Postservice examination of a thick-section AISI 347/AISI P22 transition joint revealed wedge-type creep damage at prior austenite grain boundaries in the low alloy steel heat-affected zone close to the fusion boundary. This damage had linked to form axial grain boundary microcracks oriented at 45°to the weld profile. A circumferential macrocrack had formed in these regions. The cracking observed is explained in terms of the high stresses generated due to the mismatch in coefficients of thermal expansion between the high and low alloy steels. Examination of the detail weldment microstructures was undertaken using optical and electron microscopy. Some success in predicting operating temperatures has been achieved from analysis of the carbides present.