Evaluation of hot-cracking susceptibility of miniature spot-varestraint test

摘要

The purpose of this paper is to evaluate the repair weld cracking susceptibility of various grades of service-exposed, HP-modified, heat-resisting cast alloys and to clarify the occurrence mechanism of such weld cracking. The changes in the weld cracking susceptibility under long-term aging, the changes in the base metal micro-structure and the high-temperature ductility loss behaviour are examined. The hot embrittlement behaviour of HP heat-resisting cast alloys under long-term aging and its mechanism are also clarified. The paper describes application of the miniature spot-varestraint test to investigate the hot-cracking behaviour of HP-modified, heat-resisting cast alloy welds, evaluating the changes in their hot-cracking susceptibility under long-term aging and the differences in their hot-cracking susceptibility depending on the types and contents of alloying elements. The results obtained may be summarised as follows.1 The cracking occurring during repair welding of HP-modified, heat-resisting cast alloys is HAZ cracking. The fracture surfaces bear no liquated traces and this cracking is therefore considered to be ductility-dip cracking selectively propagating through the microconstituents.2 The cracking formed in the miniature spot-varestraint test is classified into Type I ductility-dip cracking selectively propagating through the microconstituents formed at locations 1-7 mm away from the fusion boundary and Type II liquation cracking caused by local melting of microconstituents formed very near the fusion boundary.3 As-cast HP35C, HP35H, HP35CW, HP35W, HP35CT and HP43A show few ductility-dip cracks and liquation cracks, whereas the service-exposed and aged steels show more ductility-dip cracks, and the ductility-dip cracking lengths and the cracking temperature ranges also tend to be greater in the aged steels than in the as-cast and service-exposed ones. Service-exposed HP43AZ, however, contains few cracks, with particularly the ductility-dip cracks being heavily suppressed.4 The results obtained during investigation of the changes in the ductility-dip cracking susceptibility under aging suggest that the number of cracks and the total crack length are smaller in the as-cast steels, increasing under aging. HP35CT tends to show a particularly sharp increase in ductility-dip cracking susceptibility under aging. The ductility-dip cracking susceptibility of HP43AZ, however, is lower than that of the other steels, being confined to the as-cast level of the other steels.5 In as-cast HP35C, HP35H, HP35CW, HP35W, HP35CT and HP43A, the changes in microconstituent morphology under long-term aging promote cracking inside the microconstituents to give a heightened hot-cracking susceptibility. HP43AZ, through containing added Zr, contains more refined dendrite than the other steels and therefore tends to have a reduced proportion of microconstituents present at the dendrite cell boundaries. This implies that, even when service-exposed, any increase in the hot-cracking susceptibility of HP43AZ is heavily suppresses.