Effect Of Isothermal Quenching On Microstructure And Mechanical Properties Of 8Cr4Mo4V Bearing Steel

8Cr4Mo4V steel is widely used to make the main shaft bearing of key parts of aeroengines because of its excellent performance.Its performance and accuracy directly influence the reliability,working stability and environmental adaptability of aeroengines.Heat treatment is an extremely important part in the manufacturing process of 8Cr4Mo4V steel spindle bearings.8Cr4Mo4V steel is widely used in aeroengine spindle bearings with working temperature below 316℃and DN value below 2.4×10⁶ mm·r/min.With the development of the aviation industry,working temperature of bearings will be increased and the speed and load can also be increased during service,which requires more strict requirements for bearing life.Therefore,it is necessary to improve the comprehensive performance of 8Cr4Mo4V bearing steel.The lower bainite structure has better toughness and plasticity than the martensite structure.Therefore,this paper focuses on the bainite isothermal treatment of 8Cr4Mo4V steel to study the effect of lower bainite structure on the properties of 8Cr4Mo4V steel.In this paper,the melting point of different types of carbides is calculated by Jmat Pro software.Based on this,the solid solution temperature range of 8Cr4Mo4V steel is determined to be 950℃-1120℃,and the holding time is 10min-60min,in order to study the effect of solution temperature and time on the grain size,carbide dissolution,microstructure and mechanical properties of 8Cr4Mo4V steel after quenching and tempering.8Cr4Mo4V steel is subjected to isothermal quenching in the bainite transformation temperature zone.The isothermal quenching temperature is 150℃,200℃and 240℃,and the isothermal holding time is 2h,8h,16h,24h and 32h,in order to study the effect of isothermal temperature and holding time on the microstructure and mechanical properties of 8Cr4Mo4V bearing steel.Results show that the order of carbide dissolution in 8Cr4Mo4V steel is M₂₃C₆,M₆C,M₂C and MC.With the increase of solution temperature and the extension of the holding time,the amount and size of carbide in the steel gradually decrease,and the carbon element dissolves into the matrix.With the increase of the solution temperature,the grains of8Cr4Mo4V bearing steel continue to increase,and the carbides in the matrix gradually dissolve,which result in the hardness of the bearing steel after quenching initially decreasing and then increasing,but the hardness after tempering continues increase.With the increase of isothermal temperature,the transformation speed of lower bainite increases,the amount of transformation increases,and the microstructure becomes coarse.With the extension of holding time,the amount of transformation of lower bainite increases,and the transformation speed gradually decreases.There is incompleteness in bainite transformation.Under any temperature and time conditions,the full lower bainite structure cannot be obtained.During 150℃isothermal quenching,martensite transformation mainly occurs.With the extension of the holding time,the rockwell hardness decreases initially and then increases.During 200℃and 240℃isothermal quenching,the bainite transformation mainly occurs.With the extension of the holding time,the rockwell hardness initially increases and then decreases.After isothermal quenching at 150℃,the residual stress on the surface of the sample is tensile stress and compressive stress at the core,while the surface residual stress is compressive stress and tensile stress appears at the core by isothermal quenching at 200℃and 240℃.After isothermal quenching treatment,the amount of sample wearing is reduced,and the friction and wear performance is improved.The rotating bending fatigue strength of the sample is increased from 790MPa to 1050MPa.