On the bainite structure in Cr-Mo-V rotor steel

The morphology of continuously cooled and isothermally transformed bainite structures formed in a Cr-Mo-V rotor steel has been studied using transmission electron microscopy. The samples were austenitised at 955-degrees-C for an hour followed by air cooling to room temperature. The isothermal transformation reaction was carried out at 450-degrees-C for up to 100 000 s. The microconstituents observed are predominantly lower bainite with very small amount of upper bainite and martensite (formed from untransformed austenite due to water quenching). Analysis of the selected area diffraction patterns confirm that the carbide in bainite is orthorhombic cementite and the orientation relationship between ferrite and cementite is consistent with that of Bagaryatskii. The carbide particles in isothermally transformed bainite are coarser than those of continuously cooled bainite. Tempering one hour at 670-degrees-C of continuously cooled steel samples exhibited formation of fine spheroidal MC type carbides. In addition tempering leads to the enrichment of prior austenite grain boundaries by cementite particles. Tempering ten hours at 670-degrees-C exhibited microstructures almost identical to those observed in one hour tempering.