Study On Evolution Of Microstructure And Properties Of D2/U71Mn During Pure Rolling Friction And Wear

With the rapid development of railways,the running speed of trains has been increasing and the amount of transportation has increased sharply,which makes the damage problem of wheels and rails more serious.Traction,braking and operation of trains rely on the rolling contact of wheels and rails,and the change of wheel-rail contact surface plays an important role in the service status of wheel-rail.Therefore,it is of great significance to study the evolution of the microstructure and performance of the wheel and rail surfaces during the friction and wear process.In this test,the GPM-30 rolling friction and wear tester was used to perform the friction and wear test of D2 wheel steel and U71Mn rail steel(main/accompany sample)under dry rolling conditions.The vibration characteristics between wheel and rail were measured by the acceleration sensor.The damage of wheel rail surface was observed and tested by optical microscope,scanning electron microscope,EBSD,transmission electron microscope,microhardness tester and electronic balance.The formation mechanism of surface corrugation was analyzed.The wear properties of the sample,the evolution of microstructure and the change of hardness were studied.The surface corrugation of the sample is mainly caused by the vertical vibration of the system.When the load is 2740N and the speed is 1440r/min,the corrugation order is always 23 steps,which has a periodic characteristic.Finite element analysis shows that the stress and strain distributions of the sample surface have the same periodic characteristics.At the initial stage of friction and wear,the wear volume of the main/accompany sample increases slowly.After the corrugation(after3×105r),the wear of the main sample increases sharply,and the wear of the accompany sample still increases slowly.The wear of the main sample is always greater than that of the accompany sample during the whole process.The thickness of the plastic deformation layer of the main/accompany samples increase with the increase of the number of cycles,and the thickness of the deformed layer at the trough is significantly larger than that of the deformed layer at the crest,and the thickness of the deformation layer of the main sample is larger than that of the deformation layer of the accompany specimen.After wear of the main sample,the surface layer can be divided into three zones,severe plastic deformation zone,plastic deformation zone and transition zone.The cementite in the severe plastic deformation zone is severely shredded and dissolved,and the ferrite is severely refined.The cementite and ferrite dissolution of the main/accompany increased.The minimum grain size at the trough can reach the nanometer size,and the grain refinement at the trough is more obvious than that at the crest.In the wear process,the pre-eutectoid first forms a subgrain at the initial deformation stage,and then becomes a large angle grain boundary after refinement.From the surface to the matrix,the number of high-angle grain boundary in the refined ferrite structure decreases first,then increases,and finally decreases.There is a region with severe grain refinement in the subsurface.When rolling 2×105runs,the pre-eutectoid ferrite grain refinement is more serious at 200?m from the surface,and the finite element analysis results show that the shear stress of the position is the largest.After the appearance of surface corrugation,the position of pre-eutectoid ferrite subsurface grains in the peak is obviously refined to move to the surface,which is located at about 100?m at 5×105 runs.The hardness variation from the matrix to the surface of the main/accompany samples is consistent.After wear,the surface hardness of the samples is the highest.Before corrugation is formed,the surface hardness of the main/accompany samples reachs saturation at 0.6×105r and 1×105r respectively,and about 560 HV and 630 HV.After the corrugation is formed,the surface hardness of the crest basically remains unchanged,and the surface hardness of the trough continues to increase and the hardness eventually reaches saturation,which is approximately 630 HV and 680 HV,respectively.The hardness of the trough of the main/accompany samples gradually increases from the matrix to the surface;There is an obvious hardening area at the subsurface of the crest.When rolling 2×105runs and 5×105runs,the peak values of subsurface hardening are 200?m and 100?m respectively.