Study On Heat Treatment Process Optimization And Fatigue Fracture Behavior Of Bainite Rail

In this thesis,U20Mn bainite rail is taken as the research object,and the effect of segmented controlled cooling process on the mirostructure and content of each phase of rail is studied through thermal simulation experiment.The quenching and tempering processes were optimized,and the optimal heat treatment process was determined by analyzing the microstructure,mechanical properties,fatigue properties and fracture morphology.Combined with color metallography,EBSD,TEM and other characterization methods,the microstructure morphology,phase content,residual austenite and dislocation under different heat treatment processes were studied,and the strengthening and toughening mechanism of bainitic rail was clarified.The research results provide a theoretical basis for heat treatment process optimization and microstructure control of high-speed and heavy bainitic rail.The experimental results show that:through thermal simulation and heat treatment process optimization,heating austenitizing temperature 900℃ isothermal 15min,the optimal fractional controlled cooling process is 3℃/s for the first stage before phase transformation,0.5℃/s for the second stage before phase transformation,0.3℃/s for bainite phase transformation,and air cooling to room temperature at the end of phase transformation.This process corresponds to the tensile strength of experimental steel is nearly 1500Mpa,and the impact energy KV2 is more than 60J.Furthermore,after 300℃isothermal tempering for 4h,the tensile strength is nearly 1400Mpa,and the impact energy KV2 is increased to more than 85J.The tensile and impact fracture is ductile fracture.The fatigue performance shows that the fatigue growth rate of the experimental steel is 1.68 when tempering at 300℃ for 4h,which is 25.3%lower than that of the untempered steel.The crack length in the propagation zone is increased from 3.69mm to 7.6mm when tempering at 300℃,and the fatigue performance is greatly improved.After the optimization of heat treatment quenching process and 300℃ isothermal tempering process for 4 hours,the microstructure of the experim ental steel is composed of relatively fine lath bainite,martensite and thin film residual austenite,the content of each phase is 70%,22%and 8%,the average size of lath bainite in the microstructure is 147nm.The average size of thin film retained austenite is 12.15nm.70%fine lath bainite can improve the strength and toughness of rail,22%martensite can ensure the strength and hardness of rail,8%thin film residual austenite in the structure of high mechanical stability,reduce stress concentration,passive crack tip,inhibit the formation and propagation of crack,which is conducive to improve the fatigue performance of rail.At the same time,3000C isothermal tempering can make dislocation lines in the tissue more uniform and fine,which has a certain dislocation strengthening and toughening effect In addition to enhancing the strength and toughness of cr-containing alloy cementite,it is easy to cause secondary cracks and effectively hinder the expansion of main cracks.The decrease of recrystallization,the decrease of sub-crystal structure,the increase of lattice distortion,the increase of internal energy,the increase of microscopic stress,and the obstruction of dislocation sliding deformation can further enhance the strength and toughness of the material.