WELD QUALITY 539

severe embrittlement. Cracks are usually caused by met-

Table 13.2
Classification of the Weld Joint Discontinuities allurgical discontinuities. For example, cold cracks are
that Occur in Fusion Welding often the result of localized embrittlement.
Metallurgical discontinuities may also occur in the
I. Process- or Procedure-Related Discontinuities
base metal prior to fabrication. For example, lamina-
A. Geometric tions are base metal defects. These are a type of dis-
Backing left on continuity characterized by separation or weakness
Burn-through (melt-through) generally aligned parallel to the worked surface of a
Concavity metal, possibly resulting from piping blisters, seams,
inclusions, or segregations that have become elongated
Convexity
and made directional by working (e.g., rolling, forging,
Excessive weld reinforcement or drawing).
Improper reinforcement
Incomplete penetration
Incomplete fusion
Design-Related Discontinuities
Mismatch Some discontinuities found in welded joints are the
Overlap result of design decisions. Proper weldment design
Shrinkage ensures that the performance of the weldment is not
Surface irregularities
adversely affected by stress concentrations, which may
initiate a failure. For example, an abrupt change in
Undercut
shape or the cross-sectional area of a weldment, which
B. Other concentrates stress, is a discontinuity. Weldment design
Arc craters can also affect the occurrence of mechanical and metal-
Arc strikes lurgical discontinuities. Partial penetration is a recog-
Oxide films nized discontinuity that must be avoided in certain
Slag inclusions
types of weldments.
Spatter
Moreover, weldments designed with limited access
for welding make it difficult for the welder to deposit a
Tungsten inclusions
sound weld and increase the likelihood of welder-
II. Metallurgical Discontinuities performance or process-related discontinuities. Condi-
A. Cracks or fissures tions of high restraint increase the likelihood that hot
Cold or cold cracks may initiate in the weld metal or heat-
affected zone.
Delayed
Hot
Lamellar tearing
Reheat, stress-relief, or strain-age*
LOCATION AND OCCURRENCE OF
B. Porosity
DISCONTINUITIES
Scattered Discontinuities in weldments may be found in the
Cluster weld metal, the heat-affected zone, and the base metal.
Aligned They may be surface or subsurface discontinuities,
Piping requiring the application of different inspection meth-
Elongated
ods. They may be dimensional discontinuities, involving
concerns with distortion, joint mismatch, and weld size,
C. Heat-affected-zone and microstructure alteration
among others.
D. Weld-metal and heat-affected-zone segregation
It should be noted that the term fusion-type disconti-
E. Base-metal laminations and delaminations nuity is sometimes used inclusively to describe incom-
III. Design-Related Discontinuities plete fusion, incomplete joint penetration, inclusions,
A. Changes in section and other stress concentrations and similar elongated discontinuities in fusion welds.
Although many codes and standards consider fusion-
B. Type of weld joint
type discontinuities less critical than cracks, some spe-
* For further information on these discontinuities, refer to Oates, W. R., ed., cifically prohibit fusion-type defects as well as cracks.
1996, Materials and Applications—Part 1, Vol. 3 of Welding Handbook,
8th ed., Miami: American Welding Society; and Oates, W. R., and A. M. Specific joint types and welding procedures have an
Saitta, eds., 1998, Materials and Applications—Part 2, Vol. 4 of Welding effect on the type, location, and incidence of discontinu-
Handbook, 8th ed., Miami: American Welding Society. ities. The welding process, joint details, restraint on the