Crack Arrest Toughness of a Heat-Affected Zone Containing Local Brittle Zones

Abstract

The awareness of the presence of local brittle zones (LBZs) in the heat-affected zone (HAZ) of welds has led to the requirements for minimum initiation (CTOD) toughness for the HAZ for critical applications (API RP 2Z, CSA S473). Such an approach, however, is expensive to implement and limits the number of potential steel suppliers. A fracture control philosophy that is proposed to be an attractive alternative for heat-affected zones containing LBZs is the prevention of crack propagation rather than of crack initiation . Such an approach would be viable if it could be demonstrated that cracks initiated in the LBZs will be arrested without causing catastrophic failure, notwithstanding the low initiation (CTOD) toughness resulting from the presence of LBZs. Unstable propagation of a crack initiating from an LBZ requires the rupture of tougher microstructural regions surrounding the LBZ in HAZ, and therefore the CTOD value reflecting the presence of LBZ is unlikely to provide a true indication of the potential for fast fracture along the heat-affected zone. Base metal specifications (CSA S473) usually ensure that small unstable cracks propagating from the weld zone into the base metal would be arrested. Past work has also shown that unstable crack initiation resulting from interaction of surface semi-elliptical cracks parallel to the fusion boundary with the local brittle zones can get arrested once the crack has popped through the depth of the LBZ. However, the potential for arrest when a through-thickness HAZ crack runs parallel to the fusion boundary, and thus parallel to the LBZs, has not been examined previously. To investigate the likelihood of fast fracture within the HAZ, a test program has been carried out that involved performing compact plane strain (ASTM E1221) and plane stress crack arrest tests on a heataffected zone that contained LBZs, and thus exhibited unacceptable low CTOD toughness for resistance to brittle fracture initiation. The results indicated that in contrast to the initiation toughness (CTOD toughness), the crack arrest toughness was little influenced by the presence of local brittle zones. Instead, the superior toughness of the larger proportion of finer-grain HAZ surrounding the LBZ present along the crack path has a greater influence on the crack arrest toughness. It further seems that there may be potential to estimate the HAZ crack arrest toughness from more conventional smaller-scale laboratory tests, such as conventional or precracked instrumented Charpy impact tests.