| Wheel-rail is an important friction pair that affects each other,and a strategic matching of different wheel and rail materials plays a vital role in the service performance of railway system.With the fast growth of high-speed and heavy-haul railways and the development and application of new premium wheel and rail materials,the matching behaviours between wheel and rail has become more and more complicated in railway networks.Therefore,to achieve the optimum matching and selection of wheel and rail materials and enhance the service life of wheel and rail,it is necessary to carry out research on wear and damage behaviours of wheel and rail under various material/hardness matching conditions,and reveal the microstructure evolution mechanisms of wheel and rail materials.In this thesis,a series twin-disc wear experiments were performed using MJP-30A and WR-1 rigs under different material/hardness matching conditions,various creepage conditions and changing rolling direction conditions.The wear and fatigue damage behaviours of wheel and rail materials were investigated by means of Vickers hardness instrument,optical microscope,scanning electron microscope,electron backscatter diffraction,transmission electron microscopy and X-ray diffraction analyzer.And then,the microstructure evolution mechanism of wheel and rail materials during rolling contact was revealed.Based on the experimental results,the wear and fatigue damage mechanism maps were established in terms of hardness and T?/A.Finally,a preliminary study on the optimal matching and selection of wheel-rail materials was conducted.The chief conclusions are as follows:1.The effects of material hardness,creepage and rolling direction on wear and damage of wheel and rail during the rolling contact process were clarified(1)Five kinds of wheel materials(ER7,ER8,CL60,C-class and D-class)and 4 kinds of rail materials(U71Mn,U75V,U78Cr VH and hypereutectoid rail)were cross-matched at a creepage of 1%and a contact pressure of 1500 MPa.With the increase in wheel hardness,the wear rates of wheel and rail decreased,and as the rail hardness increased,the wheel wear rate slightly increased.Notably,wear rates for the wheel-rail pairs with C-class and D-class wheels were substantially lower than those for other wheel-rail pairs.As the rail/wheel hardness ratio(H_R/H_W)increased from 0.7 to 1.6,the wear rates of wheel and rail showed increasing trends.The surface damage of the harder C-class and D-class wheels,and the premium U78Cr VH and hypereutectoid rail materials were relatively slight.The U78Cr VH and hypereutectoid rails possessed significantly shorter cracks than the base materials(ER8-U71Mn).(2)With the creepage increasing from 0.17%to 9.43%,the wear rates of ER7,CL60 and C-class wheels and U75V rails showed increasing trends.As the wheel hardness reduced and the creepage increased,the wheel wear mechanism changed from mild peeling and slight fatigue wear finally to severe spalling and severe fatigue wear.As the wheel hardness and the creepage increased,the damage mechanism of rail material shifted from mild peeling and ratcheting to spalling and adhesion,and the wear mechanism achieved a transition from fatigue wear to the combination of fatigue and adhesion wear.(3)For the CL60-U75V pair,after changing the rolling direction of wheel,as the reverse rolling cycles increased,the wheel wear rate firstly decreased and then gradually increased,the wheel surface damage gradually developed into reversal peeling,meanwhile,the direction of wheel plastic deformation and fatigue crack propagation was also gradually reversed.At the initial stage of reversal,a“wave-like”deformation structure occurred near the wheel contact surface.With the reduction in the number of cycles per reversal,the wheel wear rate,the surface damage and the depth of plastic deformation decreased.2.The evolution mechanism of the microstructure of wheel and rail materials under various hardness and service conditions was revealedDuring the rolling contact process,the wheel and rail materials with various hardness underwent three-stage strain-induced dynamic recrystallization:dislocation propagation and dislocation entanglement,formation of sub-grain structure with low-angle grain boundaries and transformation into equiaxed ultrafine grain structure with high-angle grain boundaries.With the increase in the degree of recrystallization and refinement of pearlite colonies at the outermost structure,the wear rates of wheel and rail decreased.The large shear stress generated by the high creepage was beneficial to promote the plastic flow and dynamic recrystallization of the wheel and rail materials.In the initial stage of wheel reversal condition,there was an abnormal growth of the strain-induced ultrafine grains near the surface.It was suggested that reducing the content of proeutectoid ferrite in the wheel steel and the pearlitic lamellar spacing in rail steel within an appropriate range,and making the surface structure of the materials finer during the deformation process were beneficial to improve the wear resistance of the wheel and rail.3.Recommendations for the use of wheel and rail materials considering wear and rolling contact fatigue were proposedBased on the statistics of experimental results of T?/A in the range of 0?120 N/mm2,wheel hardness of 260?400 HV,and rail hardness of 280?440 HV,the wear and fatigue damage mechanism maps in terms of hardness and T?/A were constructed.The effect of hardness and contact parameters on the wear rate of wheel and rail materials was clarified.When the wheel and rail were soft(<300 HV),the wear rates of wheel and rail would increase significantly with T?/A.When the contact condition was severe(T?/A>60N/mm~2),the wear rates of wheel and rail tended to decrease with the increase in hardness.Based on the results of the rolling contact wear experiments,in consideration of reducing the wear rates and fatigue damage of wheel and rail,the recommendations for the use of wheel and rail materials were proposed:the hardness of the wheel and rail materials should be increased within a reasonable range on the premise of maintaining good performance.Specifically,wheel hardness>320HV and rail hardness>300 HV,meanwhile,in the wheel-rail pair,H_R/H_W<1,i.e.,the wheel hardness should be slightly higher than the rail hardness. |
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