 12.3Si - 5.4Mn - 1
C* = carbon content or 0.08 whichever is higher
Although arbitrary units, a value of <10 indicates high cracking resistance whereas
>30 indicates a low resistance. Within this range, the risk will be higher in a weld run
with a high depth to width ratio, made at high welding speeds or where the fit-up is
poor. For fillet welds, runs having a depth to width ratio of about one, UCS values of
20 and above will indicate a risk of cracking. For a butt weld, values of about 25 UCS
are critical. If the depth to width ratio is decreased from 1 to 0.8, the allowable UCS is
increased by about nine. However, very low depth to width ratios, such as obtained
when penetration into the root is not achieved, also promote cracking.
The high thermal expansion (approximately twice that of steel) and substantial
contraction on solidification (typically 5% more than in an equivalent steel weld)
means that aluminium alloys are more prone to cracking. The risk can be reduced by
using a crack resistant filler (usually from the 4xxx and 5xxx series alloys) but the
disadvantage is that the resulting weld metal is likely to have non-matching properties
such as a lower strength than the parent metal.
Austenitic Stainless Steel
A fully austenitic stainless steel weld is more prone to cracking than one containing
between 5-10% of ferrite. The beneficial effect of ferrite has been attributed to its
capacity to dissolve harmful impurities that would otherwise form low melting point
segregates and consequently interdendritic cracks. Therefore the choice of filler
material is important to suppress cracking so type 308 filler is used to weld type 304
stainless steel.
Best practice in avoiding solidification cracking
Apart from the choice of material and filler, the principal techniques for minimising
the risk of welding solidification cracking are:
Control joint fit-up to reduce gaps.
Before welding, clean off all contaminants from the material
Ensure that the welding sequence will not lead to a build-up of thermally
induced stresses.
Select welding parameters and technique to produce a weld bead with an
adequate depth to width ratio, or with sufficient throat thickness (fillet weld),
to ensure the weld bead has sufficient resistance to the solidification stresses
(recommend a depth to width ratio of at least 0.5:1).
Avoid producing too large a depth to width ratio that will encourage
segregation and excessive transverse strains in restrained joints. As a general
rule, weld beads whose depth to weld ratio exceeds 2:1 will be prone to
solidification cracking.
Avoid high welding speeds (at high current levels), which increase the amount
of segregation and the stress level across the weld bead.
At the run stop, ensure adequate filling of the crater to avoid an unfavourable
concave shape.
Acceptance standards
As solidification cracks are linear imperfections with sharp edges, they are not
permitted for welds meeting the quality levels B, C and D in accordance with the
requirements of BS EN 25817 (ISO 5817). Crater cracks are permitted for quality
level D.
Detection and remedial action
Surface breaking solidification cracks can be readily detected using visual
examination, liquid penetrant or magnetic particle testing techniques. Internal cracks
require ultrasonic or radiographic examination techniques.
Most codes will specify that all cracks should be removed. A cracked component
should be repaired by removing the cracks with a safety margin of approximately
5mm beyond the visible ends of the crack. The excavation is then re-welded using
filler that will not produce a crack sensitive deposit.
Defects - hydrogen cracks in steels identification
Preheating to avoid hydrogen
Hydrogen cracking may also be called cold cracking or delayed cracking. The
principal distinguishing feature of this type of crack is that it occurs in ferritic steels,
most often immediately on welding or after a short time after welding.
In this issue, the characteristic features and principal causes of hydrogen cracks are
Visual appearance
Hydrogen cracks can be usually be distinguished due to the following characteristics:
In C-Mn steels, the crack will normally originate in the heat-affected zone
(HAZ) but may extend into the weld metal (Fig 1).
Cracks can also occur in the weld bead, normally transverse to the welding
direction at an angle of 45° to the weld surface. They are essentially straight,
follow a jagged path but may be non-branching.
In low alloy steels, the cracks can be transverse to the weld, perpendicular to
the weld surface, but are non-branching and essentially planar.
Fig. 1 Hydrogen cracks originating in
the HAZ (note, the type of cracks shown
would not be expected to form in the
same weldment)
On breaking open the weld (prior to any heat treatment), the surface of the cracks will
normally not be oxidised, even if they are surface breaking, indicating they were
formed when the weld was at or near ambient temperature. A slight blue tinge may be
seen from the effects of preheating or welding heat.
Cracks that originate in the HAZ are usually associated with the coarse grain region,
(Fig 2). The cracks can be intergranular, transgranular or a mixture. Intergranular
cracks are more likely to occur in the harder HAZ structures formed in low alloy and
high carbon steels. Transgranular cracking is more often found in C-Mn steel
In fillet welds, cracks in the HAZ are usually associated with the weld root and
parallel to the weld. In butt welds, the HAZ cracks are normally oriented parallel to
the weld bead.
Fig. 2 Crack along the coarse
grain structure in the HAZ
There are three factors that combine to cause cracking:
Hydrogen generated by the welding process
A hard brittle structure which is susceptible to cracking
Residual tensile stresses acting on the welded joint
Cracking is caused by the diffusion of hydrogen to the highly stressed, hardened part
of the weldment.
In C-Mn steels, because there is a greater risk of forming a brittle microstructure in
the HAZ, most of the hydrogen cracks are to be found in the parent metal. With the
correct choice of electrodes, the weld metal will have a lower carbon content than the
parent metal and, hence, a lower carbon equivalent (CE). However, transverse weld
metal cracks can occur especially when welding thick section components.
In low alloy steels, as the weld metal structure is more susceptible than the HAZ,
cracking may be found in the weld bead.
The effects of specific factors on the risk of cracking are:
Weld metal hydrogen
Parent material composition
Parent material thickness
Stresses acting on the weld
Heat input
Weld metal hydrogen content
The principal source of hydrogen is the moisture contained in the flux i.e. the coating
of MMA electrodes, the flux in cored wires and the flux used in submerged arc
welding. Mainly the electrode type determines the amount of hydrogen generated.
Basic electrodes normally generate less hydrogen than rutile and cellulosic electrodes.
It is important to note that there can be other significant sources of hydrogen e.g.
moisture from the atmosphere or from the material where processing or service
history has left the steel with a significant level of hydrogen. Hydrogen may also be
derived from the surface of the material or the consumable.
Sources of hydrogen will include:
Oil, grease and dirt
Paint and coatings
Cleaning fluids
Parent metal composition
This will have a major influence on hardenability and, with high cooling rates, the
risk of forming a hard brittle structure in the HAZ. The hardenability of a material
is usually expressed in terms of its carbon content or, when other elements are
taken into account, its carbon equivalent (CE) value.
The higher the CE value, the greater the risk of hydrogen cracking. Generally, steels
with a CE value of <0.4 are not susceptible to HAZ hydrogen cracking as long as low
hydrogen welding consumables or processes are used.
Parent material thickness
Material thickness will influence the cooling rate and therefore the hardness level,
microstructure produced in the HAZ and the level of hydrogen retained in the weld.
The 'combined thickness' of the joint, i.e. the sum of the thicknesses of material
meeting at the joint line, will determine, together with the joint geometry, the cooling
rate of the HAZ and its hardness. Consequently, as shown in Fig. 3, a fillet weld will
have a greater risk than a butt weld in the same material thickness.
Fig.3 Combined thickness
measurements for butt and fillet joints
Stresses acting on the weld
The stresses generated across the welded joint as it contracts will be greatly
influenced by external restraint, material thickness, joint geometry and fit-up. Areas
of stress concentration are more likely to initiate a crack at the toe and root of the
Poor fit-up in fillet welds markedly increases the risk of cracking. The degree of
restraint acting on a joint will generally increase as welding progresses due to the
increase in stiffness of the fabrication.
Heat input
The heat input to the material from the welding process, together with the material
thickness and preheat temperature, will determine the thermal cycle and the resulting
microstructure and hardness of both the HAZ and weld metal.
A high heat input will reduce the hardness level.
Heat input per unit length is calculated by multiplying the arc energy by an arc
efficiency factor according to the following formula:
V = arc voltage (V)
A = welding current (A)
S = welding speed (mm/min)
k = thermal efficiency factor
In calculating heat input, the arc efficiency must be taken into consideration. The arc
efficiency factors given in BS EN 1011-1: 1998 for the principal arc welding
processes are:
Submerged arc
(single wire)
MIG/MAG and flux cored wire
TIG and plasma
In MMA welding, heat input is normally controlled by means of the run-out length
from each electrode that is proportional to the heat input. As the run-out length is the
length of weld deposited from one electrode, it will depend upon the welding
technique e.g. weave width /dwell.
Defects - hydrogen cracks in steels prevention and best practice
Preheating of a jacket structure to prevent
hydrogen cracking
In this issue, techniques and practical guidance on the avoidance of hydrogen cracks
are described.
Preheating, interpass and post heating to prevent
hydrogen cracking
There are three factors that combine to cause cracking in arc welding:
Hydrogen generated by the welding process
A hard brittle structure which is susceptible to cracking
Residual tensile stresses acting on the welded joint
In practice, for a given situation (material composition, material thickness, joint type,
electrode composition and heat input), the risk of hydrogen cracking is reduced by
heating the joint.
Preheat, which slows the cooling rate, allows some hydrogen to diffuse away and
prevents a hard, crack-sensitive structure being formed. The recommended levels of
preheat for carbon and carbon manganese steel are detailed in BS 5135. (Nb a draft
European standard Pr EN 1011-2 is expected to be introduced in 2000). The preheat
level may be as high as 200°C for example, when welding thick section steels with a
high carbon equivalent (CE) value.
Interpass and post heating
As cracking rarely occurs at temperatures above ambient, maintaining the temperature
of the weldment during fabrication is equally important. For susceptible steels, it is
usually appropriate to maintain the preheat temperature for a given period, typically
between 2 to 3 hours, to enable the hydrogen to diffuse away from the weld area. In
crack sensitive situations such as welding higher CE steels or under high restraint
conditions, the temperature and heating period should be increased, typically 250300°C for three to four hours.
Post weld heat treatment (PWHT) may be used immediately on completion of
welding i.e. without allowing the preheat temperature to fall. However, in practice, as
inspection can only be carried out at ambient temperature, there is the risk that
'rejectable,' defects will only be found after PWHT. Also, for highly hardenable steels,
a second heat treatment may be required to temper the hard microstructure present
after the first PWHT.
Under certain conditions, more stringent procedures are needed to avoid cracking than
those derived from the nomograms for estimating preheat in BS 5135. Appendix E of
this standard mentions the following conditions:
a. High restraint
b. Thick sections ( approximately 50mm)
c. Low carbon equivalent steels (CMn steels with C 0.1% and CE
approximately 0.42)
d. 'Clean’ or low sulphur steels (S approximately 0.008%), as a low sulphur and
low oxygen content will increase the hardenability of steel.
e. Alloyed weld metal where preheat levels to avoid HAZ cracking may be
insufficient to protect the weld metal. Low hydrogen processes and
consumables should be used. Schemes for predicting the preheat requirements
to avoid weld metal cracking generally require the weld metal diffusible
hydrogen level and the weld metal tensile strength as input.
Use of austenitic and nickel alloy weld metal to
prevent cracking
In situations where preheating is impractical, or does not prevent cracking, it will be
necessary to use an austenitic consumable. Austenitic stainless steel and nickel
electrodes will produce a weld metal, which at ambient temperature has a higher
solubility for hydrogen than ferritic steel. Thus, any hydrogen formed during welding
becomes locked in the weld metal with very little diffusing to the HAZ on cooling to
A commonly used austenitic MMA electrode is 23Cr: 12Ni (e.g. from BS 2926:1984).
However, as nickel alloys have a lower coefficient of thermal expansion than stainless
steel, nickel austenitic electrodes are preferred when welding highly restrained joints
to reduce the shrinkage strain. Figure 1 is a general guide on the levels of preheat
when using austenitic electrodes. When welding steels with up to 0.2%C, a preheat
would not normally be required. However, above 0.4%C a minimum temperature of
150°C will be needed to prevent HAZ cracking. The influence of hydrogen level and
the degree of restraint are also illustrated in the figure.
Fig.1 Guide to preheat
temperature when using
austenitic MMA electrodes at
a) low restraint (e.g. material
thickness <30mm)
b) high restraint (e.g. material
thickness >30mm)
Best practice in avoiding hydrogen cracking
Reduction in weld metal hydrogen
The most effective means of avoiding hydrogen cracking is to reduce the amount of
hydrogen generated by the consumable, i.e. by using a low hydrogen process or low
hydrogen electrodes.
Welding processes can be classified as very low, low, medium or high depending on
the amount of weld metal hydrogen produced:
Very low
5 - 10ml/100g
10 - 15ml/100g
Figure 2 illustrates the relative amounts of weld metal hydrogen produced by the
major welding processes. MMA, in particular, has the potential to generate a wide
range of hydrogen levels. Thus, to achieve the lower values, it is essential that basic
electrodes are used and they are baked in accordance with the manufacturer's
recommendations. For the MIG process, cleaner wires will be required to achieve
very low hydrogen levels.
Fig.2 General relationships
between potential hydrogen and
weld metal hydrogen levels for
arc welding processes
General guidelines
The following general guidelines are recommended for the various types of steel but
requirements for specific steels should be checked according to BS 5135 or BS EN
Mild steel (CE <0.4)
- Readily weldable, preheat generally not required if low hydrogen processes
or electrodes are used
- Preheat may be required when welding thick section material, high restraint
and with higher levels of hydrogen being generated
C-Mn, medium carbon, low alloy steels (CE 0.4 to 0.5)
- Thin sections can be welded without preheat but thicker sections will require
low preheat levels and low hydrogen processes or electrodes should be used
Higher carbon and alloyed steels (CE >0.5)
- Preheat, low hydrogen processes or electrodes, post weld heating and slow
cooling required.
More detailed guidance on the avoidance of hydrogen cracking is described in
BS 5135.
Practical Techniques
The following practical techniques are recommended to avoid hydrogen cracking:
Clean the joint faces and remove contaminants such as paint, cutting oils,
Use a low hydrogen process if possible
Dry the electrodes (MMA) or the flux (submerged arc) in accordance with the
manufacturer's recommendations
Reduce stresses on the weld by avoiding large root gaps and high restraint
If preheating is specified in the welding procedure, it should also be applied
when tacking or using temporary attachments
Preheat the joint to a distance of at least 75mm from the joint line ensuring
uniform heating through the thickness of the material
Measure the preheat temperature on the face opposite that being heated.
Where this is impractical, allow time for the equalisation of temperature after
removing the preheating before the temperature is measured
Adhere to the heat input requirements
Maintain heat for approximately two to four hours after welding depending on
crack sensitivity
In situations where adequate preheating is impracticable, or cracking cannot
be avoided, austenitic electrodes may be used
Acceptance standards
As hydrogen cracks are linear imperfections, which have sharp edges, they are not
permitted for welds meeting the quality levels B, C and D in accordance with the
requirements of BS EN 25817 (ISO 5817).
Detection and remedial action
As hydrogen cracks are often very fine and may be sub-surface, they can be difficult
to detect. Surface-breaking hydrogen cracks can be readily detected using visual
examination, liquid penetrant or magnetic particle testing techniques. Internal cracks
require ultrasonic or radiographic examination techniques. Ultrasonic examination is
preferred as radiography is restricted to detecting relatively wide cracks parallel to the
Most codes will specify that all cracks should be removed. A cracked component
should be repaired by removing the cracks with a safety margin of approximately
5mm beyond the visible ends of the crack. The excavation is then re-welded.
To make sure that cracking does not re-occur, welding should be carried out with the
correct procedure, i.e. preheat and an adequate heat input level for the material type
and thickness. However, as the level of restraint will be greater and the interpass time
shorter when welding within an excavation compared to welding the original joint, it
is recommended that a higher level of preheat is used (typically by 50°C).
BS 5135:1984 Arc Welding of Carbon and Carbon Manganese Steels
Pr EN 1011-1:1998 Welding - Recommendations for Welding of Metallic Materials
Part 1- General Guidance for Arc Welding
Part 2- Arc Welding of Ferritic Steels
BS EN ISO 13916: 1997 Welding - Guidance on the Measurement of Preheating
Temperature, Interpass Temperature and Preheat Maintenance Temperature
N Bailey et al, Welding steels without hydrogen cracking, Woodhead Publishing,
Defects - lamellar tearing
BP Forties
tears were
the repair
of lack of
in a brace
Lamellar tearing can occur beneath the weld especially in rolled steel plate which has
poor through-thickness ductility. The characteristic features, principal causes and best
practice in minimising the risk of lamellar tearing are described.
Visual appearance
The principal distinguishing feature of lamellar tearing is that it occurs in T-butt and
fillet welds normally observed in the parent metal parallel to the weld fusion
boundary and the plate surface , (Fig 1). The cracks can appear at the toe or root of
the weld but are always associated with points of high stress concentration.
Fracture face
The surface of the fracture is fibrous and 'woody' with long parallel sections which are
indicative of low parent metal ductility in the through-thickness direction, (Fig 2).
Fig. 1. Lamellar tearing in T
butt weld
Fig. 2. Appearance of fracture
face of lamellar tear
As lamellar tearing is associated with a high concentration of elongated inclusions
oriented parallel to the surface of the plate, tearing will be transgranular with a
stepped appearance.
It is generally recognised that there are three conditions which must be satisfied for
lamellar tearing to occur:
1. Transverse strain - the shrinkage strains on welding must act in the short
direction of the plate ie through the plate thickness
2. Weld orientation - the fusion boundary will be roughly parallel to the plane of
the inclusions
3. Material susceptibility - the plate must have poor ductility in the throughthickness direction
Thus, the risk of lamellar tearing will be greater if the stresses generated on welding
act in the through-thickness direction. The risk will also increase the higher the level
of weld metal hydrogen
Factors to be considered to reduce the risk of tearing
The choice of material, joint design, welding process, consumables, preheating and
buttering can all help reduce the risk of tearing.
Fig. 3.
between the
STRA and
content for
12.5 to
50mm thick
Tearing is only encountered in rolled steel plate and not forgings and castings. There
is no one grade of steel that is more prone to lamellar tearing but steels with a low
Short Transverse Reduction in Area (STRA) will be susceptible. As a general rule,
steels with STRA over 20% are essentially resistant to tearing whereas steels with
below 10 to 15% STRA should only be used in lightly restrained joints (Fig. 3).
Steels with a higher strength have a greater risk especially when the thickness is
greater than 25mm. Aluminium treated steels with low sulphur contents (<0.005%)
will have a low risk.
Steel suppliers can provide plate which has been through-thickness tested with a
guaranteed STRA value of over 20%.
Joint Design
Lamellar tearing occurs in joints producing high through-thickness strain, eg T joints
or corner joints. In T or cruciform joints, full penetration butt welds will be
particularly susceptible. The cruciform structures in which the susceptible plate
cannot bend during welding will also greatly increase the risk of tearing.
In butt joints, as the stresses on welding do not act through the thickness of the plate,
there is little risk of lamellar tearing.
As angular distortion can increase the strain in the weld root and or toe, tearing may
also occur in thick section joints where the bending restraint is high.
Several examples of good practice in the design of welded joints are illustrated in Fig.
As tearing is more likely to occur in full penetration T butt joints, if possible,
use two fillet welds, Fig. 4a.
Double-sided welds are less susceptible than large single-sided welds and
balanced welding to reduce the stresses will further reduce the risk of tearing
especially in the root, Fig. 4b
Large single-side fillet welds should be replaced with smaller double-sided
fillet welds, Fig. 4c
Redesigning the joint configuration so that the fusion boundary is more
normal to the susceptible plate surface will be particularly effective in
reducing the risk, Fig. 4d
Fig. 4 Recommended joint configurations to reduce the risk of lamellar tearing
Fig. 4a
Fig. 4b
Fig. 4c
Fig. 4d
Weld size
Lamellar tearing is more likely to occur in large welds typically when the leg length
in fillet and T butt joints is greater than 20mm. As restraint will contribute to the
problem, thinner section plate which is less susceptible to tearing, may still be at risk
in high restraint situations.
Welding process
As the material and joint design are the primary causes of tearing, the choice of
welding process has only a relatively small influence on the risk. However, higher
heat input processes which generate lower stresses through the larger HAZ and deeper
weld penetration can be beneficial.
As weld metal hydrogen will increase the risk of tearing, a low hydrogen process
should be used when welding susceptible steels.
Where possible, the choice of a lower strength consumable can often reduce the risk
by accommodating more of the strain in the weld metal. A smaller diameter electrode
which can be used to produce a smaller leg length, has been used to prevent tearing.
A low hydrogen consumable will reduce the risk by reducing the level of weld metal
diffusible hydrogen. The consumables must be dried in accordance with the
manufacturer's recommendations.
Preheating will have a beneficial effect in reducing the level of weld metal diffusible
hydrogen. However, it should be noted that in a restrained joint, excessive preheating
could have a detrimental effect by increasing the level the level of restraint produced
by the contraction across the weld on cooling.
Preheating should, therefore, be used to reduce the hydrogen level but it should be
applied so that it will not increase the amount of contraction across the weld.
Buttering the surface of the susceptible plate with a low strength weld metal has been
widely employed. As shown for the example of a T butt weld (Fig. 5) the surface of
the plate may be grooved so that the buttered layer will extend 15 to 25mm beyond
each weld toe and be about 5 to 10mm thick.
Fig. 5. Buttering with low strength weld metal
a) general deposit on the surface of
the susceptible plate
b) in-situ buttering
In-situ buttering ie where the low strength weld metal is deposited first on the
susceptible plate before filling the joint, has also been successfully applied. However,
before adopting this technique, design calculations should be carried out to ensure that
the overall weld strength will be acceptable.
Acceptance standards
As lamellar tears are linear imperfections which have sharp edges, they are not
permitted for welds meeting the quality levels B, C and D in accordance with the
requirements of BS EN 25817 (ISO 5817).
Detection and remedial action
If surface-breaking, lamellar tears can be readily detected using visual examination,
liquid penetrant or magnetic particle testing techniques. Internal cracks require
ultrasonic examination techniques but there may be problems in distinguishing
lamellar tears from inclusion bands. The orientation of the tears normally makes them
almost impossible to detect by radiography.
Defects/imperfections in welds - reheat
Brittle fracture in
CrMoV steel pressure
vessel probably
caused through poor
toughness, high
residual stresses and
hydrogen cracking
The characteristic features and principal causes of reheat cracking are described.
General guidelines on 'best practice' are given so that welders can minimise the risk of
reheat cracking in welded fabrications.
Visual appearance
Reheat cracking may occur in low alloy steels containing alloying additions of
chromium, vanadium and molybdenum when the welded component is being
subjected to post weld heat treatment, such as stress relief heat treatment, or has been
subjected to high temperature service (typically 350 to 550°C).
Cracking is almost exclusively found in the coarse grained regions of the heat affected
zone (HAZ) beneath the weld, or cladding, and in the coarse grained regions within
the weld metal. The cracks can often be seen visually, usually associated with areas of
stress concentration such as the weld toe.
Cracking may be in the form of coarse macro-cracks or colonies of micro-cracks.
A macro-crack will appear as a 'rough' crack, often with branching, following the
coarse grain region, (Fig. 1a). Cracking is always intergranular along the prior
austenite grain boundaries (Fig. 1b). Macro-cracks in the weld metal can be oriented
either longitudinal or transverse to the direction of welding. Cracks in the HAZ,
however, are always parallel to the direction of welding.
Fig.1a. Cracking
associated with the
coarse grained heat
affected zone
morphology of
reheat cracks
Micro-cracking can also be found both in the HAZ and within the weld metal. Microcracks in multipass welds will be found associated with the grain coarsened regions
which have not been refined by subsequent passes.
The principal cause is that when heat treating susceptible steels, the grain interior
becomes strengthened by carbide precipitation forcing the relaxation of residual
stresses by creep deformation at the grain boundaries.
The presence of impurities which segregate to the grain boundaries and promote
temper embrittlement eg sulphur, arsenic, tin and phosphorus, will increase the
susceptibility to reheat cracking.
The joint design can increase the risk of cracking. For example, joints likely to
contain stress concentration, such as partial penetration welds, are more liable to
initiate cracks.
The welding procedure also has an influence. Large weld beads are undesirable as
they produce a coarse grained HAZ which is less likely to be refined by the
subsequent pass and therefore will be more susceptible to reheat cracking.
Best practice in prevention
The risk of reheat cracking can be reduced through the choice of steel, specifying the
maximum impurity level and by adopting a more tolerant welding procedure /
Steel choice
If possible, avoid welding steels known to be susceptible to reheat cracking. For
example, A 508 Class 2 is known to be particularly susceptible to reheat cracking
whereas cracking associated with welding and cladding in A508 Class 3 is largely
unknown. The two steels have similar mechanical properties but A508 Class 3 has a
lower Cr content and a higher manganese content.
Similarly, in the higher strength, creep resistant steels, an approximate ranking of
their crack susceptibility is as follows:
5 Cr 1Mo
lower risk
2.25Cr 1 Mo
0.5Mo B
0.5Cr 0.5Mo 0.25V higher risk
Thus, in selecting a creep resistant, chromium molybdenum steel, 0.5Cr 0.5Mo 0.25V
steel is known to be susceptible to reheat cracking but the 2.25Cr 1Mo which has a
similar creep resistance, is significantly less susceptible.
Unfortunately, although some knowledge has been gained on the susceptibility of
certain steels, the risk of cracking cannot be reliably predicted from the chemical
composition. Various indices, including G1, PSR and Rs, have been used to indicate
the susceptibility of steel to reheat cracking. Steels which have a value of G of less
than 2, PSR less than zero or Rs less than 0.03, are less susceptible to reheat cracking
G1 = 10C + Cr + 3.3Mo + 8.1V - 2
= Cr +Cu + 2Mo + 10V +7Nb + 5Ti - 2
= 0.12Cu +0.19S +0.10As + P +1.18Sn + 1.49Sb
Impurity level
Irrespective of the steel type, it is important to purchase steels specified to have low
levels of trace elements (antimony, arsenic, tin and phosphorus). It is generally
accepted that the total level of impurities in the steel should not exceed 0.01% to
minimise the risk of temper embrittlement.
Welding procedure and technique
The welding procedure can be used to minimise the risk of reheat cracking by
Producing the maximum refinement of the coarse grain HAZ
Limiting the degree of austenite grain growth
Eliminating stress concentrations
The procedure should aim to refine the coarse grained HAZ by subsequent passes. In
butt welds, maximum refinement can be achieved by using a steep sided joint
preparation with a low angle of attack to minimise penetration into the sidewall, (Fig
2a). In comparison, a larger angle V preparation produces a wider HAZ limiting the
amount of refinement achieved by subsequent passes, (Fig 2b). Narrow joint
preparations, however, are more difficult to weld due to the increased risk of lack of
sidewall fusion.
Fig.2a. Welding in the flat
position - high degree of HAZ
Fig.2b. Welding in the
horizontal/vertical position low degree of HAZ refinement
Refinement of the HAZ can be promoted by first buttering the surface of the
susceptible plate with a thin weld metal layer using a small diameter (3.2mm)
electrode. The joint is then completed using a larger diameter (4 - 4.8mm) electrode
which is intended to generate sufficient heat to refine any remaining coarse grained
HAZ under the buttered layer.
The degree of austenite grain growth can be restricted by using a low heat input.
However, precautionary measures may be necessary to avoid the risk of hydrogen
assisted cracking and lack-of-fusion defects. For example, reducing the heat input will
almost certainly require a higher preheat temperature to avoid hydrogen assisted
The joint design and welding technique adopted should ensure that the weld is free
from localised stress concentrations which can arise from the presence of notches.
Stress concentrations may be produced in the following situations:
welding with a backing bar
a partial penetration weld leaving a root imperfection
internal weld imperfections such as lack of sidewall fusion
the weld has a poor surface profile, especially sharp weld toes
The weld toes of the capping pass are particularly vulnerable as the coarse grained
HAZ may not have been refined by subsequent passes. In susceptible steel, the last
pass should never be deposited on the parent material but always on the weld metal so
that it will refine the HAZ.
Grinding the weld toes with the preheat maintained has been successfully used to
reduce the risk of cracking in 0.5Cr 0.5Mo 0.25V steels.
Gouging processes
Gouging operations can be carried out using the following thermal processes:
Oxyfuel gas flame
Manual metal arc
Air carbon arc
Plasma arc
Thermal Gouging
Thermal gouging is an essential part of welding fabrication. Used for rapid removal of
unwanted metal, the material is locally heated and molten metal ejected - usually by
blowing it away. Normal oxyfuel gas or arc processes can be used to produce rapid
melting and metal removal. However, to produce a groove of specific dimensions,
particularly regarding depth and width, the welder must exercise careful control of the
gouging operation. If this does not happen, an erratic and badly serrated groove will
Thermal processes, operations and metals which may be gouged or otherwise shaped:
Thermal Process operations
process Primary Secondary
gas flame
Low carbon steels, carbon manganese steels
(structural), pressure vessel steels (carbon not
Gouging Washing
over 0.35%), low alloy steels (less than 5%Cr)
cast iron (if preheated to 400-450 deg.C)
metal arc
Low carbon steels carbon manganese steels
Grooving (structural), pressure vessel steels, low alloy
Chamfering steels, stainless steels, cast iron, nickel-based
Low carbon steels carbon manganese steels
(structural), pressure vessel steels, low and high
Air carbon
alloy steels, cast iron, nickel-based alloys, copper
and copper alloys, copper/nickel alloys,
Plasma arc Gouging Grooving Aluminium, stainless steels
Note: All processes are capable of cutting/severing operations. Preheat may or may
not be required on some metals prior to gouging
It should be emphasised that because gouging relies on molten metal being forcibly ejected, often over
quite large distances, the welder must take appropriate precautions to protect himself, other workers
and his equipment. Sensible precautions include protective clothing for the welder, shielding inside a
specially enclosed booth or screens, adequate fume extraction, and removal of all combustible material
from the immediate area.
Industrial applications
Thermal gouging was developed primarily for removal of metal from the reverse side of welded joints,
removal of tack welds, temporary welds, and weld imperfections. Figure 1 illustrates the value of
typical back-gouging applications carried out on arc welded joints., while Fig. 2 shows imperfection
removal in preparation for weld repair.
Fig.1 Typical back-gouging applications carried out on arc-welded joints
Fig. 2 Imperfection removal in preparation for weld repair
The gouging process has proved to be so successful that it is used for a wide spectrum of applications
in engineering industries:
Repair and maintenance of structures - bridges, earth-moving equipment, mining
machinery, railway rolling stock, ships, offshore rigs, piping and storage tanks
Removal of cracks and imperfections - blow holes and sand traps in both ferrous and nonferrous forgings and castings
Preparation of plate edges for welding
Removal of surplus metal - strongbacks, lifting lugs and riser pads and fins on castings,
excess weld bead profiles, temporary backing strips, rivet washing and shaping operations
demolition of welded and unwelded structures - site work
Thermal gouging is also suitable for efficient removal of temporary welded attachments such as
brackets, strongbacks, lifting lugs and redundant tack welds, during various stages of fabrication and
construction work.
Oxygen-fuel Gas Flame Gouging
Oxygen-fuel (oxyfuel) flame gouging offers fabricators a quick
and efficient method of removing metal. It can be at least four
times quicker than cold chipping operations. The process is
particularly attractive because of its low noise, ease of handling, and ability to be used
in all positions.
Process description
Flame gouging is a variant of conventional oxyfuel gas welding. Oxygen and a fuel
gas are used to produce a high temperature flame for melting the steel. When gouging,
the steel is locally heated to a temperature above the 'ignition' temperature (typically
900deg.C) and a jet of oxygen is used to melt the metal - a chemical reaction between
pure oxygen and hot metal. This jet is also used to blow away molten metal and slag.
It should be noted that compared with oxyfuel cutting, slag is not blown through the
material, but remains on the top surface of the workpiece.
The gouging nozzle is designed to supply a relatively large volume of oxygen through
the gouging jet. This can be as much as 300 litre/min through a 6mm orifice nozzle. In
oxyacetylene gouging, equal quantities of oxygen and acetylene are used to set a
neutral preheating flame. The oxygen jet flow rate determines the depth and width of
the gouge. Typical operating parameters (gas pressures and flow rates) for achieving a
range of gouge sizes (depth and width) can be seen in the Table.
Typical operating data for manual oxyacetylene flame
Gas pressure
Gas consumption
Nozzle dimensions
(mm) (mm) (Bar)
(Bar) (Litre/min) (Litre/min) (Litre/min)
10-13 10-13 0.55
When the preheating flame and oxygen jet are correctly set, the gouge has a uniform profile and its
surfaces are smooth with a dull blue colour.
Operating techniques
The depth of the gouge is determined principally by the speed and angle of the torch. To cut a deep
groove the angle of the torch is stepped up (this increases the impingement angle of the oxygen jet) and
gouging speed is reduced. To produce a shallow groove, the torch is less steeply angled, see above, and
speed is increased. Wide grooves can be produced by weaving the torch. The contour of the groove is
dependent upon the size of the nozzle and the operating parameters. If the cutting oxygen pressure is
too low, gouging progresses with a washing action, leaving smooth ripples in the bottom of the groove.
If the cutting oxygen pressure is too high, the cut advances ahead of the molten pool - this will disrupt
the gouging operation especially when making shallow grooves.
There are four basic flame-gouging techniques, which are used in the
following types of application.
Progressive gouging
This technique is used to produce uniform grooves. Gouging is conducted in
either a continuous or progressive manner. Applications include removal of
an unfused root area on the reverse side of a welded joint, part-shaping a steel
forging, complete removal of a weld deposit and preparing plate edges for
Spot gouging
Spot gouging produces a deep narrow U-shaped groove over a relatively short length. The process is
ideally suited to removal of localised areas such as isolated weld imperfections. Experienced operators
are able to observe any imperfections during gouging. These appear as dark or light spots/streaks
within the molten pool (reaction zone).
Back-step gouging
Once the material has reached ignition temperature, the oxygen stream is introduced and the torch
moved in a backward movement for a distance of 15-20mm. The oxygen is shut off and the torch
moved forward a distance of 25-30mm before restarting the gouging operation. This technique is
favoured for removal of local imperfections, which may be deeply embedded, in the base plate.
Deep gouging
It is sometimes necessary to produce a long deep gouge. Such operations are
completed using the deep gouging technique, which is basically a combination
of progressive and spot gouging.
Manual Metal Arc Gouging
The main advantage of manual metal arc (MMA) gouging is that it allows the
operator to switch easily from welding to gouging, or cutting, simply by
changing the type of electrode.
Process description
As in conventional MMA welding, the arc is formed between the tip of the electrode and the
workpiece. MMA gouging differs because it requires special purpose electrodes with thick flux
coatings to generate a strong arc force and gas stream. Unlike MMA welding where a stable weld pool
must be maintained, this process forces the molten metal away from the arc zone to leave a clean cut
The gouging process is characterised by the large amount of gas, which is generated to eject the molten
metal. However, because the arc/gas stream is not as powerful as a gas or a separate air jet, the surface
of the gouge is not really as smooth as an oxyfuel gouge or air carbon arc gouge.
According to the size of gouge specified, there is a wide range of electrode diameters available to
choose from. These grooving electrodes are also not just restricted to steels, and the same electrode
composition may be used for gouging stainless steel and non-ferrous alloys.
Power source
MMA gouging can be carried out using conventional DC and AC power sources. In DC gouging,
electrode polarity is normally negative but electrode manufacturers may well recommend electrode
polarity for their brand of electrodes and for gouging specific materials. When using an AC power
source, a minimum of 7OV open circuit (OCV) is required to stabilise the arc.
Although most MMA welding power sources can be used for gouging, the current rating and OCV
must be capable of accommodating current surges and longer arc lengths.
Typical operating data for MMA gouging
Electrode diameter
Gouging dimensions
Gouging speed
Operational characteristics
The arc is struck with an electrode, which is held at a normal angle to the workpiece (15 degrees
backwards from the vertical plane in line with proposed direction of gouging). Once the arc is
established, the electrode is immediately inclined in one smooth and continuous movement to an angle
of around 15-20 degrees to the plate surface. With the arc pointing in the direction of travel, the
electrode is pushed forward slightly to melt the metal. It should then be pulled back to allow the gas jet
to displace the molten metal and slag. This forward and backward motion is repeated as the electrode is
guided along the line to complete the gouge.
To produce a consistent depth and width of gouge, a uniform rate of travel must be maintained,
together with the angle of electrode: 10-20 degrees. If the electrode angle becomes too steep, in excess
of about 20 degrees, the amount of slag and molten metal will increase. This is a result of the arc
penetrating too deeply. Digging the electrode into the metal causes problems in controlling the gouging
operation and will produce a rough surface profile. For gouging in positions other than vertical, the
electrode is always pushed forward. With vertical surfaces, the electrode is directed and pushed
vertically downwards.
MMA gouging is used for localised gouging operations, removal of defects for example, and where it
is more convenient to switch from a welding electrode to a gouging electrode rather than use
specialised equipment. Compared with alternative gouging processes, metal removal rates are low and
When correctly applied, MMA gouging can produce relatively clean gouged surfaces. For general
applications welding can be carried out without the need to dress by grinding. However when gouging
stainless steel, a thin layer of higher carbon content material will be produced - this should be removed
by grinding.
Plasma Arc Gouging
The use of the plasma arc as a gouging tool dates back to the 1960s when the process was developed
for welding. Compared with the alternative oxyfuel and MMA gouging techniques, plasma arc has a
needle-like jet, which can produce a very precise groove, suitable for
application on almost all ferrous and non- ferrous materials.
Process description
Plasma arc gouging is a variant of the plasma arc process. The arc is formed
between a refractory (usually tungsten) electrode and the workpiece. Intense
plasma is achieved by constricting the arc using a fine bore copper nozzle. By
locating the electrode behind the nozzle, the plasma-forming gas can be
separated from the general gas supply used to cool the torch/assist the plasma gas to blow away molten
metal (dross) from the groove.
The temperature and force of the constricted plasma arc is determined by the current level and plasma
gas flow rate. Thus, the plasma can be varied to produce a hot gas stream or a high power, deeply
penetrating jet. This ability to control quite precisely the size and shape of a groove is very useful for
removing unwanted defects from a workpiece surface.
Whilst gouging, normal precautions should be taken to protect the operator and other workers in the
immediate area from the effects of intense are light and hot metal spray. Unlike the oxyfuel and MMA
processes, the plasma arc's high velocity jet will propel fume and hot metal dross some considerable
distance from the operator. When using a deeply penetrating arc, noise protection is an essential
The power source for sustaining this gouging arc must have a high open circuit voltage, usually well in
excess of 100V. The torch is connected to the negative polarity of the power source and the workpiece
must be connected to the positive. The plasma torch is the same as the one used for cutting; it will be
either gas or water-cooled and have the facility for single and dual gas operation.
Electrodes are normally tungsten for argon and argon-based gases. However, when using air as the
plasma gas, special purpose, for example hafnium tipped copper, electrodes must be used to withstand
the more aggressive, oxidising arc.
Plasma and cooling gases
Plasma gas can be argon, helium, argon - H2, nitrogen or air. Argon - 35%H2 is normally
recommended as a general- purpose plasma gas for cutting most materials. Alternative plasma gases
are argon and helium. Argon, a colder gas, will reduce metal removal rates. Helium, which generates a
hot but less intense arc than argon - H2, can produce a wider and shallower groove. Nitrogen and air
are also used as plasma gases, especially for gouging C-Mn steels. Although gas costs will be
substantially reduced, the groove surface profile will be inferior to that which can be achieved with
argon - H2 gas. Air is not recommended for gouging aluminium as this requires an inert or reducing
gas. Argon, nitrogen or air are all used as cooling gases. Use of argon will normally produce the best
quality of gouge, but nitrogen or air will reduce operating costs.
Operating techniques
Gouging is effected by moving the torch forward at a steady controlled rate. It is carried out in a
progressive manner to remove metal over a distance of 200 to 250mm. The jet can then be
repositioned, either to deepen or widen the groove, or to continue gouging for a further 200 to 250mm.
Principal process parameters are current level, gas flow rate, and speed of gouging. These settings
determine groove size and metal removal rate. In a typical gouging operation on C-Mn steel, metal is
removed at about 100 kg/hr at a speed of 0.5 m/min, and groove size will be around 12mm wide and
5mm deep.
The torch stand-off and its angle to the surface of the workpiece have a major influence on speed of
travel, groove profile and quality of surface. The torch is normally held at a distance of 20mm from the
workpiece and inclined backwards to the direction of gouging at an angle of 40 to 45 degrees. Gouging
will remove up to approximately 6mm depth of metal in a single pass.
The torch stand-off should not be reduced to less than 12mm, to avoid spatter build-up on the nozzle
from the molten particles ejected from the groove. At standoff distances greater than 25mm, arc/gas
forces are reduced and this lessens the depth of penetration of the jet. By reducing the torch angle to the
workpiece surface, the plasma jet can be encouraged to 'skate' along the surface of the workpiece; this
produces a shallower and wider groove. By increasing the angle of the torch the plasma jet is directed
into the workpiece surface, resulting in a deeper and narrower groove.
Air Carbon Arc Gouging
The main difference between this gouging technique and the others is that a separate air jet is used to
eject molten metal to form the groove.
Process description
Air carbon arc gouging works as follows.
An electric arc is generated between the tip
of a carbon electrode and the workpiece.
The metal becomes molten and a high
velocity air jet streams down the electrode
to blow it away, thus leaving a clean
groove. The process is simple to apply
(using the same equipment as MMA
welding), has a high metal removal rate,
and gouge profile can be closely controlled.
Disadvantages are that the air jet causes the
molten metal to be ejected over quite a
large distance and, because of high currents (up to 2000A) and high air pressures (80 to 100 psi), it can
be very noisy.
As air carbon arc gouging does not rely on oxidation it can be applied to a wide range of metals. DC
(electrode positive) is normally preferred for steel and stainless steel but AC is more effective for cast
iron, copper and nickel alloys. Typical applications include back gouging, removal of surface and
internal defects, removal of excess weld metal and preparation of bevel edges for welding.
The electrode is a non-consumable graphite (carbon) rod, which
has a copper coating to reduce electrode erosion. Electrode
diameter is selected according to required depth and width of
gouge. Cutting can be precisely controlled and molten
metal/dross is kept to a minimum.
Power source
A DC power supply with electrode positive polarity is most
suitable. AC power sources which are also constant current can
be used but with special AC type electrodes. The power source
must have a constant current output characteristic. If it does not,
inadvertant touching of the electrode to the workpiece will cause a high current surge sufficient to
'explode' the electrode tip. This will disrupt the operation and cause carbon pick-up. As arc voltage can
be quite high (up to 50V), open circuit voltage of the power source should be over 60V.
Air supply
The gouging torch is normally operated with either a compressed air line or separate bottled gas supply.
Air supply pressure will be up to 100psi from the airline but restricted to about 35psi from a bottled
supply. Providing there is sufficient airflow to remove molten metal, there are no advantages in using
higher pressure and flow rates.
Carbon pickup
Although the molten metal picks up carbon, the air stream will remove carbon-rich metal from the
groove to leave only minimal contamination of the sidewalls. Poor gouging technique or insufficient
airflow will result in carbon pick-up with the risk of metallurgical problems, e.g. high hardness and
even cracking.
Typical operating data for air carbon arc gouging:
diameter (mm)
Current A
Note: DC
13.0 550
13.0 850
16.0 1250
Striking the electrode tip on to the workpiece surface to initiate the arc commences gouging. Unlike
manual metal arc (MMA) welding the electrode tip is not withdrawn to establish arc length. Molten
metal directly under the electrode tip (arc) is immediately blown away by the air stream. For effective
metal removal, it is important that the air stream is directed at the arc from behind the electrode and
sweeps under the tip of the electrode. The width of groove is determined by the diameter of electrode,
but depth is dictated by the angle of electrode to the workpiece and rate of travel. Relatively high travel
speeds are possible when a low electrode angle is used. This produces a shallow groove: a steep angle
results in a deep groove and requires slower travel speed. Note, a steeply angled electrode may give
rise to carbon contamination.
Oscillating the electrode in a circular or restricted weave motion during gouging can greatly increase
gouging width. This is useful for removal of a weld or plate imperfection that is wider than the
electrode itself. It is important, however, that weave width should not exceed four times the diameter of
the electrode. The groove surface should be relatively free of oxidised metal and can be considered
ready for welding without further preparation. Dressing by grinding the sidewalls of the gouge should
be carried out if a carbon rich layer has been formed. Also, dressing by grinding or another approved
method will be necessary if working on crack-sensitive material such as high strength, low alloy steel.
Equipment for Oxyacetylene Welding
Essential equipment components
The basic oxyacetylene torch comprises:
Torch body (or handle)
Two separate gas tubes (through the handle connected to the hoses)
Separate control valves
Mixer chamber
Flame tube
Welding tip
NB The cutting torch requires two oxygen supplies to the nozzle, one mixed with fuel gas for
preheating and a separate oxygen flow for cutting.
Hoses are colour-coded red for acetylene and blue (UK) or green (US) for oxygen. Oxygen fittings on
the hose have a right-hand thread while acetylene is left-handed.
Gas regulators
The primary function of a gas regulator is to control gas pressure. It reduces the high pressure of the
bottle-stored gas to the working pressure of the torch, and this will be maintained during welding.
The regulator has two separate gauges: a high pressure gauge for gas in the cylinder and a low pressure
gauge for pressure of gas fed to the torch. The amount of gas remaining in the cylinder can be judged
from the high pressure gauge. The regulator, which has a pressure adjusting screw, is used to control
gas flow rate to the torch by setting the outlet gas pressure. Note Acetylene is supplied in cylinders
under a pressure of about 15 bars psi but welding is carried out with torch gas pressures typically up to
2 bars.
Flame traps
Flame traps (also called flashback arresters) must be fitted into both oxygen and acetylene gas lines to
prevent a flashback flame from reaching the regulators. Non-return spring-loaded valves can be fitted
in the hoses to detect/stop reverse gas flow. Thus, the valves can be used to prevent conditions leading
to flashback, but should always be used in conjunction with flashback arresters.
A flashback is where the flame burns in the torch body, accompanied by a whistling sound. It will
occur when flame speed exceeds gas flow rate and the flame can pass back through the mixing
chamber into the hoses. Most likely causes are: incorrect gas pressures giving too low a gas velocity,
hose leaks, loose connections, or welder techniques which disturb gas flow.
Identification of gas cylinders
An oxygen cylinder is colour-coded black and the acetylene cylinder is maroon. Oxygen and acetylene
are stored in cylinders at high pressure. Oxygen pressure can be as high as 230 bars. Acetylene, which
is dissolved in acetone contained in a porous material, is stored at a much lower pressure,
approximately 15 bars.
The appropriate regulator must be fitted to the cylinders to accommodate cylinder pressures. To avoid
confusion, oxygen cylinders and regulators have right-hand threads and acetylene cylinders and
regulators have left-hand ones.
Typical gas pressures and flow rates for C-Mn steel:
Selection of correct nozzles
Welding torches are generally rated according to thickness of material to be welded. They range from
light duty (for sheet steel up to 2mm in thickness) to heavy duty (for steel plate greater than 25mm in
thickness). Each torch can be fitted with a range of nozzles with a bore diameter selected according to
material thickness. Gas pressures are set to give correct flow rate for nozzle bore diameter. Proportions
of oxygen and acetylene in the mixture can be adjusted to give a neutral, oxidising or carburising
flame. (See the description of oxyacetylene processes) Welding is normally carried out using a neutral
flame with equal quantities of oxygen and acetylene.
Equipment safety checks
Before commencing welding it is wise to inspect the condition and operation of all equipment. As well
as normal equipment and workplace safety checks, there are specific procedures for oxyacetylene.
Operators should verify that:
flashback arresters are present in each gas line
hoses are the correct colour, with no sign of wear, as short as possible and not taped together
regulators are the correct type for the gas
a bottle key is in each bottle (unless the bottle has an adjusting screw)
It is recommended that oxyacetylene equipment is checked at least annually - regulators should be
taken out of service after five years. Flashback arresters should be checked regularly according to
manufacturer's instructions and, with specific designs, it may be necessary to replace if flashback has
For more detailed information the following legislation and codes of practice should be consulted:
UK Health and Safety at Work Act 1974
Pressure Systems and Transportable Gas Containers Regulations
British Compressed Gases Association, Codes of Practice
BOC Handbook
Equipment for MMA Welding
Although the manual metal arc (MMA) process has relatively basic equipment
requirements, it is important that the welder has a knowledge of operating features
and performance to comply with welding procedures for the job and, of course, for
safety reasons.
Essential equipment
The main components of the equipment required
for welding are:
Power source
Electrode holder and cables
Welder protection
Fume extraction
Tools required include: a wire brush to clean the
joint area adjacent to the weld (and the weld
itself after slag removal); a chipping hammer to remove slag from the weld deposit;
and, when removing slag, a pair of clear lens goggles or a face shield to protect the
eyes (lenses should be shatter-proof and noninflammable).
Power source
The primary function of a welding power source is to provide sufficient power to melt
the joint. However with MMA the power source must also provide current for melting
the end of the electrode to produce weld metal, and it must have a sufficiently high
voltage to stabilise the arc.
MMA electrodes are designed to be operated with alternating current (AC) and direct
current (DC) power sources. Although AC electrodes can be used on DC, not all DC
electrodes can be used with AC power sources.
As MMA requires a high current (50-30OA) but a relatively low voltage (10-50V),
high voltage mains supply (240 or 440V) must be reduced by a transformer. To
produce DC, the output from the transformer must be further rectified. To reduce the
hazard of electrical shock, the power source must function with a maximum no-load
voltage, that is, when the external (output) circuit is open (power leads connected and
live) but no arc is present. The no-load voltage rating of the power source is as
defined in BS 638 and must be in accordance with the type of welding environment or
hazard of electrical shock. The power source may have an internal or external hazardreducing device to reduce the no-load voltage; the main welding current is delivered
as soon as the electrode touches the workpiece. For welding in confined spaces, you
should use a low voltage safety device to limit the voltage available at the holder to
approximately 25V.
There are four basic types of power source:
AC transformer
DC rectifier
AC/DC transformer-rectifier
DC generator
AC electrodes are frequently operated with the simple, single-phase transformer with
current adjusted by means of tappings or sliding core control. DC rectifiers and
AC/DC transformer-rectifiers are controlled electronically, for example by thyristors.
A new generation of power sources called inverters is available. These use transistors
to convert mains AC (50Hz) to a high frequency AC (over 500 Hz) before
transforming down to a voltage suitable for welding and then rectifying to DC.
Because high frequency transformers can be relatively small, principal advantages of
inverter power sources are undoubtedly their size and weight when the source must be
Electrode holder and cables
The electrode holder clamps the end of the electrode with copper contact shoes built
into its head. The shoes are actuated by either a twist grip or spring-loaded
mechanism. The clamping mechanism allows for quick release of the stub end. For
efficiency the electrode has to be firmly clamped into the holder, otherwise poor
electrical contact may cause arc instability through voltage fluctuations. Welding
cable connecting the holder to the power source is mechanically crimped or soldered.
It is essential that good electrical connections are maintained between electrode,
holder and cable. With poor connections, resistance heating and, in severe cases,
minor arcing with the torch body will cause the holder to overheat. Two cables are
connected to the output of the power source, the welding lead goes to the electrode
holder and the current return lead is clamped to the workpiece. The latter is often
wrongly referred to as the earth lead. A separate earth lead is normally required to
provide protection from faults in the power source. The earth cable should therefore
be capable of carrying the maximum output current of the power source.
Cables are covered in a smooth and hard-wearing protective rubberised flexible
sheath. This oil and water resistant coating provides electrical insulation at voltages to
earth not exceeding 100V DC and AC (rms value). Cable diameter is generally
selected on the basis of welding current level, As these electrode types are When
welding, the welder air movement should be from duty cycle and distance of the work
from the power source. The higher the current and duty cycle, the larger the diameter
of the cable to ensure that it does not overheat (see BS 638 Pt 4). If welding is carried
out some distance from the power source, it may be necessary to increase cable
diameter to reduce voltage drop.
Care of electrodes
The quality of weld relies upon consistent performance of the electrode. The flux
coating should not be chipped, cracked or, more importantly, allowed to become
Electrodes should always be kept in a dry and well-ventilated store. It is good practice
to stack packets of electrodes on wooden pallets or racks well clear of the floor. Also,
all unused electrodes, which are to be returned, should be stored so they are not
exposed to damp conditions to regain moisture. Good storage conditions are 10
degrees C above external air temperature. As the storage conditions are to prevent
moisture from condensing on the electrodes, the electrode stores should be dry rather
that warm. Under these conditions and in original packaging, electrode storage time is
practically unlimited. It should be noted that electrodes are now available in
hermetically sealed packs that obviate the need for drying. However, if necessary, any
unused electrodes must be redried according to manufacturer's instructions.
Drying of electrodes
Drying is usually carried out following the manufacturer's recommendations and
requirements will be determined by the type of electrode.
Cellulosic coatings
As these electrode coatings are designed to operate with a definite amount of moisture
in the coating, they are less sensitive to moisture pick-up and do not generally require
a drying operation. However, in cases where ambient relative humidity has been very
high, drying may be necessary.
Rutile coatings
These can tolerate a limited amount of moisture and coatings may deteriorate if they
are overdried. Particular brands may need to be dried before use.
Basic and basic/rutile coatings
Because of the greater need for hydrogen control, moisture pick-up is rapid on
exposure to air. These electrodes should be thoroughly dried in a controlled
temperature drying oven. Typical drying time is one hour at a temperature of
approximately 150 to 300 degrees C but instructions should be adhered to.
After controlled drying, basic and basic/rutile electrodes must be held at a temperature
between 100 and 150 degrees C to help protect them from re-absorbing moisture into
the coating. These conditions can be obtained by transferring the electrodes from the
main drying oven to a holding oven or a heated quiver at the workplace.
Protective clothing
When welding, the welder must be protected from heat and light radiation emitted
from the arc, spatter ejected from the weld pool, and from welding fume.
Hand and head shield
For most operations a hand-held or head shield constructed of lightweight insulating
and non-reflecting material is used. The shield is fitted with a protective filter glass,
sufficiently dark in colour and capable of absorbing the harmful infrared and
ultraviolet rays. The filter glasses conform to the strict requirements of BS 679 and
are graded according to a shade number which specifies the amount of visible light
allowed to pass through - the lower the number, the lighter the filter. The correct
shade number must be used according to the welding current level, for example:
Shade 9 - up to 40A
Shade 10 - 40 to 80A
Shade 11 - 80 to 175A
Shade 12 - 175 to 300A
Shade 13 - 300 to 500A
For protection against sparks, hot spatter, slag and burns, a leather apron and leather
gloves should be worn. Various types of leather gloves are available, such as short or
elbow length, full fingered or part mitten.
Fume extraction
When welding within a welding shop, ventilation must dispose harmlessly of the
welding fume. Particular attention should be paid to ventilation when welding in a
confined space such as inside a boiler, tank or compartment of a ship.
Fume removal should be by some form of mechanical ventilation, which will produce
a current of fresh air in the immediate area. Direction of the air movement should be
from the welder's face towards the work. This is best achieved by localised exhaust
ventilation using a suitably designed hood near to the welding area.
Further information
Please refer to:
BS 638 Arc welding power sources, equipment and accessories
BS 679 Filters, cover lenses and backing lenses for use during welding and similar
Equipment for MIG Welding
The MIG process is a versatile welding technique, which is suitable for both thin
sheet and thick section components. It is capable of high productivity but the quality
of welds can be called into question. To achieve satisfactory welds, welders must
have a good knowledge of equipment requirements and should also recognise fully
the importance of setting up and maintaining component parts correctly.
Essential equipment
In MIG the arc is formed between the end of
a small diameter wire electrode fed from a
spool, and the workpiece. Main equipment
components are:
Power source
Wire feed system
The arc and weldpool are protected from the
atmosphere by a gas shield. This enables bare
wire to be used without a flux coating
(required by MMA). However, the absence
of flux to 'mop up' surface oxide places greater demand on the welder to ensure that
the joint area is cleaned immediately before welding. This can be done using either a
wire brush for relatively clean parts, or a hand grinder to remove rust and scale. The
other essential piece of equipment is a wire cutter to trim the end of the electrode
Power source
MIG is operated exclusively with a DC power source. The source is termed a flat, or
constant current, characteristic power source, which refers to the voltage/welding
current relationship. In MIG, welding current is determined by wire feed speed, and
arc length is determined by power source voltage level (open circuit voltage). Wire
burn-off rate is automatically adjusted for any slight variation in the gun to workpiece
distance, wire feed speed, or current pick-up in the contact tip. For example, if the arc
momentarily shortens, arc voltage will decrease and welding current will be
momentarily increased to burn back the wire and maintain pre-set arc length. The
reverse will occur to counteract a momentary lengthening of the arc.
There is a wide range of power sources available, mode of metal transfer can be:
A low welding current is used for thin-section material, or welding in the vertical
position. The molten metal is transferred to the workpiece by the wire dipping into the
weldpool. As welding parameters will vary from around 100A \ 17V to 200A \ 22V
(for a 1.2mm diameter wire), power sources normally have a current rating of up to
350A. Circuit inductance is used to control the surge in current when the wire dips
into the weldpool (this is the main cause of spatter). Modern electronic power sources
automatically set the inductance to give a smooth arc and metal transfer.
In spray metal transfer, metal transfers as a spray of fine droplets without the wire
touching the weldpool. The welding current level needed to maintain the non shortcircuiting arc must be above a minimum threshold level; the arc voltage is higher to
ensure that the wire tip does not touch the weldpool. Typical welding parameters for a
1.2mm diameter wire are within 250A \ 28V to 400A \ 35V. For high deposition rates
the power source must have a much higher current capacity: up to 500A.
The pulsed mode provides a means of achieving a spray type metal transfer at current
levels below threshold level. High current pulses between 25 and 100Hz are used to
detach droplets as an alternative to dip transfer. As control of the arc and metal
transfer requires careful setting of pulse and background parameters, a more
sophisticated power source is required. Synergic-pulsed MIG power sources, which
are advanced transistor-controlled power sources, are preprogrammed so that the
correct pulse parameters are delivered automatically as the welder varies wire feed
Welding current and arc voltage ranges for selected wire diameters operating with dip
and spray metal transfer:
Wire diameter (mm)
Dip transfer
Spray transfer
Current (A) Voltage (V) Current (A) Voltage (V)
30 - 80
15 - 18
45 - 180
16 - 21
150 - 250
25 - 33
70 - 180
17 - 22
230 - 300
26 - 35
100 - 200
17 - 22
250 - 400
27 - 35
120 - 200
18 - 22
250 - 500
30 - 40
Wire feed system
The performance of the wire feed system can be crucial to the stability and
reproducibility of MIG welding. As the system must be capable of feeding the wire
smoothly, attention should be paid to the feed rolls and liners. There are three types of
feeding systems:
Pinch rolls
Spool on gun
The conventional wire feeding system normally has a set of rolls where one is
grooved and the other has a flat surface. Roll pressure must not be too high otherwise
the wire will deform and cause poor current pick up in the contact tip. With copper
coated wires, too high a roll pressure or use of knurled rolls increases the risk of
flaking of the coating (resulting in copper build up in the contact tip). For feeding soft
wires such as aluminium dual-drive systems should be used to avoid deforming the
soft wire.
Small diameter aluminium wires, 1mm and smaller, are more reliably fed using a
push-pull system. Here, a second set of rolls is located in the welding gun - this
greatly assists in drawing the wire through the conduit. The disadvantage of this
system is increased size of gun. Small wires can also be fed using a small spool
mounted directly on the gun. The disadvantages with this are increased size,
awkwardness of the gun, and higher wire cost.
The conduit can measure up to 5m in length, and to facilitate feeding, should be kept
as short and straight as possible. (For longer lengths of conduit, an intermediate pushpull system can be inserted). It has an internal liner made either of spirally wound
steel for hard wires (steel, stainless steel, titanium, nickel) or PTFE for soft wires
(aluminium, copper).
In addition to directing the wire to the joint, the welding gun fulfils two important
functions - it transfers the welding current to the wire and provides the gas for
shielding the arc and weldpool.
There are two types of welding guns: 'air' cooled and water-cooled. The 'air' cooled
guns rely on the shielding gas passing through the body to cool the nozzle and have a
limited current-carrying capacity. These are suited to light duty work. Although 'air'
cooled guns are available with current ratings up to 500A, water cooled guns are
preferred for high current levels, especially at high duty cycles.
Welding current is transferred to the wire through the contact tip whose bore is
slightly greater than the wire diameter. The contact tip bore diameter for a 1.2mm
diameter wire is between 1.4 and 1.5mm. As too large a bore diameter affects current
pick up, tips must be inspected regularly and changed as soon as excessive wear is
noted. Copper alloy (chromium and zirconium additions) contact tips, harder than
pure copper, have a longer life, especially when using spray and pulsed modes.
Gas flow rate is set according to nozzle diameter and gun to workpiece distance, but is
typically between 10 and 30 l/min. The nozzle must be cleaned regularly to prevent
excessive spatter build-up, which creates porosity. Anti-spatter spray can be
particularly effective in automatic and robotic welding to limit the amount of spatter
adhering to the nozzle.
Protective equipment
A darker glass than that used for MMA welding at the same current level should be
used in hand or head shields.
Recommended shade number of filter for MIG/MAG welding:
Shade number
Welding current A
MIG Heavy metal MIG Light metal
Under 100
Under 100
Under 80
1001 - 175
100 - 175
80 - 125
175 - 300
175 - 250
125 - 175
300 - 500
250 - 350
175 - 300
Over 500
350 - 500
300 - 500
Over 500
Over 450
Equipment for Submerged-arc Welding
The submerged-arc welding (SAW) process is similar to MIG where the arc is formed
between a continuously fed wire electrode and the workpiece, and the weld is formed
by the arc melting the workpiece and the wire. However, in SAW a shielding gas is
not required as the layer of flux generates the gases and slag to protect the weld pool
and hot weld metal from contamination. Flux plays an additional role in adding
alloying elements to the weld pool.
Essential equipment
Essential equipment components for SAW are:
Power source
Wire gun
Flux handling
Protective equipment
As SAW is a high current welding process, the
equipment is designed to produce high deposition
Power source
SAW can be operated using either a DC or an AC power source. DC is supplied by a
transformer-rectifier and a transformer supplies AC. Current for a single wire ranges
from as low as 200A (1.6mm diameter wire) to as high as 1000A (6.0mm diameter
wire). In practice, most welding is carried out on thick plate where a single wire
(4.0mm diameter) is normally used over a more limited range of 600 to 900A, with a
twin wire system operating between 800 and 1200A.
In DC operation, the electrode is normally connected to the positive terminal.
Electrode negative (DCEN) polarity can be used to increase deposition rate but depth
of penetration is reduced by between 20 and 25%. For this reason, DCEN is used for
surfacing applications where parent metal dilution is important. The DC power source
has a 'constant voltage' output characteristic, which produces a self-regulating arc. For
a given diameter of wire, welding current is controlled by wire feed speed and arc
length is determined by voltage setting.
AC power sources usually have a constant-current output characteristic and are
therefore not self-regulating. The arc with this type of power source is controlled by
sensing the arc voltage and using the signal to control wire feed speed. In practice, for
a given welding current level, arc length is determined by wire burnoff rate, i.e. the
balance between the welding current setting and wire feed speed, which is under
feedback control.
Square wave AC square wave power sources have a constant voltage output current
characteristic. Advantages are easier arc ignition and constant wire feed speed control.
Welding gun
SAW can be carried out using both manual and mechanised techniques. Mechanised
welding, which can exploit the potential for extremely high deposition rates, accounts
for the majority of applications.
Manual welding
For manual welding, the welding gun is similar to a MIG gun, with the flux, which is
fed concentrically around the electrode, replacing the shielding gas. Flux is fed by air
pressure through the handle of the gun or from a small hopper mounted on the gun.
The equipment is relatively portable and, as the operator guides the gun along the
joint, little manipulative skill is required. However, because the operator has limited
control over the welding operation (apart from adjusting travel speed to maintain the
bead profile) it is best used for short runs and simple filling operations.
Mechanised welding - single wire
As SAW is often used for welding large components, the gun, wire feeder and flux
delivery feed can be mounted on a rail, tractor or boom manipulator. Single wire
welding is mostly practised using DCEP even though AC will produce a higher
deposition rate for the same welding current. AC is used to overcome problems with
arc blow, caused by residual magnetism in the workpiece, jigging or welding
Wire stickout, or electrode extension - the distance the wire protrudes from the end of
the contact tip - is an important control parameter in SAW. As the current flowing
between the contact tip and the arc will preheat the wire, wire burnoff rate will
increase with increase in wire stickout. For example, the deposition rate for a 4mm
diameter wire at a welding current of 700A can be increased from approximately 9
kg/hr at the normal 32mm stickout, to 14 kg/hr at a stickout length of 178mm. In
practice, because of the reduction in penetration and greater risk of arc wander, a long
stickout is normally only used in cladding and surfacing applications where there is
greater emphasis on deposition rate and control of penetration, rather than accurate
positioning of the wire.
For most applications, electrode stickout is set so that the contact tube is slightly
proud of the flux layer. The depth of flux is normally just sufficient to cover the arc
whose light can be seen through the flux.
Recommended and maximum stickout lengths:
Wire diameter mm Current range A
Wire stickout
Normal mm Maximum mm
100 to 200
150 to 300
200 to 500
250 to 600
350 to 800
400 to 900
450 to 1000
Mechanised welding - twin wire
Tandem arc connections
SAW can be operated with more than one wire. Although up to five wires are used for
high deposition rates, e.g. in pipe mills, the most common multi-wire systems have
two wires in a tandem arrangement. The leading wire is run on DCEP to produce deep
penetration. The trailing wire is operated on AC, which spreads the weld pool, which
is ideal for filling the joint. AC also minimises: interaction between the arcs, and the
risk of lack of fusion defects and porosity through the deflection of the arcs (arc
blow). The wires are normally spaced 20mm apart so that the second wire feeds into
the rear of the weld pool.
Gun angle
In manual welding, the gun is operated with a trailing angle, i.e. with the gun at an
angle of 45 degrees (backwards) from the vertical. In single wire mechanised welding
operations, the gun is perpendicular to the workpiece. However, in twin wire
operations the leading gun is normal to the workpiece, with the trailing gun angled
slightly forwards between an angle of 60 and 80 degrees. This reduces disturbance of
the weld pool and produces a smooth weld bead profile.
Flux handling
Flux should be stored in unopened packages under dry conditions. Open packages
should be stored in a humidity-controlled store. While flux from a newly opened
package is ready for immediate use, flux, which has been opened and held in a store,
should first be dried according to manufacturer's instructions. In small welding
systems, flux is usually held in a small hopper above the welding gun. It is fed
automatically (by gravity or mechanised feed) ahead of the arc. In larger installations
the flux is stored in large hoppers and is fed with compressed air. Unused flux is
collected using a vacuum hose and returned to the hopper.
Note: Care must be taken in recycling unused flux, particularly regarding the removal
of slag and metal dust particles. The presence of slag will change the composition of
the flux, which, together with the wire, determines the composition of the weld metal.
The presence of fine particles can cause blockages in the feeding system.
Protective equipment
Unlike other arc welding processes, SAW is a clean process, which produces
minimum fume and spatter when welding steels. (Some noxious emissions can be
produced when welding special materials.) For normal applications, general workshop
extraction should be adequate.
Protective equipment such as a head shield and a leather apron are not necessary.
Normal protective equipment (goggles, heavy gloves and protective shoes) are
required for ancillary operations such as slag removal by chipping or grinding.
Special precautions should be taken when handling flux - a dust respirator and gloves
are needed when loading the storage hoppers.
Equipment for TIG Welding
Job Knowledge for Welders No. 6 describes the TIG welding process. Using an inert
gas shield instead of a slag to protect the weldpool, this technology is a highly
attractive alternative to gas and manual metal arc welding and has played a major role
in the acceptance of high quality welding in critical applications.
Essential equipment
In TIG, the arc is formed between the end of a small
diameter tungsten electrode and the workpiece. The main
equipment components are:
Power source
Backing system
Protective equipment
Power source
The power source for TIG welding can be either DC or AC but in both the output is
termed a drooping, or constant current, characteristic; the arc voltage / welding
current relationship delivers a constant current for a given power source setting.
If the arc voltage is slightly increased or decreased, there will be very little change in
welding current. In manual welding, it can accommodate the welder's natural
variations in arc length and, in the event of the electrode touching the work, an
excessively high current will not be drawn which could fuse the electrode to the
The arc is usually started by HF (High Frequency) sparks
which ionise the gap between the electrode and the workpiece.
HF generates airborne and line transmitted interference, so
care must be taken to avoid interference with control systems
and instruments near welding equipment. When welding is
carried out in sensitive areas, a non-HF technique, touch
starting or 'lift arc', can be used. The electrode can be short
circuited to the workpiece, but the current will only flow
when the electrode is lifted off the surface. There is, therefore, little risk of the
electrode fusing to the workpiece surface and forming tungsten inclusions in the weld
metal. For high quality applications, using HF is preferred.
DC power source
DC power produces a concentrated arc with most of the heat in the workpiece, so this
power source is generally used for welding. However, the arc with its cathode roots
on the electrode (DC electrode negative polarity), results in little cleaning of the
workpiece surface. Care must be taken to clean the surface prior to welding and to
ensure that there is an efficient gas shield.
Transistor and inverter power sources are being used increasingly for TIG welding.
The advantages are:
The smaller size makes them easily transported
Arc ignition is easier
Special operating features, e.g. current pulsing, are readily included
The output can be pre-programmed for mechanised operations
The greater stability of these power sources allows very low currents to be used
particularly for micro-TIG welding and largely replaced the plasma process for microwelding operations.
AC power source
For materials such as aluminium, which has a tenacious oxide film on the surface, AC
power must be employed. By switching between positive and negative polarity, the
periods of electrode positive will remove the oxide and clean the surface.
The figure shows current and voltage waveforms for (sine wave) AC TIG welding.
Disadvantages of conventional, sine wave AC compared with DC are:
The arc is more diffuse
HF is required to reignite the arc at each current reversal
Excessive heating of the electrode makes it impossible to maintain a tapered
point and the end becomes balled
Square wave AC, or switched DC, power sources are particularly attractive for
welding aluminium.
By switching between polarities, arc reignition is made easier so that the HF can be
reduced or eliminated. The ability to imbalance the waveform to vary the proportion
of positive to negative polarity is important by determining the relative amount of
heat generated in the workpiece and the electrode.
To weld the root run, the power source is operated with the greater amount of positive
polarity to put the maximum heat into the workpiece.
For filler runs a greater proportion of negative polarity should be used to minimise
heating of the electrode. By using 90% negative polarity, it is possible to maintain a
pointed electrode. A balanced position (50% electrode positive and negative
polarities) is preferable for welding heavily oxidised aluminium.
There is a wide range of torch designs for welding, according to the application.
Designs, which have the on/off switch and current control in the handle, are often
preferred to foot controls. Specialised torches are available for mechanised
applications, e.g. orbital and bore welding of pipes.
For DC current, the electrode is tungsten with between 2 and 5% thoria to aid arc
initiation. The electrode tip is ground to an angle of 600 to 900 for manual welding,
irrespective of the electrode diameter. For mechanised applications as the tip angle
determines the shape of the arc and influences the penetration profile of the weld
pool, attention must be paid to consistency in grinding the tip and checking its
condition between welds.
For AC current, the electrode is either pure tungsten or tungsten with a small amount
(up to 0.5%) of zirconia to aid arc reignition and to reduce electrode erosion. The tip
normally assumes a spherical profile due to the heat generated in the electrode during
the electrode positive half cycle.
Gas shielding
A gas lens should be fitted within the torch nozzle, to ensure laminar gas flow. This
will improve gas protection for sensitive welding operations like welding vertical,
corner and edge joints and on curved surfaces.
Backing system
When welding high integrity components, a shielding gas is used to protect the
underside of the weld pool and weld bead from oxidation. To reduce the amount of
gas consumed, a localised gas shroud for sheet, dams or plugs for tubular components
is used. As little as 5% air can result in a poor weld bead profile and may reduce
corrosion resistance in materials like stainless steel. With gas backing systems in pipe
welding, pre-weld purge time depends on the diameter and length of the pipe. The
flow rate/purge time is set to ensure at least five volume changes before welding.
Stick on tapes and ceramic backing bars are also used to protect and support the weld
bead. In manual stainless steel welding, a flux-cored wire instead of a solid wire can
be used in the root run. This protects the underbead from oxidation without the need
for gas backing.
A pre-placed insert can be used to improve the uniformity of the root penetration. Its
main use is to prevent suck-back in an autogenous weld, especially in the overhead
position. The use of an insert does not make welding any easier and skill is still
required to avoid problems of incomplete root fusion and uneven root penetration.
Protective equipment
A slightly darker glass should be used in the head or hand shield than that used for
MMA welding.
Recommended shade number of filter for TIG welding:
Shade number Welding current A
Less than 20
20 to 40
40 to 100
100 to 175
175 to 250
250 to 400
Equipment for
Plasma Welding
Plasma welding derives its unique
operating characteristics from the torch
design. As in TIG welding, the arc is
formed between the end of a small
diameter tungsten electrode and the
workpiece. However, in the plasma torch,
the electrode is positioned behind a fine
bore copper nozzle. By forcing the arc to
pass through the nozzle, the characteristic
columnar jet, or plasma, is formed.
As described in Job Knowledge for Welders, No 7, three
different operating modes can be produced by the choice of
the nozzle bore diameter, current level and plasma gas flow
Microplasma (0.1 to 15A) is equivalent to microTIG but the columnar arc
allows the welder to operate with a much longer arc length. The arc is stable at
low welding current levels producing a 'pencil-like' beam, which is suitable for
welding very thin section material.
Medium current plasma (15 to 100A) similar to conventional TIG is also
used for precision welding operations and when a high level of weld quality is
Keyhole plasma (over 100A) produced by increasing the current level and the
plasma gas flow. It generates very powerful arc plasma, similar to a laser
beam. During welding, the plasma arc slices through the metal producing a
keyhole, with the molten weld pool flowing around the keyhole to form the
weld. Deep penetration and high welding speeds can be achieved with this
operating mode.
As the plasma arc is generated by the special torch arrangement and system controller,
the equipment can be obtained as an add-on unit to conventional TIG equipment to
provide additional pilot arc and separate plasma and shielding gases. Alternatively,
purpose-built plasma equipment is available. Despite similarities in plasma and TIG
equipment, there are several important differences in the following components:
Power source
Backing system
Protective equipment
Power source
The power source for plasma welding is almost exclusively DC and, as in TIG, the
drooping, or constant current, output characteristic will deliver essentially constant
current for a given power source setting. The power source is ideal for mechanised
welding as it maintains the current setting even when arc length varies and, in manual
welding, it can accommodate the natural variations of the welder.
The plasma process is normally operated with electrode negative polarity to minimise
heat produced in the electrode (approximately 1/3rd of the heat generated by the arc is
produced at the cathode with 2/3rds at the anode). Special torches are available,
however, for operating with electrode positive polarity which rely on efficient cooling
to prevent melting of the electrode. The positive electrode torch is used for welding
aluminium, which requires the cathode to be on the material to remove the oxide film.
AC is not normally used in the plasma process because it is difficult to stabilise the
AC arc. Problems in reigniting the arc are associated with constriction by the nozzle,
the long electrode to workpiece distance and balling of the electrode caused by the
alternate periods of electrode positive polarity. The square wave AC (inverter,
switched DC) power source, with an efficiently cooled torch, makes the use of the AC
plasma process easier; rapid current switching promotes arc reignition and, by
operating with very short periods of electrode positive polarity, electrode heating is
reduced so a pointed electrode can be maintained.
The plasma system has a unique arc starting system in which HF is only used to ignite
a pilot arc held within the body of the torch. The pilot arc formed between the
electrode and copper nozzle is automatically transferred to the workpiece when it is
required for welding. This starting system is very reliable and eliminates the risk of
electrical interference through HF.
The torch for the plasma process is considerably more complex than the TIG torch
and attention must be paid, not only to initial set up, but also to inspection and
maintenance during production.
In the conventional torch arrangement, the electrode is positioned behind the watercooled copper nozzle. As the power of the plasma arc is determined by the degree of
nozzle constriction, consideration must be given to the choice of bore diameter in
relation to the current level and plasma gas flow rate. For’soft’ plasma, normally used
for micro and medium current operating modes, a relatively large diameter bore is
recommended to minimise nozzle erosion.
In high current keyhole plasma mode, the nozzle bore diameter, plasma gas flow rate
and current level are selected to produce a highly constricted arc, which has sufficient
power to cut through the material. The plasma gas flow rate is crucial in generating
the deeply penetrating plasma arc and in preventing nozzle erosion; too low a gas
flow rate for the bore diameter and current level will result in double arcing in the
torch and the nozzle melting.
The suggested starting point for setting the plasma gas flow rate and the current level
for a range of the bore diameters and the various operating modes is given.
The electrode is tungsten with an addition of between 2 and 5% thoria to aid arc
initiation. Normally, the electrode tip is ground to an angle of 15 degrees for
microplasma welding. The tip angle increases with current level and for high current,
keyhole plasma welding, an angle of 60 degrees to 90 degrees is recommended. For
high current levels, the tip is also blunted to approximately 1mm diameter. The tip
angle is not usually critical for manual welding. However, for mechanised
applications, the condition of the tip and the nozzle will determine the shape of the arc
and penetration profile of the weld pool penetration, so particular attention must be
paid to grinding the tip. It is also necessary to check periodically the condition of the
tip and nozzle and, for critical components, it is recommended the torch condition is
checked between welds.
Electrode set-back
To ensure consistency, it is important to maintain a constant electrode position behind
the nozzle; the torch manufacturer provides guidance on electrode setback and a
special tool. The maximum current rating of each nozzle has been established for the
maximum electrode setback position and the maximum plasma gas flow rate. Lower
plasma gas flow rates can be used to soften the plasma arc with the maximum current
rating of the nozzle providing electrode setback distance is reduced.
Plasma and shielding gas
The usual gas combination is argon for the plasma gas and argon-2 to 8% H2 for the
shielding gas. Irrespective of the material being welded, using argon for the plasma
gas produces the lowest rate of electrode and nozzle erosion. Argon - H2 gas mixture
for shielding produces a slightly reducing atmosphere and cleaner welds. Helium
gives a hotter arc; however, its use for the plasma gas reduces the current carrying
capacity of the nozzle and makes formation of the keyhole more difficult. Helium argon mixtures, e.g. 75% helium - 25% argon, are used as the shielding gas for
materials such as copper.
Plasma gas flow rate must be set accurately as it controls the penetration of the weld
pool but the shielding gas flow rate is not critical.
Backing system
The normal TIG range of backing bar designs or shielding gas techniques can be
employed when using micro and medium current techniques. When applying the
keyhole mode a grooved backing bar must be used, with or without gas shielding or
total shielding of the underside of the joint. Because the efflux plasma normally
extends about 10mm below the back face of the joint, the groove must be deep
enough to avoid disturbance of the arc jet; if the efflux plasma hits the backing bar,
arc instability will disturb the weld pool, causing porosity.
Protective equipment
Protective equipment for plasma welding is as described for TIG in Job Knowledge
for Welders No 17. Regarding protection from arc light, a similar Shade number to
TIG at the same welding current level should be used in head or hand shield. The
glass will be slightly darker than that used for MMA welding at the same current
Recommended shade number of filter for plasma welding:
Shade Number
Welding Current, A
Micro Plasma
0.5 to 1
1 to 2.5
2.5 to 5
5 to 10
10 to 15
15 to 30
30 to 60
Less than 150
60 to 125
150 to 250
125 to 225
Above 250
225 to 450
See BS 639:1989 for further information on shade numbers.
The Manual Metal Arc
Manual metal arc welding was first invented in Russia in 1888. It involved a bare
metal rod with no flux coating to give a protective gas shield. The development of
coated electrodes did not occur until the early 1900s when the Kjellberg process was
invented in Sweden and the Quasi-arc method was introduced in the UK. It is worth
noting that coated electrodes were slow to be adopted because of their high cost.
However, it was inevitable that as the demand for sound welds grew, manual metal
arc became synonymous with coated electrodes. When an arc is struck between the
metal rod (electrode) and the workpiece, both the rod and workpiece surface melt to
form a weld pool. Simultaneous melting of the flux coating on the rod will form gas
and slag which protects the weld pool from the surrounding atmosphere. The slag will
solidify and cool 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. Weld penetration is low and the quality of the weld
deposit is highly dependent on the skill of the welder.
Types of flux/electrodes
Arc stability, depth of penetration, metal deposition rate and positional capability are
greatly influenced by the chemical composition of the flux coating on the electrode.
Electrodes can be divided into three main groups:
Cellulose electrodes contain a high proportion of cellulose in the coating and are
characterised by a deeply penetrating arc and a rapid burn-off rate giving high
welding speeds. Weld deposit can be coarse and with fluid slag, deslagging can be
difficult. These electrodes are easy to use in any position and are noted for their use in
the 'stovepipe' welding technique.
Deep penetration in all positions
Suitability for vertical down welding
Reasonably good mechanical properties
High level of hydrogen generated - risk of cracking in the heat affected zone
Rutile electrodes contain a high proportion of titanium oxide (rutile) in the coating.
Titanium oxide promotes easy arc ignition, smooth arc operation and low spatter.
These electrodes are general-purpose electrodes with good
welding properties. They can be used with AC and DC
power sources and in all positions. The electrodes are
especially suitable for welding fillet joints in the
horizontal/vertical (H/V) position.
Moderate weld metal mechanical properties
Good bead profile produced through the viscous slag
Positional welding possible with a fluid slag (containing fluoride)
Easily removable slag
Basic electrodes contain a high proportion of calcium carbonate (limestone) and
calcium fluoride (fluorspar) in the coating. This makes their slag coating more fluid
than rutile coatings - this is also fast-freezing which assists welding in the vertical and
overhead position. These electrodes are used for welding medium and heavy section
fabrications where higher weld quality, good mechanical properties and resistance to
cracking (due to high restraint) are required.
Low weld metal produces hydrogen
Requires high welding currents/speeds
Poor bead profile (convex and coarse surface profile)
Slag removal difficult
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 the 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.
Power source
Electrodes can be operated with AC and DC power supplies. Not all DC electrodes
can be operated on AC power sources, however AC electrodes are normally used on
Welding current
Welding current level is determined by the size of electrode - manufacturers
recommend the normal operating range and current. Typical operating ranges for a
selection of electrode sizes are illustrated in the table. As a rule of thumb when
selecting a suitable current level, an electrode will require about 40A per millimeter
(diameter). Therefore, the preferred current level for a 4mm diameter electrode would
be 160A, but the acceptable operating range is 140 to 180A.
What's new
Transistor (inverter) technology is now enabling very small and comparatively low
weight power sources to be produced. These power sources are finding increasing use
for site welding where they can be readily transported from job to job. As they are
electronically controlled, add-on units are available for TIG and MIG welding which
increase the flexibility. Electrodes are now available in hermetically sealed containers.
These vacuum packs obviate the need for baking the electrodes immediately prior to
use. However, if a container has been opened or damaged, it is essential that the
electrodes be redried according to the manufacturer's instructions.
The oxyacetylene process
Process features
Oxyacetylene welding, commonly referred to as gas welding, is a process, which
relies on combustion of oxygen and acetylene. When mixed together in correct
proportions within a hand-held torch or blowpipe, a relatively hot flame is produced
with a temperature of about 3,200°C. The chemical action of the oxyacetylene flame
can be adjusted by changing the ratio of the volume of oxygen to acetylene.
Three distinct flame settings are used, neutral, oxidising and carburising.
Neutral flame
Oxidising flame
Carburising flame
Welding is generally carried out using the neutral flame setting, which has equal
quantities of oxygen and acetylene. The oxidising flame is obtained by increasing just
the oxygen flow rate while the carburising flame is achieved by increasing acetylene
flow in relation to oxygen flow. Because steel melts at a temperature above 1,500°C,
the mixture of oxygen and acetylene is used, as it is the only gas combination with
enough heat to weld steel. However, other gases such as propane, hydrogen and coal
gas can be used for joining lower melting point non-ferrous metals, and for brazing
and silver soldering.
Oxyacetylene equipment is portable and easy to use. It comprises oxygen and
acetylene gases stored under pressure in steel cylinders. The cylinders are fitted with
regulators and flexible hoses which lead to the blowpipe. Specially designed safety
devices such as flame traps are fitted between the hoses and the cylinder regulators.
The flame trap prevents flames generated by a 'flashback' from reaching the cylinders;
principal causes of flashbacks are the failure to purge the hoses and overheating of the
blowpipe nozzle.
When welding, the operator must wear protective clothing and tinted coloured
goggles. As the flame is less intense than an arc and very little UV is emitted, generalpurpose tinted goggles provide
sufficient protection.
Operating characteristics
The action of the oxyacetylene flame on
the surface of the material to be welded
can be adjusted to produce a soft, harsh
or violent reaction by varying the gas
flows. There are of course practical
limits as to the type of flame, which can
be used for welding. A harsh forceful flame will cause the molten weld pool to be
blown away, while too soft a flame will not be stable near the point of application.
The blowpipe is therefore designed to accommodate different sizes of 'swan neck
copper nozzle which allows the correct intensity of flame to be used. The relationship
between material thickness, blowpipe nozzle size and welding speed, is shown in the
chart. When carrying out fusion welding the addition of filler metal in the form of a
rod can be made when required. The principal techniques employed in oxyacetylene
welding are leftward, rightward and all positional rightward. The former is used
almost exclusively and is ideally suited for welding butt, fillet and lap joints in sheet
thicknesses up to approximately 5mm. The rightward technique finds application on
plate thicknesses above 5mm for welding in the flat and horizontal-vertical position.
The all-positional rightward method is a modification of the rightward technique and
is ideally suited for welding steel plate and in particular pipework where positional
welding, (vertical and overhead) has to be carried out. The rightward and allpositional rightward techniques enable the welder to obtain a uniform penetration
bead with added control over the molten weld pool and weld metal. Moreover, the
welder has a clear view of the weld pool and can work in complete freedom of
movement. These techniques are very highly
skilled and are less frequently used than the
conventional leftward technique.
Solid wire MIG
Metal inert gas (MIG) welding was first
patented in the USA in 1949 for welding aluminium. The arc and weld pool formed
using a bare wire electrode was protected by helium gas, readily available at that time.
From about 1952 the process became popular in the UK for welding aluminium-using
argon as the shielding gas, and for carbon steels using CO2. CO2 and argon-CO2
mixtures are known as metal active gas (MAG) processes. MIG is an attractive
alternative to MMA, offering high deposition rates and high productivity.
Process characteristics
MIG is similar to MMA in that heat for welding is produced by forming an arc
between a metal electrode and the workpiece; the electrode melts to form the weld
bead. The main difference is that the metal electrode is a small diameter wire fed from
a spool. As the wire is continuously fed, the process is often referred to as semiautomatic welding.
Metal transfer mode
The manner, or mode, in which the metal transfers from the electrode to the weld pool
largely, determines the operating features of the process. There are three principal
metal transfer modes:
Short circuiting
Droplet / spray
Short-circuiting and pulsed metal transfer are used for low current operation while
spray metal transfer is only used with high welding currents. In short-circuiting or
'dip' transfer, the wire dipping into the weld pool transfers the molten metal forming
on the tip of the wire. This is achieved by setting a low voltage; for a 1.2mm diameter
wire, arc voltage varies from about 17V (100A) to 22V (200A). Care in setting the
voltage and the inductance in relation to the wire feed speed is essential to minimise
spatter. Inductance is used to control the surge in current, which occurs when the wire
dips into the weld pool.
For droplet or spray transfer, a much higher
voltage is necessary to ensure that the wire
does not make contact i.e. short-circuit, with
the weld pool; for a 1.2mm diameter wire, the
arc voltage varies from approximately 27V
(250A) to 35V (400A). The molten metal at
the tip of the wire transfers to the weld pool
in the form of a spray of small droplets (about
the diameter of the wire and smaller).
However, there is a minimum current level,
threshold, below which droplets are not
forcibly projected across the arc. If an open arc technique is attempted much below
the threshold current level, the low arc forces would be insufficient to prevent large
droplets forming at the tip of the wire. These droplets would transfer erratically across
the arc under normal gravitational forces. The pulsed mode was developed as a means
of stabilising the open arc at low current levels i.e. below the threshold level, to avoid
short-circuiting and spatter. Spray type metal transfer is achieved by applying pulses
of current, each pulse having sufficient force to detach a droplet. Synergic-pulsed
MIG refers to a special type of controller, which enables the power source to be tuned
(pulse parameters) for the wire composition and diameter, and the pulse frequency to
be set according to the wire feed speed.
Shielding gas
In addition to general shielding of the arc and the weld pool, the shielding gas
performs a number of important functions:
Forms the arc plasma
Stabilises the arc roots on the material surface
Ensures smooth transfer of molten droplets from the wire to the weld pool
Thus, the shielding gas will have a substantial effect on the stability of the arc and
metal transfer and the behaviour of the weld pool, in particular, its penetration.
General-purpose shielding gases for MIG welding are mixtures of argon, oxygen and
C02, and special gas mixtures may contain helium. The gases, which are normally
used for the various materials, are:
Argon +2 to 5% oxygen
Argon +5 to 25% CO2
Argon / helium
Argon based gases, compared with CO2, is generally more tolerant to parameter
settings and generates lower spatter levels with the dip transfer mode. However, there
is a greater risk of lack of fusion defects because these gases are colder. As CO2
cannot be used in the open arc (pulsed or spray transfer) modes due to high backplasma forces, argon based gases containing oxygen or CO2 are normally employed.
MIG is widely used in most industry sectors and accounts for almost 50% of all weld
metal deposited. Compared to MMA, MIG has the advantage in terms of flexibility,
deposition rates and suitability for mechanisation. However, it should be noted that
while MIG is ideal for 'squirting' metal, a high degree of manipulative skill is
demanded of the welder.
Submerged-arc Welding
The first patent on the submerged-arc welding (SAW) process was taken out in 1935
and covered an electric arc beneath a bed of granulated flux. Developed by the E O
Paton Electric Welding Institute, Russia, during the Second World War, Saw’s most
famous application was on the T34 tank.
Process features
Similar to MIG welding, SAW involves formation of an arc between a continuously
fed bare wire electrode and the workpiece. The process uses a flux to generate
protective gases and slag, and to add alloying elements to the weld pool. A shielding
gas is not required. Prior to welding, a thin layer of flux powder is placed on the
workpiece surface. The arc moves along the joint line and as it does so, excess flux is
recycled via a hopper. Remaining fused slag layers can be easily removed after
welding. As the arc is completely covered by the flux layer, heat loss is extremely
low. This produces a thermal efficiency as high as 60% (compared with 25% for
manual metal arc). There is no visible arc light, welding is spatter-free and there is no
need for fume extraction.
Operating characteristics
SAW is usually operated as a fully mechanised or automatic process, but it can be
semi-automatic. Welding parameters: current, arc voltage and travel speed all affect
bead shape, depth of penetration and chemical composition of the deposited weld
metal. Because the operator cannot see the weld pool, greater reliance must be placed
on parameter settings.
Process variants
According to material thickness, joint type and size of component, varying the
following can increase deposition rate and improve bead shape.
SAW is normally operated with a single wire on either AC or DC current. Common
variants are:
Twin wire
Triple wire
Single wire with hot wire addition
Metal powdered flux addition
All contribute to improved productivity through a marked increase in weld metal
deposition rates and/or travel speeds.
Fluxes used in SAW are granular fusible minerals containing oxides of manganese,
silicon, titanium, aluminium, calcium, zirconium, magnesium and other compounds
such as calcium fluoride. The flux is specially formulated to be compatible with a
given electrode wire type so that the combination of flux and wire yields desired
mechanical properties. All fluxes react with the weld pool to produce the weld metal
chemical composition and mechanical properties. It is common practice to refer to
fluxes as 'active' if they add manganese and silicon to the weld, the arc voltage and the
welding current level influence the amount of manganese and silicon added. The main
types of flux for SAW are:
Bonded fluxes - produced by drying the ingredients, then bonding them with a
low melting point compound such as a sodium silicate. Most bonded fluxes
contain metallic deoxidisers, which help to prevent weld porosity. These
fluxes are effective over rust and mill scale.
Fused fluxes - produced by mixing the ingredients, then melting them in an
electric furnace to form a chemical homogeneous product, cooled and ground
to the required particle size. Smooth stable arcs, with welding currents up to
2000A and consistent weld metal properties, are the main attraction of these
SAW is ideally suited for longitudinal and
circumferential butt and fillet welds.
However, because of high fluidity of the weld
pool, molten slag and loose flux layer,
welding is generally carried out on butt joints
in the flat position and fillet joints in both the
flat and horizontal-vertical positions. For
circumferential joints, the workpiece is
rotated under a fixed welding head with
welding taking place in the flat position.
Depending on material thickness, either
single-pass, two-pass or multipass weld
procedures can be carried out. There is virtually no restriction on the material
thickness, provided a suitable joint preparation is adopted. Most commonly welded
materials are carbon-manganese steels, low alloy steels and stainless steels, although
the process is capable of welding some non-ferrous materials with judicious choice of
electrode filler wire and flux combinations.
TIG Welding
Tungsten inert gas (TIG) welding became an overnight success in the 1940s for
joining magnesium and aluminium. Using an inert gas shield instead of a slag to
protect the weld pool, the process was a highly attractive replacement for gas and
manual metal are welding. TIG has played a major role in the acceptance of
aluminium for high quality welding and structural applications.
Process characteristics
In the TIG process the arc is formed between a pointed tungsten electrode and the
workpiece in an inert atmosphere of argon or helium. The small intense arc provided
by the pointed electrode is ideal for high quality and precision welding. Because the
electrode is not consumed during welding, the welder does not have to balance the
heat input from the arc as the metal is deposited from the melting electrode. When
filler metal is required, it must be added separately to the weld pool.
Power source
TIG must be operated with a drooping, constant current power source - either DC or
AC. A constant current power source is essential to avoid excessively high currents
being drawn when the electrode is short-circuited on to the workpiece surface. This
could happen either deliberately during arc starting or inadvertently during welding.
If, as in MIG welding, a flat characteristic power source is used, any contact with the
workpiece surface would damage the electrode tip or fuse the electrode to the
workpiece surface. In DC, because arc heat is distributed approximately one-third at
the cathode (negative) and two-thirds at the anode (positive), the electrode is always
negative polarity to prevent overheating and melting. However, the alternative power
source connection of DC electrode positive polarity has the advantage in that when
the cathode is on the workpiece, the surface is cleaned of oxide contamination. For
this reason, AC is used when welding materials with a tenacious surface oxide film,
such as aluminium.
Arc starting
The welding arc can be started by scratching the surface, forming a short-circuit. It is
only when the short-circuit is broken that the main welding current will flow.
However, there is a risk that the electrode may stick to the surface and cause a
tungsten inclusion in the weld. This risk can be minimised using the 'lift arc' technique
where the short-circuit is formed at a very low current level. The most common way
of starting the TIG arc is to use HF (High Frequency). HF consists of high voltage
sparks of several thousand volts, which last for a few microseconds. The HF sparks
will cause the electrode - workpiece gap to break down or ionise. Once an
electron/ion cloud is formed, current can flow from the power source.
Note: As HF generates abnormally high electromagnetic emission (EM), welders
should be aware that its use can cause interference especially in electronic
equipment. As EM emission can be airborne, like radio waves, or transmitted along
power cables, care must be taken to avoid interference with control systems and
instruments in the vicinity of welding.
HF is also important in stabilising the AC arc; in AC, electrode polarity is reversed at
a frequency of about 50 times per second, causing the arc to be extinguished at each
polarity change. To ensure that the arc is reignited at each reversal of polarity, HF
sparks are generated across the electrode/workpiece gap to coincide with the
beginning of each half-cycle.
Electrodes for DC welding are normally pure tungsten with 1 to 4% thoria to improve
arc ignition. Alternative additives are lanthanum oxide and cerium oxide, which are
claimed to give superior performance (arc starting and lower electrode consumption).
It is important to select the correct electrode diameter and tip angle for the level of
welding current. As a rule, the lower the current the smaller the electrode diameter
and tip angle. In AC welding, as the electrode will be operating at a much higher
temperature, tungsten with a zirconia addition is used to reduce electrode erosion. It
should be noted that because of the large amount of heat generated at the electrode, it
is difficult to maintain a pointed tip and the end of the electrode assumes a spherical
or 'ball' profile.
Shielding gas
Shielding gas is selected according to the material being welded. The following
guidelines may help:
Argon - the most commonly used shielding gas, which can be used for
welding a wide range of materials including steels, stainless steel, aluminium
and titanium.
Argon + 2 to 5% H2 - the addition of hydrogen to argon will make the gas
slightly reducing, assisting the production of cleaner-looking welds without
surface oxidation. As the arc is hotter and more constricted, it permits higher
welding speeds. Disadvantages include risk of hydrogen cracking in carbon
steels and weld metal porosity in aluminium alloys.
Helium and helium/argon mixtures - adding helium to argon will raise the
temperature of the arc. This promotes higher welding speeds and deeper weld
penetration. Disadvantages of using helium or a helium/argon mixture are the
high cost of gas and difficulty in starting the arc.
TIG is applied in all industrial sectors but is especially suitable for high quality
welding. In manual welding, the relatively small arc is ideal for thin sheet material or
controlled penetration (in the root run of pipe welds). Because deposition rate can be
quite low (using a separate filler rod) MMA or MIG may be preferable for thicker
material and for fill passes in thick-wall pipe welds.
TIG is also widely applied in mechanised systems either autogenously or with filler
wire. However, several 'off the shelf' systems are available for orbital welding of
pipes, used in the manufacture of chemical plant or boilers. The systems require no
manipulative skill, but the operator must be well trained. Because the welder has less
control over arc and weld pool behaviour, careful attention must be paid to edge
preparation (machined rather than hand-prepared), joint fit-up and control of welding
Plasma Welding
Process characteristics
Plasma welding is very similar to TIG as the
arc is formed between a pointed tungsten
electrode and the workpiece. However, by
positioning the electrode within the body of
the torch, the plasma arc can be separated
from the shielding gas envelope. Plasma is
then forced through a fine-bore copper nozzle, which constricts the arc. Varying bore
diameter and plasma gas flow rate can produce three operating modes:
Micro plasma: 0.1 to 15A.
The micro plasma arc can be operated at very low welding currents. The
columnar arc is stable even when arc length is varied up to 20mm.
Medium current: 15 to 200A.
At higher currents, from 15 to 200A, the process characteristics of the plasma
arc are similar to the TIG arc, but because the plasma is constricted, the arc is
stiffer. Although the plasma gas flow rate can be increased to improve weld
pool penetration, there is a risk of air and shielding gas entrainment through
excessive turbulence in the gas shield.
Keyhole plasma: over 100A.
By increasing welding current and plasma gas flow, a very powerful plasma
beam is created which can achieve full penetration in a material, as in laser or
electron beam welding. During welding, the hole progressively cuts through
the metal with the molten weld pool flowing behind to form the weld bead
under surface tension forces. This process can be used to weld thicker material
(up to 10mm of stainless steel) in a single pass.
Power source
The plasma arc is normally operated with a DC, drooping characteristic power source.
Because its unique operating features are derived from the special torch arrangement
and separate plasma and shielding gas flows, a plasma control console can be added
on to a conventional TIG power source. Purpose-built plasma systems are also
available. The plasma arc is not readily stabilised with sine wave AC. Arc reignition
is difficult when there is a long electrode to workpiece distance and the plasma is
constricted, Moreover, excessive heating of the electrode during the positive halfcycle causes balling of the tip which can disturb arc stability.
Special-purpose switched DC power sources are available. By imbalancing the
waveform to reduce the duration of electrode positive polarity, the electrode is kept
sufficiently cool to maintain a pointed tip and achieve arc stability.
Arc starting
Although the arc is initiated using HF, it is first formed between the electrode and
plasma nozzle. This 'pilot' arc is held within the body of the torch until required for
welding then it is transferred to the workpiece. The pilot arc system ensures reliable
arc starting and, as the pilot arc is maintained between welds, it obviates the need for
HF, which may cause electrical interference.
The electrode used for the plasma process is tungsten-2%thoria and the plasma nozzle
is copper. The electrode tip diameter is not as critical as for TIG and should be
maintained at around 30-60 degrees. The plasma nozzle bore diameter is critical and
too small a bore diameter for the current level and plasma gas flow rate will lead to
excessive nozzle erosion or even melting. It is prudent to use the largest bore diameter
for the operating current level.
Note: too large a bore diameter, may give problems with arc stability and maintaining
a keyhole.
Plasma and shielding gases
The normal combination of gases is argon for the plasma gas, with argon plus 2 to 5%
hydrogen for the shielding gas. Helium can be used for plasma gas but because it is
hotter this reduces the current rating of the nozzle. Helium's lower mass can also
make the keyhole mode more difficult.
Micro plasma welding
Micro plasma was traditionally used for welding thin sheets (down to 0.1 mm
thickness), and wire and mesh sections. The needle-like stiff arc minimises arc
wander and distortion. Although the equivalent TIG arc is more diffuse, the newer
transistorised (TIG) power sources can produce a very stable arc at low current levels.
Medium current welding
When used in the melt mode this is an alternative to conventional TIG. The
advantages are deeper penetration (from higher plasma gas flow), and greater
tolerance to surface contamination including coatings (the electrode is within the body
of the torch). The major disadvantage lies in the bulkiness of the torch, making
manual welding more difficult. In mechanised welding, greater attention must be paid
to maintenance of the torch to ensure consistent performance.
Keyhole welding
This has several advantages, which can be exploited: deep penetration and high
welding speeds. Compared with the TIG arc, it can penetrate plate thicknesses up to
l0mm, but when welding using a single pass technique, it is more usual to limit the
thickness to 6mm. The normal methods are to use the keyhole mode with filler to
ensure smooth weld bead profile (with no undercut). For thicknesses up to 15mm, a
vee joint preparation is used with a 6mm root face. A two-pass technique is employed
and here, the first pass is autogenous with the second pass being made in melt mode
with filler wire addition.
As the welding parameters, plasma gas flow rate and filler wire addition (into the
keyhole) must be carefully balanced to maintain the keyhole and weld pool stability,
this technique is only suitable for mechanised welding. Although it can be used for
positional welding, usually with current pulsing, it is normally applied in high speed
welding of thicker sheet material (over 3 mm) in the flat position. When pipe welding,
the slope-out of current and plasma gas flow must be carefully controlled to close the
keyhole without leaving a hole
Ceramics - materials, joining and
Ceramics are an incredibly diverse family of materials whose members span
traditional ceramics (such as pottery and refractories) to the modern day engineering
ceramics (such as alumina and silicon nitride) found in electronic devices, aerospace
components and cutting tools.
Whilst the most extravagant claims of the 1980s in favour of advanced ceramic
materials (such as the all ceramic engine) have largely proved inaccurate, it is true to
say that ceramics have established themselves as key engineering materials.
When used in conjunction with other materials, usually metals, they provide added
functionality to components thereby improving application performance, once the
appropriate joint design and technology have been identified.
Ceramic materials
Ceramics exhibit very strong ionic and/or covalent bonding (stronger than the metallic
bond) and this confers the properties commonly associated with ceramics: high
hardness, high compressive strength, low thermal and electrical conductivity and
chemical inertness.
This strong bonding also accounts for the less attractive properties of ceramics, such
as low ductility and low tensile strength. The wider range of properties, however, is
not widely appreciated. For example, whilst ceramics are perceived as electrical and
thermal insulators, ceramic oxides (initially based on Y-Ba-Cu-O) are the basis for
high temperature superconductivity. Diamond, beryllia and silicon carbide have a
higher thermal conductivity than aluminium or copper.
Control of the microstructure can overcome inherent stiffness to allow the production
of ceramic springs, and ceramic composites have been produced with a fracture
toughness about half that of steel.
The main compositional classes of engineering ceramics are the oxides, nitrides and
carbides. The Table gives the general properties of the most used ceramics.
Table 1 Properties of ceramics
Ceramic Melting Density Strength
of thermal
(x 10-6/°C)
Aluminium oxide (Al2O3) and zirconia (ZrO2) are the most commonly used
engineering grade oxide ceramics, with alumina being the most used ceramic by far in
terms of both tonnage and value.
Silicon nitride (Si3N4), and aluminium nitride (AlN) are the main advanced
engineering ceramics in this category. There is a wide range of grades and types of
these materials, particularly of silicon nitride with each grade having specific
Silicon carbide (SiC) is widely used for its high thermal conductivity, corrosion
resistance and hardness, although as an engineering ceramic its toughness is lower
than that of some silicon nitride grades. Boron carbide (B4C) is the third hardest
industrial material (after diamond and cubic boron nitride) and is used for components
needing very high wear performance.
Ceramic-based composites
Ceramics are used as the reinforcement of composite systems such as GRP (glass
reinforced plastics) and metal matrix composites such as alumina-reinforced
aluminium (Al/Al2O3). Advanced ceramic materials are also used as the matrix
materials in composites. Currently the most widely available materials are based on
SiC and carbon.
There are many possible techniques for joining ceramics to themselves and to
dissimilar materials. These technologies range from mechanical fixturing to direct
bonding. Fig.1 gives an overview of these methods.
Fig.1. An overview
of processes for
joining ceramics
The selection of one of these techniques to manufacture a particular component will
depend on a number of factors including:
Desired component function e.g. strength, electrical insulation or wear
Materials to be joined
Operational temperature
Applied stress
Required level of joint hermeticity
Component design
Whilst all these considerations must be taken into account, generally the two
important factors are the similarity of the materials to be joined and the required
temperature capability. Fig. 2 gives the temperature capability of a number of joining
capability of a
number of joining
When joining ceramics to metals it is necessary to create an interface between the
materials. In general the interface must accommodate the following:
The difference in coefficient of thermal expansion (CTE)
Bond type i.e. ionic/covalent for ceramics ranging to the metallic bond
Crystallographic lattice mismatch between the ceramic and metal
Compared to metals and plastics, ceramics are hard, non-combustible and inert. Thus
they can be used in high temperature, corrosive and tribological applications. These
applications rely on combinations of properties that are unique to industrial ceramics
and which include:
Retention of properties at high temperature
Low coefficient of friction (particularly at high loads and low levels of
Low coefficient of expansion
Corrosion resistance
Thermal insulation
Electrical insulation
Low density
Engineering ceramics are used to fabricate components for applications in many
industrial sectors, including ceramic substrates for electronic devices (Fig. 3),
turbocharger rotors (Fig. 4), and tappet heads for use in automotive engines. Other
examples of where advanced ceramics are used include oil-free bearings in food
processing equipment, aerospace turbine blades, nuclear fuel rods, lightweight
armour, cutting tools, abrasives, thermal barriers and furnace/kiln furniture.
Fig.3. Ceramic substrates for
electronic devices
Fig.4. Ceramic turbocharger rotor
assembly made from silicon nitride
Courtesy of NGK/NTK Spark Plug Co
When selecting a material for use in a specific component the applicability and
suitability of the candidate materials need to be considered in detail. When a ceramic
material is being selected the fitness-for-purpose criteria that should be applied
Operational environment - atmosphere, temperature, applied stress, fatigue,
exposure time
Predictable excursions beyond the usual, including mechanical impact or rapid
Design - ceramic materials are relatively intolerant of abrupt changes in crosssection such as notches, holes and corners
Joining - the role of the joint, its operational conditions and performance
requirements and the joining techniques suitable for manufacture
Cost - as with all materials selection and component design questions, the cost
and availability of the raw materials and all necessary fabrication techniques
must be considered in the light of their suitability to provide a component with
the required performance profile at a viable cost
Future development is likely to come from improved processing and fabrication
techniques that will lower component costs or improve behaviour, an increasing
demand for higher performance materials necessitating the use of more ceramics.
Whilst it is difficult to predict new materials, improvements in existing ones can be
readily foreseen. The most significant area of development is likely to be in the
ceramic matrix composites.
Whilst existing composites based on SiC will improve as porosity levels are reduced
by improved processing techniques, the development of high temperature oxide-based
composites is likely to provide a competitor material system with wider applicability
in the near future. In the future we can expect to see a still greater contribution to
industrial growth and technological development from these materials.
Welding techniques for thermoplastics
The purpose of this article is to give an overview of the variety of techniques
available to industry for the thermal joining of thermoplastics.
The techniques used can be divided into three distinct groups based on the method
used to introduce heat to the weld. These are:
By mechanical movement,
By an external heat source
From electromagnetism
Welding techniques where heat is generated by
mechanical movement
Linear vibration
In linear vibration welding the parts to be joined are brought into contact under
pressure before being rubbed together in a linear reciprocating motion. The resulting
friction melts the material at the interface after which the vibration stops; the parts are
then aligned and held together until the weld solidifies.
Most thermoplastic materials can be welded using this technique, which is used
extensively in the automotive industry for joining components such as two-part
bumpers, fuel tanks, air ducts and inner door panels.
Fig. 1. Spin welding machine
In spin welding the joint areas are always circular and the motion is rotational. The
technique has been exploited for applications as diverse as the manufacture of
polyethylene floats, aerosol bottles, transmission shafts and PVC pipes and fittings.
Ultrasonic welding involves the use of high frequency mechanical energy to soften or
melt the thermoplastic at the joint line. Parts to be joined are held together under
pressure and then subjected to ultrasonic vibrations, usually at a frequency of 20 or
40kHz. Ultrasonic welding is a fast process, with weld times typically less than a
second, and can be easily automated. It is a popular choice for assembling
components in the automotive, medical, electronic and packaging markets.
Welding techniques using an external heat source
Hot plate
Hot plate welding is possibly the simplest plastic joining technique, used for various
applications ranging from small automotive fluid reservoir vessels to pipelines in
excess of 1000mm in diameter.
The technique involves heating the ends of the parts to be joined against an
electrically heated platen until they are sufficiently molten. The heater plate is then
removed and the parts pressed together. A cooling cycle follows, allowing the weld to
develop strength.
Hot bar and impulse
This technique is mainly used for joining thermoplastic films with a thickness of less
than 0.5mm. It works on the principle that if two films are pressed against a heated
metal bar, they will soften and allow a joint to be made between them. Weld times are
rapid, around two seconds for 100mm film.
The principle of impulse welding is the same. Here the heat comes from a brief burst
of electrical energy through a nickel chromium wire triggered as the films are pressed
together. This method is used in packaging for the rapid sealing of polyethylene bags.
Hot gas
In hot gas welding of thermoplastics, the parts to be joined, typically sheet sections up
to 30mm in thickness, are prepared in a V-butt or T-butt configuration before a stream
of hot gas is directed towards the joint area. This causes melting of the joint area and
also of a consumable filler rod of the same polymer type as the parts being joined.
The weld is formed from the fusing together of the joint with the filler material.
The main advantage of hot gas welding is that the equipment is easily portable.
However, the process is slow and weld quality depends greatly on the skill of the
operator. Training and Certification of operators is recommended to achieve high
Fig. 2. Extrusion welding
Extrusion welding is similar to hot gas welding, sharing some of its characteristic
advantages and disadvantages. Molten thermoplastic filler material is fed into the joint
preparation from the barrel of a mini hand-held extruder based on an electric drill.
The molten material emerges from a PTFE shoe shaped to match the profile being
welded. At the leading edge of the shoe a stream of hot gas is used to pre heat the
substrate prior to the molten material being deposited, ensuring sufficient heat is
available to form a weld.
The process is used typically for assembly of large fabrications such as chemical
storage vessels, with wall thicknesses up to 50mm.
Welding techniques, which directly use
Fig. 3. Overview of welding processes for thermoplastics, grouped by heating
Resistive implant
This involves trapping an electrically conducting implant between the two parts to be
joined before applying a high electric current to cause resistive heating. As the
implant heats, the surrounding thermoplastic material softens and melts. Application
of pressure ensures the molten surfaces fuse together to form a weld.
A widely used application of resistive implant welding is the electrofusion technique
for joining thermoplastic pipes using specially designed socket couplers containing an
integral electrical heating coil.
Induction is similar to resistive implant welding, as an implant is generally needed at
the joint line. However, in this process a work coil connected to a high frequency
power supply is placed close to the joint. As high frequency electric current passes
through the work coil, a dynamic magnetic field is generated whose flux interacts
with the implant. Eddy currents are induced in the implant, heating it and the
surrounding joint area.
High frequency (dielectric)
High frequency (dielectric or radio frequency) welding relies on the ability of the
plastic being joined to generate heat in a rapidly alternating electric field. Hence the
technique is generally restricted to PVC, EVA and polyurethane’s.
During the process, the parts to be joined are subjected to a high frequency electric
field applied between two metal bars. The dynamic electric field causes molecular
vibration in the plastic. Some of the resulting oscillatory motion is converted into
thermal energy, causing the material to heat.
Products manufactured by high frequency welding include stationery wallets,
inflatables, tarpaulins and blood bags.
During infrared welding the parts to be joined are brought into very close proximity
with an electrically heated platen. The technique is similar to hot plate welding
although no actual physical contact is made with the heat source. After sufficient time
has elapsed the parts become molten and can be forced together to form a weld.
Infrared welding is generally faster than hot plate welding with typical welding times
being reduced by around 50%. The fact that heating is achieved without physical
contact eliminates the possibility of contamination entering the weld from the surface
of the hot plate. The technique is used for joining thermoplastic pipes.
The laser welding technique uses a focused beam of intense radiation, usually in the
infrared area of the electromagnetic spectrum, to melt the plastic in the joint region.
The type of laser used and the absorption characteristics of the plastic determine the
extent of welding possible.
ClearWeldTM transmission welding, recently patented by TWI, uses a colourless
infrared absorbing medium at the joint interface of two transmissive plastics. Thus
two optically clear plastics may be laser welded with an almost invisible joint.
Laser welding has the advantage of being a quick, clean, non-contact process, which
generates minimum flash and distortion