Study On Rolling Contact Fatigue Behavior And Microstructural Evolution Of Wheel Steels

With the rapidly development of high railway,the rolling contact fatigue(RCF)failure is one of problems,which influence the safety of high-speed trains operation and increase of maintenance costs.Therefore,it is important to analysis RCF failure mechanism and improve fatigue property of wheel materials.The RCF failure is an urgent technological problem for railway industry at home and abroad.The actual wheel steel and the laboratory wheel samples are studied by using of optical microscope(OM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM),etc.During RCF process,microstructural evolution of the wheel steel is studied.The influence of pre-wear on RCF property of wheel steels is analyzed.The microstructural change at edge of RCF cracks of different wheel samples and its effect on the RCF crack propagation are analyzed.The mechanism of the formation of the white etching microstructure at subsurface of the wheel steel is proposed.On the basis of microstructural evolution,the mechanism of improving RCF resistance of wheel steels by tempered sorbite is analyzed.The following results are drawn:(1)During RCF process,the refinement process of proeutectoid ferrite and eutectoid ferrite of wheel steel is mainly divided into four stages.In the first stage,the dislocations are formed in the two kinds of ferrite grains,the dislocation tangles are formed in proeutectoid ferrite grains,and the dislocation tangles change into dislocation cells;in the second stage,the dislocation cells in proeutectoid ferrite grains gradually transform into low angle grain boundaries(θ<10°),a large amount of dislocation cells are formed in the eutectoid ferrite grains;in the third stage,The subgrains in proeutectoid ferrite change into high angles grain boundaries(θ>10°),and the proeutectoid ferrite grains are refined,the subgrains are formed in eutectoid ferrite grains;in the fourth stage,the subgrains in eutectoid ferrite grains transform into high angles grain boundaries,the eutectoid ferrite grains are refined.(2)During RCF process,plastic deformation of proeutectoid ferrite grains is quickly,which leads to high dislocation density of proeutectoid ferrite grains and grain refinement,but strength/hardness of proeutectoid ferrite grains is still lower than that of lamellar pearlite.Therefore,the plastic deformation is mainly concentrated at the interface of proeutectoid ferrite grains and pearlite or in proeutectoid ferrite grains.The RCF cracks are mostly formed at the interface of proeutectoid ferrite grains and pearlite and in the proeutectoid ferrite grains.(3)Pre-wear has a double effect on the RCF life of wheel steels.After pre-wear,at surface,the dislocation density is increased and grain refinement lead to the formation of a gradient hardening layer at surface,which can increase the resistance of RCF crack initiation and improve the RCF life.However,when fatigue wear cracks are formed on the worn surface during pre-wear process,the fatigue wear cracks will act as a crack source to accelerate the initiation of RCF cracks,and reduce the RCF life.(4)After RCF failure,the proeutectoid ferrite grains and eutectoid ferrite grains in pearlite are obviously refined at the edge of RCF cracks of wheel steels.Before the formation of RCF cracks,the accumulation of plastic strain of the wheel samples during RCF process is the main reason for the grain refinement at the edge of the RCF cracks.The stress field at the crack tip also contributes to the further refinement of the grains at the edge and tip of RCF cracks.The fatigue cracks mainly propagate along the grain refinement zone and the unrefined zone.(5)The RCF cracks of cast wheel steel after fatigue failure are at the interface between white etching microstructure(WEM)and matrix microstructure.In no RCF cracks area,there are a large amount of WEM.This indicates that the formation of WEM is prior to the RCF cracks.The subsurface WEM is composed of nanocrystalline ferrite grains(10～20 nm)and residual ultrafine cementite particles.The hardness of WEM is about920 HV,which is obviously higher than that of matrix microstructure.However,the WEM exhibits the poor plasticity.A soft zone model for the formation of WEM is proposed based on the microscopic characteristics of continuous plastic deformation leading to nanocrystallization and dissolution of cementite.There are soft areas(such as more proeutectoid ferrite)in the original microstructure.Concentrated and continuous plastic deformation occurs under the action of subsurface contact stress,which results in the refinement of proeutectoid ferrite and pearlite successively,the dissolution of cementite,the improvement of corrosion resistance,and the appearance of white under optical microscope.Under the action of shear stress,the RCF cracks propagates at the interface WEM and matrix microstructure,which results in the non-uniform distribution of WEM at both edges of RCF cracks.(6)The tempered sorbite of wheel steel exhibits better RCF resistance the traditional normalizing microstructure.The main reason is that the uniform microstructure of tempered sorbite lead to the plastic deformation layer to be uniform during wear and RCF process.Therefore,the fatigue cracks are not easy to initiate,and the uniform microstructure also can resist fatigue crack propagation.