Creep damage of weldments in 1Cr-0.5Mo steels during long term service at elevated temperature

Abstract

1. 1) The cracking of 1Cr-0.5Mo steel stub welds from the E.C.N.S.W. 3rd stage superheater outlet header occurs almost exclusively within the HAZ. The cracking is intergranular and follows prior austenite grain bo undaries. There is extensive cavitation along prior austenite grain boundaries adjacent to and ahead of these cracks. 2. 2) There is an enhanced concentration of MnS inclusions confined almost exclusively to the HAZ of those stub welds from the E.C.N.S.W. header exhibiting cracking. This is attributed to an effective “overheating” of the HAZ during weld formation, leading to dissolution and then re-precipitation of MnS particles at pr ior austenite grain boundaries during heating and cooling cycles, respectively. 3. 3) Techniques of scanning electron microscopy, and transmission electron microscopy of extraction replicas from both fracture surfaces and polished and etched sections, have been used to identify the presence of a large volume fraction of sub-micron MnS particles at prior austenite grain boundaries in the HAZ of cracked weldments. Coarse carbides have also been identified at these boundaries, the most common carbides being M 7C 3 and M 3C.