Effect Of Ultrasonic Rolling Surface Modification Gradient Field On Fatigue Strength Size Effect Of 51CrV4 Spring Steel

As one of the key components of the high-speed train,the fatigue performance of the bogie spiral spring will directly affect the safety of the train in high-speed operation.However,for the bogie spiral spring components which often exist in the form of large size,the fatigue life of the bogie spiral spring components is often reduced due to the size effect,which brings serious hidden dangers to the safety and reliability of the train.Therefore,in this paper,the effect of ultrasonic surface rolling process(USRP)with different parameters on the surface modification gradient field of 51 CrV4 spring steel is studied by controlling static pressure,rolling times and spindle speed,and then the USRP parameters with the best comprehensive properties of surface modification gradient field are selected to study the fatigue behavior of 51 CrV4 spring steel samples with geometric dimensions of ?3.4 mm and ?6.3 mm.Finally,SEM,FIB,TEM and HRTEM were used to study the mechanism of the effect of surface modification gradient field and fatigue crack initiation location on fatigue properties.As a result,the application of anti-fatigue manufacturing technology in spring steel is established and improved,to provide a new technical reference for improving the fatigue performance of large-size components.The main conclusions are as follows:(1)During the treatment of 51 CrV4 spring steel by USRP,the gradient field of surface modification changed obviously by changing the parameters such as static pressure,rolling times and spindle speed.When the static pressure,rolling times and spindle speed are 663 N,3 times and 180 r/min respectively,the comprehensive performance of surface modified gradient field is the best.(2)Compared with the fatigue limit(718 MPa)obtained from the 51 CrV4 spring steel specimen with the size of ?6.3 mm treated by USRP,the fatigue limit(848 MPa)of the USRP treated sample was increased by 18.1%,while the fatigue limit(751MPa)of the untreated ?3.4 mm specimen was only increased by 4.6%.This is because the surface modified gradient field and internal fatigue source generated by ?6.3 mm specimen after USRP treatment reduce the fatigue limit gap between it and the without USRP ?3.4 mm specimen.(3)After the fatigue test of the samples treated by USRP,it is found that the fatigue life increases at first and then decreases with the increase of the distance between the fatigue crack initiation position and the surface.This is because: when the fatigue crack initiation position is transferred from the surface to the interior of the specimen,the fatigue crack initiation resistance in the interior is greater than that on the surface,which increases the fatigue life,but when the fatigue crack initiation location is transferred from the interior to the center,the stress triaxiality at the fatigue source increases,makes the crack tip region subject to greater stress constraints,which makes the multiplication and movement of dislocations in this region more difficult,resulting in a decrease in the density of movable dislocations.The decrease of dislocation density leads to the increase of stress concentration,which accelerates the crack growth rate and reduces the gap between fatigue life.