Effect of cold deformation on the bainite transformation kinetics, microstructural evolution and mechanical properties of austempered GCr15 bearing steel

Abstract

In this work, the effect of prior cold deformation on the bainite transformation kinetics of austempered GCr15 bearing steel was investigated by means of dilatometry. It is found that prior cold deformation retards bainite transformation by increasing the incubation and growth time and decreasing the transformation rate, indicating that the effect of carbon content in parent austenite on bainite transformation kinetics is larger than the refinement of prior austenite grain size. Meanwhile, it also found from microstructures analysis that the prior cold deformation is beneficial to refine prior austenite grain size, facilitate dissolution of spherical carbides and improve its aspect ratio. After the heat treatment of austenitizing at 850 °C for 30 min and following austempering at 240 °C for 30 min, a marginal improvement of tensile strength and significant improvement of impact toughness were achieved in specimen with 30% prior cold deformation, which is increased by 6.2% and 29.3%, as compared to that of the specimen without prior cold deformation. In addition, the effect of prior cold deformation on microstructural evolution is also discussed.