Training of CSWIP
Welding Inspectors
Weld and Base Metal Discontinuities
22 August 2009
Prepared by Diver Kim
Fusion Weld Joint
Fusion Zone
• A mixture of filler metal and base metal that has completely
melted
• High degree of homogeneity among the component metals
that have been melted during welding
• The mixing of these components is motivated largely by
convection in the molten weld pool
2
Fusion Weld Joint
Weld Interface
• The narrow boundary that separates the fusion zone and the heat
affected zone
• This interface consists of a thin band of base metal that was melted
or partially melted (localized melting within the grains) during the
welding process, but immediately solidified before any mixing could
take place
Heat Affected Zone (HAZ)
• The metal in this region has experienced temperature below its
melting point, but high enough to change the microstructure
• This metal consists of the base metal which has undergone a heat
treatment due to the welding temperatures, so that its properties
have been altered.
• The amount of metallurgical damage in the HAZ depends on the
amount of heat input, peak temp reached, distance from fusion
zone, time at elevated temp, cooling rate, and the metal’s thermal
properties
3
Fusion Weld Joint
Heat Affected Zone (HAZ) cont’d
• The effect on the mechanical properties is usually negative, and
it is most often the region of the weld joint where failure occurs
Unaffected Base Metal Zone
• Where no metallurgical change has occurred
• The base metal surrounding the HAZ is likely to be in a state of
high residual stress, due to the shrinkage in the fusion zone
4
Discontinuities and Defects
• Discontinuity- Lack of homogeneity
• Defect- Rejectable
• Determinants of discontinuity significance
1. Linearity: Ratio of largest and least dimensions (Length > 3 Times)
2. End condition (or sharpness)
Most severe
Cracks
↓
Incomplete fusion
↓
Inadequate penetration
↓
Solid inclusions
Least severe
Gaseous inclusions
Varying severity
Shape discontinuities
Miscellaneous discontinuities
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Cracks and Cracking
Two factors for cracking
1. Load (magnitude, application mod)
2. Mechanical properties (grain type,
size)
Three factors for delayed
cracking
1. Hydrogen in HAZ
2. High residual stress
3. A microstructure of relatively low
ductility
Detection
1. Surface: VT, MT, PT
2. Internal: UT, RT
6
Cracks – Hot Cracking
• Occurrences
1. Solidification
(A tear rather than a crack)
2. Deep and narrow
(Depth exceeding width)
3. Impurities, etc.
• Detections
1. Visual exam (Break the surface)
2. MT or PT for even tight cracks
7
Cracks – Cold Cracking
•Occurrences
1. Develop at or near ambient
temperature below the lower
transformation (A1) temperature.
2. Hot cracks- Welder
3. Cold cracks-Design/Supervision
•Delayed cracks:
1. After the weldment cools to ambient
temperature
2. QT steel requires exam after 4 days
3. Hydrogen involved in cold cracking
8
Hydrogen Induced HAZ Cracking
Causes:
• Hardened HAZ coupled with the
presence of hydrogen diffused
from weld metal
• Susceptibility increases with the
increasing thickness of section
especially in steels with high
carbon equivalent composition
• Can also occur in weld metal
• Increase welding heat beneficial
• Preheating sometimes necessary
• Control of moisture in
consumables and cleanliness of
weld prep desirable
9
Reheat Cracking
• Occurs in creep resisting and some thick
section structural low alloy steels during
post weld heat treatment
Causes:
• Poor creep ductility in HAZ
coupled with thermal stress
• Accentuated by severe notches
such as preexisting cracks, or
tears at weld toes, or unfused
root of partial penetration weld
• Heat treatment may need to
include low temperature soaking
• Grinding or peening weld toes
after welding can be beneficial
X 35
X 200
10
Incomplete Fusion
1. Molten weld metal flows over
unmelted base metal or adjacent
weld beads
2. A significant welding problem –
linearity, end condition
3. Primary causes: Electrode size or
improper manipulation, or both
Detections
1.Visual exam for overlap, which
results from molten weld metal
flowing over unmelted base
metal
2. A problem of semi-, automatic
3. RT or UT for subsurface exam
11
Incomplete Fusion and Penetration
Lack of side-wall fusion
Lack of root fusion
Lack of penetration
Lack of inter-run fusion
12
Incomplete Joint Penetration
1. Results from utilizing an
unsuitable welding procedure
Incomplete fusion: welder-based
problem
2. Readily detected by visual
inspection, be identified during
back gouging
3. Unless it is possible to view the
back of a joint, directly or by
boroscope – small diameter
pipe and tube
13
Solid Inclusion
Detection
- Normally revealed by RT
• Oxide Inclusion
Non-ferrous metals / Ferrous
metals
2. Metallic Inclusion
Tungsten / Copper
3. Slag Inclusion
Wagon tracks
14
Linear Slag Inclusion
Cause:
•Incomplete removal of
slag in multi-pass welds
often associated with the
presence of undercut or
irregular surfaces in
underlying passes
15
Isolated Slag Inclusion
Causes:
•Normally by the presence
of mill scale and/or rust on
prepared surfaces, or
electrodes with cracked
or damaged coverings
•Can also arise from
isolated undercut in
underlying passes of multipass welds
16
Tungsten Inclusions
A tungsten particle embedded in a weld.
(Typically GTAW only)
Cause:
Tungsten electrode too small, amperage
too high, AC balance on +, Upslope too
high, electrode tip not snipped, electrode
dipped into the weld pool or touched
with the fill rod, electrode split.
• Prevention: Eliminate the cause
• Repair: Grind out and re-weld Very hard
entrained particle
• Imparts local mechanical and thermal
stresses
17
Copper Inclusions
Causes:
•Melting of copper contact
tube in MIG welding due to
incorrect welding conditions
X 275
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Gaseous Inclusion (porosity)
Cavities
• Detection
Surface: VT
Internal: UT, RT
• Single or Isolated pores
•
Uniformly and non-uniformity
scattered porosity
• Starting porosity
• Linear or longitudinal porosity
• Special event porosity- by lack of
cleanliness
• Cluster Porosity
19
Worm Holes
• Resulting from the entrapment of gas
between the solidifying dendrites of weld
metal, often showing ‘herringbone’ array ( B )
Causes:
• The gas may arise from contamination of
surfaces to be welded, or be prevented
from escaping from beneath the weld by
joint crevices
20
Uniformly Distributed Porosity
• Resulting from the entrapment of gas
in solidified weld metal
Causes:
• Gas may originate from dampness
or grease on consumables or
workpiece, or by nitrogen
contamination from the
atmosphere
• If the weld wire used contains
insufficient deoxidant it is also
possible for carbon monoxide to
cause porosity
21
Restart Porosity
Causes:
• Unstable arc conditions at
weld start, where weld pool
protection may be
incomplete and temperature
gradients have not had time
to equilibrate, coupled with
inadequate manipulative
technique to allow for this
instability
22
Surface Porosity
Causes:
• Excessive contamination
from grease, dampness,
or atmosphere
entrainment
• Occasionally caused by
excessive sulfur in
consumables or parent
metal
23
Crater Pipes
Resulting from shrinkage at the
end crater of a weld run
• Causes:
• Incorrect manipulative
technique or current
decay to allow for crater
shrinkage
24
Shape Discontinuities
• Shape refers to the weld profile (in cross section)
• Size under run is considered a shape discontinuity
25
Shape Discontinuities
26
Shape Discontinuities
• The Significant of the
weld reinforcement
angle with respect to the
load carrying capacity.
• The load capacity of
fillet welds, is based on
the weld throat.
27
Linear Misalignment
Cause:
•Incorrect assembly or
distortion during
fabrication
28
Excessive Reinforcement
Causes:
•Deposition of too much
weld metal, often
associated with in
adequate weld
preparation
•Incorrect welding
parameters
•Too large of an electrode
for the joint in question
29
Overlap
Causes:
•Poor manipulative
technique
•Too cold a welding
conditions (current and
voltage too low)
30
Undercut
• Results from the washing
away of edge preparation
when molten
Causes:
•Poor welding technique
•Imbalance in welding
conditions
31
Excessive Penetration
Causes:
•Incorrect edge
preparation providing
insufficient support at the
weld root
•Incorrect welding
conditions (too high of
current)
•The provision of a backing
bar can alleviate this
problem in difficult
circumstances
32
Root Concavity
Causes:
•Shrinkage of molten pool
at weld root, due to
incorrect root preparation
or too cold of conditions
•May also be caused by
incorrect welding
technique
33
Miscellaneous Faults (Arc Strikes)
Super-fast cooling, drop of weld
metal, gas pores
Cause:
• Accidental contact of an
electrode or welding torch
with a plate surface
remote from the weld
• Usually result in small hard
spots just beneath the
surface which may
contain cracks, and are
thus to be avoided
34
Miscellaneous Faults (Spatters)
Causes:
• Incorrect welding
conditions and/or
contaminated
consumables or
preparations, giving rise to
explosions within the arc
and weld pool
• Globules of molten metal
are thrown out, and
adhere to the parent
metal remote from the
weld
35
Miscellaneous Discontinuities
• Base Metal caused Discontinuities
1. Seams and Laps:
Result from rolling practices
2. Lamination:
Metal composition, or processing
3. Delamination:
Lamination physically separates
36
Miscellaneous Discontinuities
•Lamellar Tearing
Shrinkage associated with
cooling weld metal causes
lamination prone steel to tear.
Causes:
• Poor ductility in throughthickness direction in rolled
plate due to non-metallic
inclusions
• Occurs mainly in joints
having weld metal
deposited on plate surfaces
• Prior buttering of surface
beneficial for susceptible
plate
37
Miscellaneous Discontinuities
•Lamellar Tearing
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