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 pro perties of a low alloy Cr-Mo-Ni-V steel applied for turbine rotor, ret ired after 15.8 years of service was evaluated and supplemented with t he results from microstructural and fractographic examinations. Maximu m embrittlement, manifested as decrease in upper shelf energy and incr ease in ductile-to-brittle transition temperature had occurred at a se rvice temperature of 714K and the material serviced at 811 K exhibited relatively better toughness. A re-aging treatment at 811 K for 24 h p artially recovered the loss in toughness. The dominant carbides identi fied were M23C6 and M6C, enriched with chromium and iron, and molybden um and iron, respectively and M2C with molybdenum and vanadium as the major metallic constituents. Coarsening of carbides occurred with incr easing 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 relat ively less coarse grain boundary precipitates. Homogeneous precipitati on of fine carbides of M2C type also occurred at this temperature thro ugh carbide reactions. Re-aging of the embrittled material led to part ial dissolution of grain boundary carbides and also to copious precipi tation of M2C type of carbide. Observed changes in mechanical properti es are partly attributed to the in-service evolution of carbides and p artly attributed to segregation of impurities at grain boundary.