Effect Of Surface Ultrasonic Rolling Processing On Surface Integrity And Fatigue Properties Of 60Si2CrVAT Spring Steel

As the key component of the train,the performance of the bogie spring can affect the safety of the railway transportation directly.However,decarburization has always been prevalent in the production of spring steel and in the process of product preparation,and the existence of the decarburization layer has many adverse effects.The influence to surface integirty especially decarburization layer was studied by controlling the rolling times of pretreatment,and fatigue test was did.By controlling spindle speed,static pressure and rolling times,the surface integrity was st,and fatigue test was did.SEM,X ray stressometer and EBSD methods were used to study the surface integrity and fatigue limit,combined with surface analysis of the mechanism,thus to establish and perfect the application of anti fatigue technology in the spring,to provide a theoretical basis and technical guidance for reducing decarburization of spring steel fatigue life adverse effects.The following main conclusions are obtained:(1)Before heat treatment,hot-rolled 60Si2 CrVAT steel is treated with SURP pretreatment,the decarburization layer(including fully decarburization layer and partly decarburization layer)is obviously decreased.For the round bar specimen with diameter 22 mm,after 6 times pretreatment,the fully decarburization layer can not be observed,and the total decarburization layer is reduced from 130 μm to 55 μm.(2)In the process of SURP surface strengthening,the surface integrity changes obviously with the change of spindle speed,static pressure,rolling times and so on.(3)After heat treatment,the ultrasonic surface rolling strengthening treatment,the area includes complete decarburization,partial decarburization layer and a surface layer part without decarburization,can reduce surface roughness and improve the surface hardness and residual compressive stress in the deep level.The strengthening mechanism is as follows: plastic deformation occurs,hardening occurs,martensite microstructure is refined,dislocation,martensite lath and martensite bundle number are increased,martensite number is reduced and grain is refined.(4)Whether preconditioning or surface strengthening can improve the fatigue performance of 60Si2 CrVAT steel in different degrees.In addition,the fatigue strength is slightly improved,and the fatigue performance is improved remarkably after surface strengthening.The effective diameter of the fatigue specimens of 7mm spring steel,rolling by ultrasonic pretreatment,the fatigue strength increased from 469.23 MPa to 476.92,only 1.64% of the increase of heat treatment after ultrasonic surface rolling processing,fatigue strength increased from 469.23 MPa to 580.36 MPa up to 23.68%.Without pre hardening treatment and ultrasonic rolling pretreatment,the fatigue sources are located in the surface decarburization zone.After the heat treatment,the fatigue specimens are processed by ultrasonic rolling,and the fatigue source is produced at the secondary surface or the heart part far away from the decarburization layer.Strengthen the surface pretreatment and ultrasonic rolling after heat treatment,the influence mechanism of improvement of fatigue performance is from the change of surface integrity to the former only reduces the decarburized layer thickness,therefore enhance the fatigue strength is small,and the latter from the surface roughness,microstructure,strength,residual stress and other aspects to improve the surface performance,fatigue strength is significantly improved.