Page 109 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Fracture Modes and Welding Defects

Reheat cracking
Reheat cracking occurs primarily in the HAZ of thick-section
high strength low alloy (HSLA) steels, 300 series stainless
steels and nickel-based alloys. During PWHT or elevated
service temperatures intergranular cracking can occur due to
stress relaxation in coarse grained regions under high
restraint or residual stresses. The failure normally initiates
at a stress concentration such as a notch or change in cross-
section. Reheat cracking can be avoided by:
. adequate preheating to reduce the stress levels in the HAZ;
. using joint designs that require less restraint during
welding in thick sections;
. removing stress concentrations caused by sharp changes in
cross-section, such as sharp undercut, mechanical damage
and poorly blended weld toes.
Lamellar tearing
Lamellar tearing occurs mainly in thick-section T-joints and
closed corner joints in carbon and carbon manganese steels
with a high sulphur content and/or high levels of restraint. It
does not occur in cast or forged steels; only in rolled plate. It
has the appearance of a ‘steplike’ crack (Fig. 7.2) and occurs
in wrought (rolled) plates due to a combination of:
. contraction stresses from the cooling weld acting through
the parent plate thickness plus

Figure 7.2 Lamellar tearing

93

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200907QG Welding chap7.3d Page 93 of 107