Study On Evolution Of Microstructure And Mechanical Properties And Fracture Mechanism Of High-speed Train Bearing

In recent years,high-speed trains have developed rapidly in China.Bearing plays an important role in the operation safety as the key component of high-speed trains.However,the bearing is completely dependent on imports when the speed exceeds 300km/h.In addition,the strict service conditions of bearing make it easy to damage,which seriously affects the safe operation of trains.Although the normal maintenance means can effectively avoid accidents,the maintenance cycle is long and there is no index parameter to reflect the evolution of bearing performance in service.Therefore,it is imperative to strengthen the basic research of high-speed railway bearing and promote localization.In this paper,the high-speed bearing produced by FAG company of Germany was selected as the research object.The chemical elements of the bearing outer ring,inner ring and roller were analyzed by Optical Emission Spectrometer(OES)and compared with domestic GCr15 bearing.The composition phase of the high-speed bearing components were analyzed by X-ray diffractometer(XRD).The microstructure was analyzed by optical microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM).The mechanical properties were tested by microhardness tester and MTS universal tensile tester.On this basis,the microstructure of the bearing outer rings which run 0,1.2,2.4 Mkm(million kilometers)respectively were characterized by the microscopic characterization methods.The mechanical properties of the bearing outer ring under different service period were tested by microhardness tester and MTS universal tensile tester.Finally,tensile unloading test and in-situ tensile test were carried out on the bearings.The stress state in the tensile process was calculated by using the finite element software ABAQUS,and the damage and fracture mechanism of high-speed train bearings were revealed through comprehensive analysis.The results show that:(1)The chemical elements of the outer ring,inner ring and roller of the high-speed train bearing are mainly composed of Fe,C,Cr,which are similar to the domestic GCr15 bearing steel.The surface of the roller is carburized,and the carbon content is higher than that of the center.The microstructure is composed of low bainite,(Fe,Cr)₃C carbides and a small amount of residual austenite.The granular carbides are uniformly distributed on the bainite,while the acicular carbides are distributed between the long axis of bainite at the angle of 50-60°.The size of the bainite cluster in the outer ring is 10-20?m.(2)The microhardness of the bearing inner ring is evenly distributed with an average of 677.14HV.The tensile strength is 2387.63MPa and the yield strength ratio is 0.93.The microhardness of the bearing roller along the diameter direction is"concave"distribution,the average microhardness is 657.06HV.The tensile strength is 2284.46MPa and the yield strength ratio is 0.93 with the low plasticity.The microhardness of the bearing outer ring is evenly distributed with an average of670.47HV.The tensile strength is 2312.23MPa and the yield strength ratio is 0.92.(3)With the increasing of the service period,the lower bainite become coarser.The coarsening rate of lower bainite reaches 21.4%and 50.0%respectively after the bearing outer ring services 1.2 and 2.4 Mkm.The acicular carbides slightly increases.The dislocation density and microhardness of the inner surface of the bearing outer ring increase gradually,and the microhardness of the inner surface of the outer ring of the 2.4Mkm bearing is higher than that of the center.With the increasing of the service period,the yield strength and tensile strength of the bearing outer ring gradually decrease,and the reduction rate gradually increases.(4)Microcracks are the main damage form of bearing outer ring.The microcracks nucleate in the three locations,which are the boundary of bainite clusters,between the bainite lath and around carbide.The bainite clusters are the weakest link of the bearing.A large number of microcracks nucleate on the boundary of bainite cluster.Under the action of external stress,the microcracks propagate along the boundary of bainite cluster.With the increase of external stress,the cracks propagate unsteadily and lead to the quasi-cleavage fracture of the sample.