Research On The Influence Mechanism Of Low Temperature Environment On Rolling Wear And Contact Fatigue Damage Of Wheel/rail Materials

With the continues development of Chinese railway,railway lines served in the cold region are becoming more and more,such as the Harbin-Dalian,the Qinghai-Tibet railway lines.The decrease in the environmental temperature would induce the variation of the wear and the failure mechanism of the wheel and the rail materials,which makes the service behaviours of the wheel and the rail materials be different from those at the room temperature.At present,relevant studies on the wear and the damage behaviours of the wheel and the rail materials are mainly focused in the room temperature environment and are lacking in the low temperature environment.Therefore,it is necessary to study the wear and the damage behaviours of the wheel and the rail materials at the low temperature.The results obtained in the present study could provide the theoretical and the technological supports for the new developed materials and improving the service reliability of the wheel and the rail materials served in the low temperature environment.In the present study,through the wheel-rail rolling-sliding wear testing machine,the rolling wear and the contact fatigue damage of the wheel and the rail materials under the low and the alternating temperature conditions were investigated.The wear transition mechanism of the wheel-rail at the low temperature was revealed through analyzing the tribo-chemical behaviours at the wheel-rail interface.The influence formula of the contact pressure,the slip ratio and the rail hardness on the rolling wear and the damage of the wheel and the rail materials was investigated.A wear map and a damage mechanism diagram of the wheel and the rail materials under the low temperature condition were founded.Furthermore,a finite element wear model of the wheel-rail three-bodies contact considering the effect of the wear debris layer was founded.The wear and the damage behaviours of the wheel and the rail materials with and without the wear debris layer at the wheel-rail interface were analyzed via the proposed model.Besides,the effect of the variation of the mechanical properties of the rail induced by the low temperature on the rolling contact fatigue damage of the rail was analyzed.Based on the experimental results,suggestions for the operation and the development of the wheel and the rail materials in the low temperature environment were put forward.The main results of this thesis are as follows:(1)As the environmental temperature declined from the room temperature(20℃)to-40 ℃,the oxidation at the wheel-rail interface was weakened,which accelerated the accumulation of the wear debris at the wheel-rail interface.As the temperature was lower than-20℃,a large area of the wear debris layer was adhered on the rail surface.Then,the wear mechanism of the wheel-rail transferred from the fatigue wear and the slight adhesive wear at the room temperature to the adhesive wear at-40℃.The formation of the wear debris layer palliated the wear and the damage of the wheel and the rail materials.Due to the low wear rate,the wheel material would experience more cyclic loading,accumulate severe plastic deformation,and be refined extremely.At-40℃,the wheel surface material had undergone a severe plastic deformation and the local material was transformed into a white etching layer structure.The refined ferrite and the white etching layer were the main crack initiation sources of the wheel material at-40℃.Under the alternating temperature condition,under the cooling condition,the wear of the wheel decreased and the wear of the rail increased.Under the heating condition,the wear debris layer formed on the rail surface at-40℃ was rapidly broken,which increased the wear and deteriorated the fatigue damage of the wheel and the rail materials.(2)At-40℃,the wear rate of the wheel material increased linearly with the Tγ/A value at first and then remained nearly stable.The wear rate of the rail material had a close relationship with the contact stress at-40℃ but not the Tγ/A value.At-40℃,the wear maps both of the ER7 wheel material and the U71Mn rail material could be divided into two regions: the mild wear and the severe wear.At-40℃,the wear mechanism of the wheel and the rail materials would undergo two transitions: with the increase in the contact stress,the broken of the wear debris layer at the wheel-rail interface would cause the wear transition from the adhesive wear to the fatigue wear;With the increase in the slip ratio,the frictional heat at the wheel-rail interface increased,which led to the wear transition from the fatigue wear to the oxidative wear.At-40℃,as the slip ratio lower than 3.0%,with the increase in the contact stress,the main damage mechanism of wheel material gradually changed from pitting to peeling.Under the large slip ratio condition(12.0%),with the increase in the contact stress,the main damage mechanism transferred from slight peeling to peeling and eventually large spalling.(3)Whether at the room temperature or at-40℃,with the increase in the initial hardness of the rail,the wear rate of the rail material decreased.Therefore,the hypereutectoid rail with higher strength and higher hardness still had a better wear resistance at the low temperature.At-40℃ and 1500 MPa,the crack propagation depth and angle of the hypereutectoid rail with higher hardness were deeper and larger.Therefore,the hypereutectoid rail material with higher hardness had worse rolling contact fatigue performance than that of U75VH rail and U78CrVH rail in the low temperature environment.(4)Compared with the wheel-rail two-bodies contact model,the three-bodies contact wear model considering the effect of the wear debris layer was more realistic and was consistent with the experimental results in the friction coefficient,the wear and the damage behaviours.The wear debris layer,which was considered as a deformable body,adjusted the relative sliding displacement between the wheel and rail rollers through the plastic deformation.Therefore,the wear and the damage of the wheel and the rail materials were palliated.The change in the mechanical properties of the rail at low temperature would increase the RCF crack initiation life of rail.The severe contact fatigue damage of the rail material at the low temperature could be attributed to the fatigue wear induced by the broken of the wear debris layer.