Microstructure evolution under different austenitizing temperatures and its effect on mechanical properties and mechanisms in a newly high aluminum bearing steel

A new type of lightweight high aluminum bearing steel had been developed based on the widely used high carbon chromium bearing steel. Via multi-scale experimental characterizations and theoretical calculations, here we mainly focused on the evolution of microstructure including precipitates under different quenching temperatures. Accordingly, the mechanical properties, mechanisms and fracture features of the new high-Al steel after tempering were characterized in detail. As austenitizing temperature rose, matrix of steel varied from multiphase structure of martensite, retained austenite (RA) and ferrite to the full martensite and RA. RA was found in all four samples with the austenitizing temperature from 850 °C to 1000 °C. In terms of precipitates, a transformation from M3C to M7C3 were observed. As a phase newly introduced by Al, AlFe3C (κ carbides) would dissolve into matrix at 900 °C. Eventually, the yield strength of the researched steel could reach 1430 MPa with an unnotched impact energy of 35 J. Theoretical results showed that high strength of researched steel was mainly caused by solid solution effect of carbon and high density of dislocation. In terms of toughness and fracture observation, high impact energy was attributed to the highest density of grain boundary, and also, κ carbides were found at the origin of secondary crack. This study highlighted the significant effect of Al addition and quenching process based on the traditional bearing steel, which showed a novel method and idea to reduce significantly the weight of bearings.