DEFECTS, CAUSES &
REMEDIES
DISCONTINUITY
The defect in acceptable limit
It may cause minor effects
100% quality welding is not possible
DEFECT
When discontinuity is large enough to affect the function of
the joint, it is termed a defect
Defects in Fusion Zone
Solidification Cracking Defects in Base Metal
• Contraction Stresses Spatter
• Segregation Lamellar Tearing
• Improper bead profile
Porosity Weld Joint Design
• Gas Lack of Fusion and
• Shrinkage Penetration
Hard Phase Transformation Undercut and Over weld
Defects in HAZ
Hydrogen Embrittlement
Grain Coarsening
Hard Phase Transformation
Typical Solidification Cracking
Carbon migration from the parent into the
weld metal, a process which starts during
post weld heat treatment and continues
with operation at service temperature
seams to be cause of cracking.
• Improper Bead Profile and Low melting Constituents
Low melting point compounds often
melted out of the BM (S in steel from
coal used to make the steel) last to
freeze and do so at weld centerline
leading to crack-like fissure known as
centerline cracking. Addition of Mn to
steel ties up S as MnS in interior of
grain and away from grain boundaries.
Porosity
Cluster Porosity
Warm Hole
Arc Craters
• Definition: A depression left at the termination of the weld where the weld pool
is left unfilled.
• Cause: Improper weld termination techniques
• Prevention: improving weld technique, use run off tabs
• Repair: If no cracks exist, simply fill in the crater. Generally welding from beyond
the crater back into the crater.
Hydrogen damages
Due to atmospheric hydrogen, fluxes etc
Hydrogen tendency to diffuse or penetrate into the steel matrix , especially HAZ
Due to hydrogen entrapment, it will lead to Martensite and brittleness
In general this crack is inter-granular in nature
Major Failures – Pressure vessel
Robert Jenkins Pressure vessel failure
The vessel was 34m long ; 2.7m in dia.
with a wall thickness of 28mm
The vessel was constructed according
to ASME Section VIII
The vessel material was a C-Mn ASTM
515 Grade 70
Failed during pressure testing at the
company's works on the 6 Nov. 1970
Vessel contained 171,000 l of water
Press. was 29 bar, 85% of req. value
A cold crack 115mm long at fillet weld
toe (weld HAZ) caused the failure
Typical Cold Crack
Weld metal structure
Principal Micro structure types of weld metal
AF
Acicular Ferrite (highly desirable) GF
- (AF) small feathery laths with low aspect ratio
- transformation occurs at 800C
Ferrite with aligned carbides
- (AC) martensite + austenite + carbides with AC
high aspect ratio
- poor toughness & low strength
Ferrite with carbide aggregates (FC)
- High heat input welds with low cooling rates
- equiaxed ferrite / carbide (pearlite) produced
Polygonal ferrite GF
- Grain boundary ferrite (GF)
- nucleated at Grain boundaries in high heat
input welds FC AF
- poor strength & very low toughness
Spatter generation in GMAW
Definition: Small particles of weld metal expelled from the welding operation which
adhere to the base metal surface
Low choking effect
increased spattering
coarse-scaled weld surface
stable, instantaneous arc striking
lots of short-circuits
High chocking effect
little spattering
fine-scaled weld appearance
delayed arc striking, irregular globular transfer
few short-circuits
Lamellar tearing
Serious cracks which develop in HAZ or
BM after welding! Called lamellar because
crack appearance is a series of straight lines
Rolled-out inclusions in BM lead to planes of
weakness in the short transverse direction in the
rolled plate
Arc Strike
• Definition: A localized
coalescence outside the
weld zone.
• Cause: Carelessness
• Repair: Where applicable, arc strikes must be sanded
smooth and tested for cracks. If found, they must be
remove and repaired using a qualified repair procedure
and inspected as any other weld.
Undercut
• Definition: A groove cut at the
toe of the weld and left unfilled.
• Cause: High amperage, electrode
angle, long arc length, rust
• Prevention: Set machine on scrap metal. Clean metal
before welding.
• Repair: Weld with smaller electrode, sometimes must be
low hydrogen with preheat. Sometimes must gouge first.
Undercut
(cont......)
Undercut typically has an allowable limit. Different
codes and standards vary greatly in the allowable
amount.
Plate - the lesser of 1/32” or 5% (typ.)
SUCK BACK
Suck back or root concavity is not a serious weld discontinuity. If the metal
cross-section is the same as the thickness of the base metal, no problem!
Large weld gap and weld metal shrinks or base metal parts move while
solidification occurs?
Overlap
Overlap is measured with
a square edge such as a
6” rule. No amount of
overlap is typically
allowed.
• Lack of fusion defects
Very serious, crack-like weld defects. Occur when welding process is out of
control. Severity proportional to the projection of the defect perpendicular to the
applied stress.
Inter-bead lack of fusion. Weld metal running ahead of the electrode.
Side-wall lack of fusion. Not enough energy directed at the sides of the prepared
edges/
• Geometric causes: lack of root fusion
Lack of root fusion.
Really a crack-like
defect. Results from
inadequate welding
procedure in the root
pass.
Radiographic images
appear like wagon-
tracks because
either side of the
root faces are un-
fused.
Burn-through
• Definition: When an undesirable open hole has been
completely melted through the base metal. The hole may or
may not be left open.
• Cause: Excessive heat input.
• Prevention: Reduce heat input by increasing travel speed,
use of a heat sink, or by reducing welding parameters.
• Repair: Will be defined by standards. Otherwise, removal
and rewelding may be required. Some standards may
require special filler metal and/or PWHT.
Geometric causes: melt through
Melt-through. Certainly a stress concentration and damaging in fatigue applications.
Melt through caused by excessive heat input in the first pass.
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