Study On Bainite Isothermal Quenching And Tempering Stability Of New Medium Carbon Bearing Steel

As a new impact-resistant rolling mill bearing steel, when G55SiMoV steel was treated by conventional martensitic quenching and tempering, its impact toughness is still difficult to meet the demand, which often cause early breakage during use, and affect its performance into full play. This paper developed the bainite isothermal quenching process of G55SiMoV steel, and optimized the best process and parameters, through the study on the effect of parameters on the organization and performance. As a result, the bearing parts after processing get a good combination of strength and toughness.The influence of bainite isothermal quenching process on microstructure and properties of G55SiMoV steel was investigated and compared with conventional martensitic quenching and pre-quenching in this paper. The results show:with the austenitizing temperature increase, prior austenite grain and lath of martensite and bainite coarsen, the bainite content increase, when the austenitizing temperature over930℃, the microstructure obvious coarsen, hardness and impact toughness reduce; With the isothermal quenching temperature increase, the bainite needles become longer and wider, when the isothermal quenching temperature over300℃, the morphology of bainite changes form needles to featheriness, the residual austenite content increase, the hardness reduce and the impact toughness increase; With the extension of isothermal time, the content of bainite and residual austenite increase, the hardness reduce, and the impact toughness increase. Optimizing process for mechanical properties is that:austenitizing temperature shall not be higher than930℃, isothermal quenching temperature should be lower than300℃, isothermal time for at leastl5min. The microstructure of G55SiMoV steel treated by this process are mixed microstructure of bainite and martensite, and the proportion of bainite and martensite easy to be controlled, so that, G55SiMoV steel not only has the best cooperate of hardness and impact toughness, but also has good comprehensive mechanical properties, especially its toughness and wear resistance performance are improved greatly.In addition, this process has good stability and reproducibility. Pre-quenching microstructure are also mixed microstructure of bainite and martensite, but the pre-quenching process is more complex, so the pre-formed martensite content is not easy to control, resulting microstructure and performance unstable. Conventional martensitic quenching is simple, hardness and wear-resisting performance also can meet the requirements, but the main drawback is that toughness is poor, likely to cause the fracture failure in the process of using. Therefor, bainite isothermal quenching process is best for G55SiMoV steel making in rolling mill bearingThe temper stability of G55SiMoV steel was studied by series tempering test. The study find:the G55SiMoV steel in the low-temperature tempering after being bainite isothermal quenching, the microstructure do not change significantly, the hardness remain stable, and impact toughness is higher; When temper temperature at300℃, impact toughness decrease dramatically and brittleness appear; as temper temperature rises further, hardness greatly reduce substantially. Compared with the martensite quenching process, the G55SiMoV steel treated by bainite isothermal quenching has better temper stability.