Effect of cooling rate after isothermal stage of vacuum austempering on microstructure and hardness of M50 bearing steel

By means of the observation of martensite, bainite and carbides, and the measurement of surface residual stress and hardness, the effect of cooling rate after isothermal stage of vacuum austempering on the microstructure and mechanical properties of M50 steel was studied. Results show that after the isothermal stage of 200 °C × 120 min, the average surface cooling rates of 4.25 °C/min, 5.69 °C/min and 7.33 °C/min were obtained when the inflation pressures of N2 were 0.05 MPa, 0.10 MPa and 0.15 MPa, and the microstructure of the austempered M50 steel was composed of lower bainite, martensite/residual austenite and carbides. When the average cooling rate was 4.25 °C/min, a large amount of lower bainite was formed in the steel, and the microstructure was significantly refined. When the average cooling rate increased to 5.69 °C/min and 7.33 °C/min, the nucleation rate of martensite increased, and the amount of martensite obtained after austempering increased. The surface residual stresses of the steel with the cooling rates of 4.25 °C/min, 5.69 °C/min and 7.33 °C/min were −54 MPa, 208 MPa and 336 MPa respectively. As the cooling rate increases, the hardness of the austempered steel increases, but that of the tempered steel decreased. This is related to the formation of more martensite and its tempering decomposition.