The repair weldability of two types of heat-resistant austenitic stainless steel castings, HP–Nb modified alloys and 20–32Nb alloys, has been evaluated after extended service exposure of these materials at temperatures on the order of 815 °C (1500 °F). The initial part of this investigation has focused on characterization of the microstructure of these alloys after service exposure times of up to 12 years. Microstructure evolution from the as-cast condition to the service-exposed condition is described. In the as-cast condition, both eutectic NbC and Cr-rich M 7 C 3 carbides were observed in the HP–Nb alloys. In contrast, only eutectic NbC carbides were present in the as-cast 20–32Nb alloys due to a significantly higher atomic ratio of Nb/C as compared to the HP–Nb alloys. In the service-exposed condition, an Ni–Nb silicide and Cr-rich M 23 C 6 were identified in both alloys. The HP–Nb alloys exhibited a much higher total volume fraction of microconstituents than the 20–32Nb alloys and the M 23 C 6 phase was more prevalent than Ni–Nb silicide. In contrast, the Ni–Nb silicide was dominant in the service-exposed 20–32Nb alloys. This difference resulted in a significant increase in the repair welding liquation cracking susceptibility of the 20–32Nb service-exposed alloy.