| At present,with the rapid development of high-speed railway transportation industry in China,the high speed and heavy load of train operation put forward higher requirements on the performance of wheel and rail.Wheel/rail failure caused by friction and wear between wheel/rail has become a key factor affecting the safe operation of railway.When the train starts,turns and braking,the huge friction between wheel/rail will generate martensitic transformation white layer on the rail surface.In wheel-rail contact operation,the phase change white layer can increase the surface hardness,improve the wear resistance and form cracks inside the white layer,causing the phase change white layer to peel off,seriously affecting the safety and comfort of train operation.In this paper,the local laser quenching treatment for U75 VG and U75 VH rail steel is carried out in the laboratory.The rolling wear test of the system is carried out by GPM-40 rolling contact fatigue wear testing machine.By means of digital camera,optical microscope,scanning electron microscope and microhardness tester,the spalling process of white layer with different wear cycles,different creep rates and different types of phase change was systematically analyzed.The results show that: 1)After surface laser quenching treatment for U75 VH and U75 VG,martensitic transformation white layer with a width of about 2.5mm and a depth of about 250μm is obtained.2)The microstructure hardness of the sample was improved after laser treatment.The hardness of phase transition white layer is about 860 HV,the original hardness of U75 VG steel is about 320 HV,and the original hardness of U75 VH steel is about 360 HV.3)The surface crack initiation,propagation and stripping laws of U75 VG and U75 VH rail steel with two kinds of phase transition white layers are basically the same in the wear test.The stripping speed and degree increase with the increase of creep rate and wear time.Under the same running cycle,U75 VG is prone to crack initiation and the stripping degree is also serious compared with U75 VH.The wear resistance of U75 VH rail steel is better than that of U75 VG rail steel.4)In the process of wear and running,cracks are easy to initiate on the surface of the initial end of the white layer,and the cracks are also easy to expand on the interface between the edge of the white layer and the matrix and in the white layer,that is,along the pearlite and martensite interface or martensite body,and then the cracks expand and converge and peel off.Crack initiation and propagation at the end of phase change white layer are the same as that at the beginning of operation.With the increase of operation cycle,phase change white layer is gradually rolled into the adjacent pearlite matrix,and the crack propagation becomes slow,and only the pearlite matrix covering the white layer is shallowly exfoliated.The direction of plastic flow is different at both ends.5)The characteristics of crack initiation,propagation and stripping at the beginning and end of phase change white layer strip are the same as those of point white layer strip.The difference is that it is easy to crack and propagate on both sides of the phase change white layer strip.There are many cracks at the beginning of the phase change white layer strip and the most serious peeling,followed by the two sides. |
