Achieving ultrafine martensite-bainite duplex microstructures by combining prior cold deformation with chronological control of phase transformation in GCr15 bearing steel

In this work, a strategy is proposed by combining prior cold deformation with chronological control of phase transformation to achieve ultrafine martensite-bainite duplex microstructures in GCr15 bearing steel, which is the specimens with 0∼45% prior cold deformation are subjected to first martensite transformation at below martensite start (Ms) temperature to form different fraction of first transformed martensite due to the decreasing of Ms temperature caused by prior cold deformation and subsequent transfer to isothermal holding at above Ms temperature for bainite transformation. It is found that prior cold deformation can further accelerates bainite transformation by shortening incubation and growth period, and increasing bainite transformation rate. The specimen with 30% prior cold deformation after heat-treated by austenitizing at 850 o C for 30min, first martensite transformation at 200 o C for 5min and subsequent bainite transformation at 240 o C for 30min, a marginal improvement of strength and significant improvement of impact toughness are achieved, which is increased by 2.8% and 20.9% compared to the specimen with 0 prior cold deformation. Meanwhile, ultrafine bainite platelets of ∼220nm are achieved, which is refined by 35% compared to the specimen with 0 prior cold deformation. In addition, the effect of prior cold deformation on the evolution of martensite-bainite duplex microstructures is also discussed.