| In recent years,the problems of excessive tread wear and premature contact fatigue damage have occurred frequently in China’s high-speed train wheels.Most of the wheels have fatigue failure in service of 50,000 to 300,000 kilometers,far lower than the normal service life of 2.4 million kilometers,which affects the operation quality of high-speed trains,disturbs the normal running order of train,and increases the wheel operation and maintenance costs.However,the cause of damage is unclear and the mechanism is unknown.Studying the damage mechanism has important application value for ensuring the driving order and operation safety of high-speed trains.At the same time,it also provides a theoretical basis for scientific prevention and reasonable remediation of such wheel damage,effective design of wheel materials and processing technology,improving wheel quality and reducing maintenance costs.In this paper,based on the practical problem of premature fatigue damage of ER8 steel wheel treads of high-speed trains,the microstructure characterization methods such as optical microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM),as well as the micro-area performance test methods such as nanoindentation and in-situ tensile are used.Through the failure analysis of the actual damaged wheels,it is found that abnormal microstructures exist in the wheels with premature contact fatigue damage.The correlation between abnormal microstructure and premature fatigue damage of wheels was studied,and the types and formation mechanisms of abnormal microstructure were analyzed.Through the wear and rolling contact fatigue tests of wheel small samples with abnormal microstructure,the wear and contact fatigue properties and the evolution of abnormal microstructure were obtained.By means of 1:1 ratio physical wheel rolling test platform,the simulation evaluation was carried out,and the whole process of premature fatigue failure of wheels with abnormal microstructure was reproduced.Combined with finite element simulation,the influence of abnormal microstructure on wheel stress state was studied,and the mechanism of premature fatigue damage of wheels caused by abnormal microstructure was analyzed in depth.The main results are as follows:(1)The upper bainite structure on the wheel tread is the main cause of premature contact fatigue damage.The cementite in the upper bainite is distributed between the ferrite laths in the form of particles,chain beads or short rods,which is different from the normal microstructure of the wheel matrix(proeutectoid ferrite + pearlite structure,F+P),which destroys the continuity of the wheel matrix microstructure and causes the nonuniform of the wheel microstructure and properties.(2)The serious segregation of carbon element in the local area of the wheel and the high content of vanadium in the whole wheel are the main reasons for the formation of upper bainite and its residue in the finished wheel.The segregation of carbon element promotes the transformation of segregation microstructure to the upper bainite at lower cooling rate.At the same time,the increase of vanadium element makes the continuous cooling transformation curve of wheel steel shift to the right,and also increases the formation depth of upper bainite microstructure in the actual production of wheel.(3)During the wear and contact fatigue test process of ER8 steel samples with the upper bainite microstructure,plastic deformation will occur in the surface microstructure of all samples,but the deformation is not uniform.The pearlite lamellar spacing in the normal microstructure of the wheel is obviously reduced and refined,and the lamellar carbides are fractured and dissolved,while the ferrite in the upper bainite microstructure is only elongated,and the lath width is slightly reduced.Most of the cementite is still in the original state of granular or shot rod,and the refinement and dissolution are not obvious,that is,the overall deformation and grain refinement are not as good as the adjacent normal microstructure.(4)The original hardness and elastic limit of the upper bainite microstructure are higher than those of the adjacent normal microstructure.The plastic deformation ability and hardening degree of the upper bainite microstructure are less than those of the normal microstructure during the wear and contact fatigue test process.Under the action of wheel-rail contact stress,the incongruity of elastic-plastic deformation and hardness difference between the upper bainite and the surrounding normal microstructure result in stress concentration at the boundary of the two microstructures,which in turn induces and promotes the initiation and propagation of fatigue cracks,and accelerates the occurrence of fatigue wear and contact fatigue damage of wheel tread.(5)The finite element simulation results show that the Mises stress of the wheel tread is discontinuously distributed at the junction of the upper bainite and the surrounding normal microstructure,and there is an obvious stress mutation,resulting in the stress concentration.Moreover,the stress concentration factor is the highest when the upper bainite is distributed vertically in the tread area.When the wheel tread contains upper bainite microstructure,the wheel-rail contact stress at the tread will increase.(6)The wear mechanism of the samples containing upper bainite is mainly fatigue wear,and the wear resistance is lower than that of the samples containing normal microstructure which wear mechanism is mainly adhesive wear.The rolling contact fatigue limit of the wheel samples containing upper bainite is 1074 MPa,which is lower than that of the wheel samples with normal microstructure(1112 MPa).After 45,000 km(about 1.56 × 107 cycles),the results of real wheel rolling bench test also confirmed that although the wheel containing upper bainite microstructure has a certain ability to hinder fatigue crack propagation in the wear process,due to fine grain,solid solution and dispersion strengthening,it is much lower than the wheel with the same composition containing normal microstructure. |
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