UNDERCUT
An irregular groove at the toe of a run in the parent
metal or in a previously deposited weld metal due to welding, it is characterised
by its depth, length and sharpness.
Causes
Melting of top edge due to high welding current
(especially at free edge) or high travel speed
Attempting a fillet weld in horizontal vertical (PB)
position with leg length >9mm
Excessive/incorrect weaving
Incorrect electrode angle
Incorrect shielding gas selection (MAG)
Prevention
Reduce power input, especially approaching a free
edge where overheating can occur
Weld in the flat position or use multi run techniques
Reduce weaving width or switch to multi-runs
Direct arc towards thicker member
Ensure correct gas mixture for material type and
thickness (MAG)
Comments
Care must be taken during weld repairs of undercut
to control the heat input. If the bead of a repair weld is too small, the
cooling rate following welding will be excessive and the parent metal may have
an increased hardness and the weld may be susceptible to hydrogen cracking.
EXCESS
WELD METAL
Excess weld metal is the extra metal that produces
excessive convexity in fillet welds and a weld thickness greater than the
parent metal plate in butt welds. This feature of a weld is regarded as an
imperfection only when the height of the excess weld metal is greater than a
specified limit.
Causes
Excess arc energy (MAG/ SAW)
Shallow edge preparation
Faulty electrode manipulation or build-up sequence
Incorrect electrode size
Too slow a travel speed
Incorrect electrode angle
Wrong polarity used (electrode polarity DC-VE (MMA,
SAW)
Prevention
Reduction of heat input
Deepen edge preparation
Improve welder skill
Reduce electrode size
Ensure correct travel speed is used
Ensure correct electrode angle is used
Ensure correct polarity i.e. DC +VE Note: DC-VE must
be used for TIG
Comments
The term ‘reinforcement’ used to designate this
feature of the weld is misleading since the excess metal does not normally
produce a stronger weld in a butt joint in ordinary steel. This imperfection
can become a problem, as the angle of the weld toe can be sharp, leading to an
increased stress concentration at the toes of the weld and fatigue cracking.
EXCESS
PENETRATION
Projection of the root penetration bead beyond a
specified limit can be local or continuous.
Causes
Weld heat input too high
Incorrect weld preparation i.e. excessive root gap,
thin edge preparation, lack of backing
Use of electrode unsuited to welding position
Lack of welder skill
Prevention
Reduce arc voltage and/or welding current; increase
welding speed
Improve work piece preparation
Use correct electrode for position
Retrain welder
Comments
Note that the maintenance of a penetration bead
having uniform dimensions requires a great deal of skill, particularly in pipe
butt welding. This can be made more difficult if there is restricted access to
the weld or a narrow preparation. Permanent or temporary backing bars can be
used to assist in the control of penetration.
OVERLAP
An imperfection at the toe of a weld caused by metal
flowing on to the surface of the parent metal without fusing to it
Causes
Poor electrode manipulation (MMA)
High heat input/low travel speed causing surface
flow of fillet welds
Incorrect positioning of weld
Wrong electrode coating type resulting in too high a
fluidity
Prevention
Retrain welder
Reduce heat input or limit leg size to 9mm max leg
size for single pass fillets.
Change to flat position
Change electrode coating type to a more suitable
fast freezing type which is less fluid
Comments
For a fillet weld overlap is often associated with
undercut, as if the weld pool is too fluid the top of the weld will flow away
to produce undercut at the top and overlap at the base. If the volume of the weld
pool is too large in case of a fillet weld in horizontal-vertical (PB) position,
weld metal will collapse due to gravity, producing both defects (undercut at
the top and overlap at the base), this defect is called ‘sagging’.
LINEAR
MISALIGNMENT
Misalignment between two welded pieces such that
while their surface planes are parallel, they are not in the required same
plane.
Causes
Inaccuracies in assembly procedures or distortion
from other welds
Excessive out of flatness in hot rolled plates or
sections
Comments
Misalignment is not really a weld imperfection, but
a structural preparation problem. Even a small amount of misalignment can
drastically increase the local shear stress at a joint and induce bending
stress.
ANGULAR
DISTORTION
Misalignment between two welded pieces such that
their surface planes are not parallel or at the intended angle
Causes
and prevention
Same as for linear misalignment
INCOMPLETELY
FILLED GROOVE
A continuous or intermittent channel in the surface
of a weld due to insufficient deposition of weld filler metal
Causes
Insufficient weld metal
Irregular weld bead surface
Prevention
Increase the number of weld runs
Retrain welder
Comments
This imperfection differs from undercut; it reduces
the load-bearing capacity of a weld, whereas undercut produces a sharp
stress-raising notch at the edge of a weld.
Irregular width
Excessive variation in width of the weld
Causes
Severe arc blow
Irregular weld bead surface
Prevention
Switch from DC to AC; keep as short as possible arc
length
Retrain welder
Comments
Although this imperfection may not affect the integrity
of completed weld, it can affect the width of HAZ and reduce the load-carrying
capacity of the joint (in the case of fine-grained structural steels) or impair
corrosion resistance (in the case of duplex stainless steels).
ROOT
CONCAVITY
A shallow groove that occurs due to shrinkage at the
root of a butt weld
Causes
Insufficient arc power to produce positive bead
Incorrect prep/fit-up
Excessive backing gas pressure (TIG)
Lack of welder skill
Slag flooding in backing bar groove
Prevention
Raise arc energy
Work to WPS
Reduce gas pressure
Retrain welder
Tilt work to prevent slag flooding
Comments
A backing strip can be used to control the extent of
the root bead.
BURN
- THROUGH
A collapse of the weld pool resulting in a hole in
the weld
Causes
Insufficient travel speed
Excessive welding current
Lack of welder skill
Excessive grinding of root face
Excessive root gap
Prevention
Increase the travel speed
Reduce welding current
Retrain welder
More care taken, retrain welder
Ensure correct fit-up
Comments
This is a gross imperfection, which occurs basically
due to lack of welder skill. It can be repaired by bridging the gap formed into
the joint, but requires a great deal of attention.
Comments
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