Study On Surface Microtructure And Fatigue Performance Of D2 Wheel Steel In The Process Of Friction And Wear

With the rapid development of high-speed railway in our country,the speed of the train increases continuously.The fatigue failure is one of the main failure modes of the wheels.One of the causes of fatigue failure of wheel is the change of surface microstructure during the operation of the wheel.Therefore,the study of the evolution of surface microstructure and fatigue properties during the wear process of wheel steel is of great significance to the optimization of wheel processing technology design.The wear machine was used to investigate the evolution of worn-surface microstructure and fatigue performance under pure rolling,0.5%slip ratio and pure sliding conditions,and the influence of pro-rolling wear on rolling contact fatigue life of D2 wheel steel.After testing,a transmission electron microscope(TEM),scanning electron microscope with electron backscatter diffraction(EBSD),LECIA microscopy,i-XRD residual stress tester and micro-hardness testers were used to characterize the microstructure and fatigue cracks.The surface microstructure and hardness of the pure rolling sample were in a steady state after 8 x 104 cycles;however,the 0.5%slip ratio sample reached a steady state after 7 x 103 cycles.Regardless of whether the test uses the slip ratio,the orientation of lamellar pearlites gradually became parallel to the surface and a portion of lamellar cementites was fragmented and dissolved during the formation of steady-state microstructure.The slip ratio accelerates this process.The hardening mechanism of the samples shows a decrease in the lamellar spacing of pearlite and the refinement of proeutectoid ferrite(PF).As the number of cycles increased,plastic deformation of samples became increasingly severe and the wear mechanism of the samples was fatigue wear in steady-state.The sample surfaces formed shallow cracks,which gradually peeled off.The slip ratio accelerated the initiation and propagation of fatigue cracks because of the high friction stress on the contact surface.Most fatigue cracks initiated at the interface of pearlite and PF and in the PF region.Pro-rolling wear under dry friction in air will increase the contact fatigue life in subsequent oil lubrication conditions.The contact fatigue life of the 1×104 and 3×105 cycles pro-rolling wear specimens was improved by 2 times and 7 times respectively compared with the original machined specimens.The microscopic analysis shows that the pro-rolling wear removed the mechanical damage layer on the machined surface,reduced the surface roughness.And the surface formd a fine grain layer after pro-rolling wear.The surface was obviously strengthened.The compressive stress layer was formed on the surface after pro-rolling wear.Therefore,the pro-rolling wear played a role in suppressing fatigue crack formation and improving contact fatigue life.During the sliding wear process,the thickness of the deformation layer and microhardness of the wheel samples gradually increased with the increase of cycles.The cementite in the deformed layer were fragmented,and the ferrite grains were refined to form a low-angle grain boundary.The ferrite grains mainly formed the texture in the<110>crystal direction.The closer the distance to the surface,the ferrite grains were gradually refined,and the proportion of high-angle grain boundaries gradually increased.The surface formed the white etching layer(WEL)with a thickness of about 10?m.The hardness of WEL was 850 HV.The fatigue crack on the surface of samples gradually became serious with the increase of cycles.The fatigue crack originated from the surface and propagataed along the interface between the fine grain layer and the original microstructure.