Study On Damage Fatigue Performance Of EA4T Axle Steel Strengthened By Induction Hardening

With the increase of train running speed,the axle,as one of the important bearing parts of the train,is greatly increased in the risk of surface damage caused by the impact of foreign bodies such as gravel and ballast,which destroys the structural integrity of axle components,leads to fatigue fracture and endangers the safety of train operation.In order to improve the fatigue performance of axle and improve the safety level of axle,induction hardening treatment was carried out on axles steel,and reasonable induction hardening process parameters were explored.The damage defects were prepared on the surface of axle steel by different methods（electric discharge machining（EDM）,gas gun impact）,and the influence of induction hardening on the damage fatigue performance of axle steel was investigated.Gradient microstructure was formed in EA4T axle steel sample after induction hardening.Fine tempered martensite structure was formed on the surface,the transition zone was tempered martensite and ferrite,and the core was tempered sortensite structure.The maximum microhardness and residual compressive stress of the hardened layer were located on the subsurface,reaching 551 HV_0.3and 497 MPa,respectively.In the transition zone,the residual compressive stress decreased and became tensile stress,and the microhardness decreased sharply to the hardness value without treatment.The depth of hardened layer increases with the decrease of scanning speed and the increase of power.Rotating bending fatigue tests were carried out on samples with different hardened layer depths.The fatigue strength increased firstly and then decreased with the increase of hardened layer depth.The average fatigue strength of sample with hardened layer depth of 1000μm increased the most,reaching 61.20%compared with unhardened sample.The fatigue strength of the hardened annular notched sample was 51.27%higher than that of the unhardened notched sample.The notch sensitivity of induction hardened sample was 0.2643,which is lower than that of unhardened sample.The impact defect sample was prepared by impinging tungsten steel balls on the surface of the hardened sample,and the damage defect sample of the same size was prepared by EDM.The fatigue strength decreased with the increase of the defect size.The fatigue strength of impact defect sample was higher than that of EDM defect sample with the same defect size due to the residual stress generated by high-speed impact.Because of the gradient microstructure and residual compressive stress,the fatigue strength of the hardened sample was obviously higher than that of the unhardened sample.The cracks of EDM defect sample originated from the micro-notches at the bottom of the crater,while the cracks of the impact defect sample originated from the micro-notches at the bottom of the crater and the residual tensile stress area at the edge,and there were cracks extending along the direction of the adiabatic shear bands（ASBs）at the bottom of the crater.The notch sensitivity of EDM defect sample and impact defect sample was generally lower than that of unquenched sample.Due to strain hardening and residual stress accumulation caused by impact,crack initiation of the impact defect sample was delayed,and the notch sensitivity was lower than that of EDM defect sample.