Analysis Of Formation And Exfoliation Of White Etching Layer Under Sliding Wear Of D2/U71Mn Wheel And Rail Material

Friction and wear is an inevitable phenomenon in the surface of wheel and rail under cyclic stress.With the rapid development of high-speed railway in China,wheel-rail failure caused by wear has become an important factor affecting railway safety.Especially when the train starts,brakes or curves to a curve,severe sliding wear occurs between the wheels and rails,and the contact stress is affected by the centrifugal force.As a result,the surface of the wheel and rails undergo severe plastic deformation and even a White Etching Layer is formed.Although the high hardness of the White Etching Layer will increase the wear resistance of the wheel and rail,it will also produce cracks,causing spalling.Therefore,it is of great significance to study the formation and exfoliation of White Etching Layers under sliding wear.In this paper,the sliding wear test of D2/U71Mn wheel and rail material was carried out under the condition of the laboratory although MRH-5A ring-block wear tester.Digital camera,optical microscope,scanning electron microscope(SEM)and transmission electron microscope(TEM),EDS,micro-hardness tester and simulation software systematically were used to study and analyze the formation and exfoliation of White Etching layers at different wear rotations.In the sliding wear process with different rotations,the wear loss weight of the ring(U71Mn)remains basically unchanged,while the wear loss weight of the block(D2)gradually increases,that is,the wear resistance of the ring is better than the block.the wear mechanism of the block is gradually changed from adhesive wear to abrasive wear with the increase of the number of sliding wear rotations,However,the surface of the ring is always slightly abrasive,but the oxidation wear on the surface gradually increases.Nanocrystalline White Etching Layer will form on the wear surface of ring-block specimen In the sliding wear process.The White Etching Layer is composed of ferrite nanocrystalline and a small amount of cementite particles.The mechanism of formation is plastic deformation mechanism.Observed from the cross section,formation process of the White Etching Layer of block surface is mainly divided into five steps:1)The crescent-shaped plastic deformation layer appears in the wear furrow,and the pearlite organization is refined;2)The formation of a relatively uniform serious plastic deformation layer on the wear surface,cementite is broken into short rod or even granular;3)Formed the White Etching Layer with the thickness of less than 1 ?m in the furrow,its inner structure is nano-sized ferrite and cementite small particles;4)Thicker crescent-shaped White Etching Layer in furrows;5)Crescent-shaped White Etching Layers in adjacent furrows are interconnected to a thickness of 10 ?m.The formation process of the White Etching Layer on the surface of the ring is basically the same as that of the block,except that the thickness of the White Etching Layer is different due to the different degree of deformation at the same rotation.The exfoliation process of the White Etching Layer of ring-block surface is as follows:The crack initiation is generated mainly at the ridge.The surface cracks extend and meet in the furrow along the direction of 30°?45° with the friction force,In the subsurface along the interface of the White Etching Layer and deformation layer or the White Etching Layer inside,and finally make the surface metal layered or even foil(with the White Etching Layer)exfoliating.A simulation study of the temperature field of the contact surface of the ring-block under the experimental conditions found that when the flash temperature during the sliding wear does not exceed 60 ms,the temperature rise of the ring-block surface does not exceed 300 ?,and the temperature rising will not affect the formation process of White Etching Layer structure.in that,no recrystallized White Etching Layer is formed.The surface of the White Etching Layer has the highest microhardness,about 820 HV,which is about 3 times higher than the hardness of the matrix.The microhardness of the surface of the White Etching Layer to the heart structure showed a decreasing trend.