Study On Composite Heat Treatment Process Of 8Cr4Mo4V Bearing Steel

8Cr4Mo4V is an important aviation bearing steel.It is mainly used to manufacture aero-engine bearings.It is composition is equivalent to the M50 steel in the American ASTMA600standard.In order to improve the heat treatment structure of 8Cr4Mo4V steel and improve its performance,the composite heat treatment technology of the combination of isothermal quenching and oil cold quenching was studied in this paper.The phase transition of 8Cr4Mo4V steel was analyzed,and the optimum process of oil cold quenching and isothermal quenching tempering was studied respectively.On this basis,the composite heat treatment process was designed,through a series of experiments,performance tests,the microstructure of 8Cr4Mo4V steel in different heat treatment states was analyzed by SEM/EDS,TEM/EDS,XRD.Therefore,the heat treatment structure and properties of 8Cr4Mo4V steel are clarified,while the optimal heat treatment processes of oil quenching,isothermal quenching,compound quenching and tempering of8Cr4Mo4V steel are determined.The research results show that the best oil quenching heat treatment process for 8Cr4Mo4V steel is 1090?×20min solid solution+oil quenching+3 times 530?×120min tempering.At this time,the hardness of the steel is 61.87HRC,the grain size is grade 9,and the structure is tempered sorbite,residual carbides and a large number of dispersed precipitates.Among them,the MC and M₂C type residual carbides have sizes of 3.5?m and 2.0?m,respectively,located near the grain boundaries,and the M₇C₃and M₆C type residual carbides have sizes below 1.0?m and are located in the crystal.The composition phases include?-Fe,VC,(Mo,V)₂C,(Fe,Cr)₆C,(Fe,Cr)₇C₃and(Fe,Cr)₂₃C₆phases.The best austempering heat treatment process is 1090?×20min solid solution+200?×120min austempering+3 times 540?×120min tempering.The structure is tempered sorbite,residual carbides and a large number of dispersed precipitation phases.The hardness is 61.10HRC.The composition phases include?-Fe,VC,(Mo,V)₂C,(Fe,Cr)₆C,(Fe,Cr))₇C₃,(Fe,Cr)₃C,Cr₂₃C₆.8Cr4Mo4V steel after 1090?×20min solution,200?austempering and subsequent oil-cooling quenching to obtain B/M composite structure,including acicular bainite,flake martensite body,retained austenite and residual carbides.Among them,the types and distribution of residual carbides are the same as those of oil-cooled quenching.Both bainite and martensite are refined.With the extension of the austempering time,the bainite content gradually increases and the size gradually increases.The content of stenite gradually decreases,the size gradually decreases,and the hardness first increases and then decreases.Under the 200?×50min austempering+oil quenching process,the difference in size between bainite and martensite is the smallest,the structure is the smallest,and the hardness is the highest.The tempered hardness is61.87HRC,the wear resistance is the best,and its structure is tempered sorbite,residual carbides and a large number of nano-precipitated phases.The composition phase includes?-Fe,VC,(Mo,V)₂C,(Fe,Cr)₇C₃,(Fe,Cr)₆C,(Fe,Cr)₃C and(Fe,Cr)₂₃C₆.Compared with the quenched state,The(Fe,Cr)₇C₃phase increased significantly.The combined effect of fine-grain strengthening,solid solution strengthening and second phase dispersion strengthening is the key to determining the performance of steel.The bainite formed by austempering has not grown sufficiently,and the retained austenite content during oil quenching is limited,and it is body segmentation and limited martensite size are the cause of the refinement of the steel structure and the main reason why the hardness of the composite quenched steel is greater than that of the oil quenched and austempered steel.In short,it can effectively control the B/M composite structure and improve the mechanical properties of 8Cr4Mo4V steel through a reasonable combination of austempering and oil quenching and controlling the austempering time.