Research On The Effect Of Surface Defect On Rolling Wear And Contact Fatigue Performace Of Rail

In the process of rolling contact between wheel and rail,rail is an important component to provide continuous and smooth operation surface for trains.The surface defects on rail head caused by hard body from open evironment may have an influence on railway operation.In order to ensure the rail service performance after being defected,the effects of surface defects on the wear and rolling contact fatigue(RCF)behaviors of rail should be studied,and the critical size and dangerous morphology of surface defects need to be put forward,which are necessary to be a theoretical basis for surface defects removing from the rail head.In this thesis,the rolling contact tests on rails with different types of defects were carried out using a MMS-2A twin disc testing device.From the macro aspect,the wear behavior and RCF characters caused by the defects were investigated by means of optical microscope(OM),Vickers hardness instrument,laser confocal scanning microscope(LCSM).From the micro aspect,the rail microstructure and RCF fatigue were also analyzed by electron backscatter diffraction(EBSD)and scanning electron microscope(SEM).The main accomplished research contents and the chief conclusions are as follows:1.Effect of surface dents on wear and RCF behaviors of rail(1)The contact stress near the dent increases with the size of dent larging,which aggravats the fatigue damage of rail materials.The cracks with large angle appears around the dent when the depth of spherical dent on rail surface reaches 348 um.The surface damage transfers from peeling to massive spalling due to the sharpening of the edge morphology of ballast dent,and the propagation angle of the fatigue cracks also increase sharply.The cracks with large propagation angle and long length occurs near ballast dent where the dent depth is more than 140?m,which resulting from the martensitic transformation at the bottom of dent.The experimental results show that the crack growth rate near the dent is 1.69 times that of on the non-defected rail.The simulation result of the crack growth rate near the dent from ABAQUS is similar to the experimental results.(2)RCF cracks are distributed unevenly in the front and rear areas of defects.Under the action of tangential force,the plastic deformation layer accumulates in the front area of defect,and the grains in the deformation area are seriously deformed.At the same time,the grain is refined and then forms many large-angle grain boundary(LAGB)grains with high strain.Ferthermore,the high stress stretches the refined grains into fibrous texture,providing a precondition for the crack propagation in the front area of defect.2.Effect of decarburized layer on wear and RCF behaviors of rail(1)Under dry condition,the wear rate of decarburized rails increases sharply compared with that of non-decarburized rails.The decarburized thickness has a significant effect on the surface damage behaviors of rails.Cracks on decarburized rails propagate along the ferrite line or the boundary between ferrite and pearlite.When the thickness of decarburized layer reaches the critical value of 0.5mm,the crack growth rate increases sharply.(2)Under the wet condition,the crack growth rate of decarburized rail increases rapidly,and the surface damage morphology of decarburized rail is obviously different from that of non-decarburized rail.With the cycle increasing,the surface damage of decarburized rail gradually evolves from small peeling to large peeling,and the propagation angle increases with transgranular growth.3.Fatigue damage mechanism analysis and critical size of surface defect on rails(1)Two different types of defects change the surface damage behavior and RCF damage mechanism of rail by structure transforming.The white ething layer(WEL)occurs at the bottom of dent,as a result,the adhesive damage is usually distributed in uniform strips.The pearlite transfers into ferrite due to the decarburization defect,as a result,the adhesive damage on the rail surface appears to be unevenly distributed.Based on the characteristics of severe plastic deformation and stress concentration in front of the dent,the driving force of crack propagation in front of the dent is large.Therefore,the crack usually growth with large angle.Based on the characteristics of high ferrite content and low shear strength of the decarburized materials,the crack on decarburized materials tends to propagate in length with multiple directions.(2)For the spherical dent with a depth of over 400 microns,the depth of dent should be ground within 200 microns,and the maximum width should not exceed 3mm;For an irregularity dent with a depth of over 140 microns,it should be ground to the bottom of the defect completely and continue to grind to the depth of 100 microns,which ensures the changed structure to be removed completely.For the decarbonization layer with the thickness of more than 0.5mm on the rail surface,it should be ground immediately within 0.3mm.