The structure of the heat-affected zone in welds of a Ni-29 wt.% Mo commercial alloy (Hastelloy B2)

Abstract

The microstructure of a welded pipe of Hastelloy B2 removed from a coal gasification plant was examined. Although this alloy is susceptible to severe embrittlement if ordering occurs, no significant ordering was found in the heat-affected zone (HAZ) or the weld, and the material was ductile. However, intergranular corrosion was found in the HAZ, and fine (e.g., 0.1 mm) particles of Mo-rich M 12C carbide and the intermetallic compound NiMo were found in the grain boundaries. These are apparently depleting the adjacent matrix of Mo, rendering the grain boundary region susceptible to corrosion. A single- and a double-pass autogeneous weld made on the base-plate material were examined. No ordering was detected in the HAZ, which is consistent with the measured temperature-time curves of regions adjacent to the weld and with the known ordering kinetics. In the HAZ, fine grain boundary particles of M 12C carbide were detected, and NiMo may also be present.