EFFECT OF HOT CRACKS ON EAC CRACK INITIATION AND GROWTH IN NICKEL-BASE ALLOY WELD METALS

摘要

The differences in the EAC susceptibility between different weld geometries and weld metals have been distinguished by the doped steam test method. Pure weld metals of Alloy 182 and 82 are clearly more susceptible to EAC than the pure weld metals of Alloy 152 and 52, which did not show any crack initiation. The dissimilar metal welds (DMW) with diluted microstructures are less susceptible than the pure weld metals of Alloy 182 and 82. No crack initiation/extension from hot cracks was observed in any of the studied weld metals. At the hot crack tips no crack growth was observed in any of the studied samples. This is related to the segregated micro structure of the hot crack tips. In accelerated doped steam tests selective dissolution takes place and metallic Ni or NiO forms a continuous layer in the middle of the cracks surrounded by the Cr-rich oxide layer. Selective dissolution typical for EAC was not observed inside the hot cracks or at their crack tips. EAC initiation occurred in the Alloy 600 base metal of the DMWs and selective dissolution inside the EAC cracks in Alloy 600 was extensive. The results are discussed based on the selective dissolution creep model of EAC.