Effect Of Carbon Content On Microstructure And Properties Of Bainite Steel

In this paper,four kinds of different test steels with carbon content of 0.22 wt.%,0.48 wt.%,0.55 wt.%,and 0.68 wt.% were smelted to imitate the sublayer of G23Cr2Ni2 Si Mo carburized nano-bainitic bearing steel.The test steel was isothermal at 200? for 2h,4h,8h,12 h,24h and 48 h respectively.The change of microstructure was characterized by DIL,SEM and TEM and the hardness and impact toughness of the sample were tested.The effects of carbon content and isothermal time on the microstructure and properties of the bearing steel were revealed.Bainite and retained austenite were obtained by bainite transformation at different isothermal temperature.The effect of different isothermal temperature and carbon content on the microstructure and properties of bainite steel was studied.The carbon content from the surface to the center of the carburized bearing steel decreases gradually,and the microstructure of the transition layer is composed of martensite,bainite,retained austenite and cementite.Under the isothermal temperature of 200 ?,with the increase of isothermal time from 2 h to 48 h,the content of bainite ferrite in the microstructure increases gradually,the content of martensite decreases gradually,and the content of retained austenite increases first and then decreases.The hardness of the transition layer is affected by the volume fraction of each phase of the test steel,the hardness decreases first and then increases with the increase of isothermal time.The carbon content in the core of bearing steel is low,and the transformation time is very short at this temperature,so the hardness has no obvious relationship with the isothermal time,which is about 41 HRC.The impact toughness of the four steels increases with the increase of isothermal time.At the same time,with the increase of carbon content,the hardness of the steel increases and the impact toughness decreases.Bainitic ferrite and retained austenite are obtained by isothermal transformation at different temperatures.The Volume fraction of retained austenite increases with the increase of isothermal temperature.With the increase of carbon content and the decrease of transformation temperature,the thickness of bainite ferrite lath decreases.Because the increase of carbon content and the decrease of isothermal temperature can increase the strength of austenite,which makes the coarsening of bainite lath more difficult.When the bainite transformation is completed,the hardness of the steel increases with the increase of isothermal temperature.