Microstructure and Hydrogen-Induced Failure Mechanisms in Fe and Ni Alloy Weldments

The microstructure and fracture morphology of AISI 8630-IN625 and ASTM A182-F22-IN625 dissimilar metal weld interfaces were compared and contrasted as a function of postweld heat treatment (PWHT) duration. For both systems, the microstructure along the weld interface consisted of a coarse grain heat-affected zone in the Fe-base metal followed by discontinuous martensitic partially mixed zones and a continuous partially mixed zone on the Ni side of the fusion line. Within the partially mixed zone on the Ni side, there exists a 200-nm-wide transition zone within a 20-μm-wide planar solidification region followed by a cellular dendritic region with Nb-Mo–rich carbides decorating the dendrite boundaries. Although there were differences in the volume of the partially mixed zones, the major difference in the metal weld interfaces was the presence of M₇C₃ precipitates in the planar solidification region, which had formed in AISI 8630-IN625 but not in ASTM A182-F22-IN625. These precipitates make the weldment more susceptible to hydrogen embrittlement and provide a low energy fracture path between the discontinuous partially mixed zones.