THE STRUCTURE OF THE HEAT-AFFECTED ZONE IN WELDS OF A NI-29 WT-PERCENT MO COMMERCIAL ALLOY (HASTELLOY-B2)

The microstructure of a welded pipe of Hastelloy B2 removed from a coa l gasification plant was examined. Although this alloy is susceptible to severe embrittlement if ordering occurs, no significant ordering wa s found in the heat-affected zone (HAZ) or the weld, and the material was ductile. However, intergranular corrosion was found in the HAZ, an d fine (e.g., 0.1mm) particles of Mo-rich M12C carbide and the interme tallic compound NiMo were found in the grain boundaries. These are app arently depleting the adjacent matrix of Mo, rendering the grain bound ary region susceptible to corrosion. A single- and a double-pass autog eneous weld made on the base-plate material were examined. No ordering was detected in the HAZ, which is consistent with the measured temper ature-time curves of regions adjacent to the weld and with the known o rdering kinetics. In the HAZ, fine grain boundary particles of M12C ca rbide were detected, and NiMo may also be present.