Effects of Preformed Martensite on Microstructure and Mechanical Properties of Nanobainite Bearing Steel

Previous studies have indicated that the preformed martensite has a significant effect on the subsequent bainite transformation of nanobainite bearing steel, but its effects on the microstructure and mechanical properties are still unclear. In this paper, the effects of preformed martensite and its volume fraction on microstructure and mechanical properties of nanobainite bearing steel were investigated by using scanning electron microscope, transmission electron microscope, X-ray diffractometer and wear resistance experiments. It was demonstrated that the introduction of preformed martensite can refine bainitic microstructure and increase the volume fraction of retained austenite, but reduces the mechanical stability of retained austenite. When the volume fraction of preformed martensite is less than 20%, the hardness and wear resistance of nanobainite bearing steel gradually increases with the increase in the volume fraction of preformed martensite, but the toughness of steel decreases gradually. When the volume fraction of preformed martensite reaches 25%, the properties of nanobainite bearing steel, especially the wear resistance, obviously deteriorate. The optimized volume fraction of preformed martensite is similar to 10%-15% by comprehensive comparison the mechanical properties of nanobainite bearing steel.