Evolution of carbides on surface of carburized M50NiL bearing steel

The dissolution and precipitation behaviors of the carbides in carburized M50NiL steel were derived from different solution and tempering treatments. Totally four kinds of carbides of (V, Cr)-rich MC, (Mo, Fe)-rich M2C, Fe-rich M3C and (Fe, Cr)-rich M7C3 were obtained from the carburized M50NiL steel after different heat treatments. The key carbides of carburized M50NiL steel were proved to be tough V-rich MC and Cr-rich M7C3. The highest hardness (634 HV) and the optimal surface structure with 1.0% volume fraction of uniformly distributed MC carbides were obtained after the carburized M50NiL steel was solution-treated at 1150 °C and tempered at 500 °C. The quantitative statistics show that 63% of the MC carbides were less than 200 nm under that heat treatment. The variety of carbides changed with solution and tempering conditions. When the solution temperature increased from 1050 to 1150 °C, the undissolved carbides were proved to be Fe-rich M7C3, Mo-rich MC and (Mo, Fe)-rich M2C. Besides, the equivalent content of V-rich MC was found increased when the tempering temperature changed from 500 to 550 °C. The combination of high-temperature solution and low-temperature tempering is recommendable heat treatment for the high hardness as well as the tiny and uniformly distributed carbides.