Microstructure evolution during service exposure of two cast, heat-resisting stainless steels — HP–Nb modified and 20–32Nb

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 M7C3 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 M23C6 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 M23C6 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.