Effect of Austempering Temperature on Microstructure and Mechanical Properties of M50 Bearing Steel

By means of microstructure observation and mechanical property tests, the effect of austempering temperature on the bainite transformation, carbide precipitation, and the hardness, impact toughness of M50 bearing steel were studied. Results show that when the austempering temperature increases from 180 °C, 200 °C, 220 °C, 240 °C to 260 °C, the bainite content in the microstructure of the steel is 3.5%, 11.9%, 15.4%, 22.1% and 36.1% respectively. The effect of austempering temperature on bainite content is mainly related to the incubation period of bainite transformation, the driving force of bainitic ferrite nucleation and the stability of supercooled austenite. With the increase of austempering temperature, the carbide content decreases but the average carbide size generally increases in the austempered and tempered steel. When the austempering temperature increases from 180 °C to 200 °C, the hardness of the sample decreases from 62.45 HRC to 62.04 HRC, and the impact toughness of the sample increases from 144.75 kJ/m2 to the maximum value of 171.50 kJ/m2 at 200 °C. When the austempering temperature increases from 200 °C to 260 °C, the hardness and impact toughness of the steel both decrease, which is related to the weakening of the dispersion strengthening caused by the increase of carbide size, and to the increase of bainite content, the increase of bainite size, and the self-inhibited behavior of bainite transformation.