Service-induced changes in the microstructure and mechanical properties of a Cr-Mo-Ni-V turbine steel

The effect of service temperature on the degradation of mechanical properties of a low alloy Cr-Mo-Ni-V steel applied for turbine rotor, retired after 15.8 years of service was evaluated and supplemented with the results from microstructural and fractographic examinations. Maximum embrittlement, manifested as decrease in upper shelf energy and increase in ductile-to-brittle transition temperature had occurred at a service temperature of 714K and the material serviced at 811 K exhibited relatively better toughness. A re-aging treatment at 811 K for 24 h partially recovered the loss in toughness. The dominant carbides identified were M23C6 and M6C, enriched with chromium and iron, and molybdenum and iron, respectively and M2C with molybdenum and vanadium as the major metallic constituents. Coarsening of carbides occurred with increasing temperature through carbide reactions. The embrittled material showed the presence of coarse M23C6 carbides along the prior-austenite grain boundaries. However, material serviced at 811 K contained relatively less coarse grain boundary precipitates. Homogeneous precipitation of fine carbides of M2C type also occurred at this temperature through carbide reactions. Re-aging of the embrittled material led to partial dissolution of grain boundary carbides and also to copious precipitation of M2C type of carbide. Observed changes in mechanical properties are partly attributed to the in-service evolution of carbides and partly attributed to segregation of impurities at grain boundary.