Study On The Corrosion Fatigue Property And Life Prediction Of Ultrasonic Rolled EA4T Axle Steel

In recent years,with the rapid development of high-speed railway,especially the construction of high-speed railway in acid rain areas,high-speed train operation was facing with severe test in corrosive environment.As one of the key components of high-speed train,the safety and reliability of axles have been challenged.In order to ensure the safety and reliability of axles,it was necessary to study the corrosion fatigue properties of axles and improve their corrosion fatigue properties by using surface strengthening treatment process.In this study,EA4T axle steel was selected as the research object,and fatigue test of EA4T axle steel samples were carried out by rotating bending fatigue testing machine in air and corrosion environment.Corrosion fatigue properties,surface damage process and crack propagation of EA4T axle steel were studied.Then the surface of EA4T axle steel samples were strengthened by ultrasonic rolling,and the fatigue properties,corrosion fatigue properties and crack propagation behavior of ultrasonic rolling treatment EA4T axle steel were studied.Finally,different life prediction models were used to predict the corrosion fatigue life,The applicability of different models to corrosion fatigue life prediction of EA4T axle steel were compared.The main contents of this paper are as follows:(1)The fatigue test of EA4T axle steel samples were carried out by rotating bending fatigue testing machine in air environment and corrosion environment.The effect of corrosion environment on the fatigue properties,surface damage process and crack propagation of EA4T axle steel were studied.The result show that there was no fatigue limit in corrosive environment,and the fatigue strength corresponding to 10~7cycles was 245MPa,which was 31%lower than that in air environment;Gumbel distribution statistic was more suitable than Weibull distribution statistic to describe the change of corrosion crack length with cycles;The threshold value of crack growth in corrosive environment was significantly reduced,which was 6.29MPa m in air environment and 4.1MPa m in corrosive environment.The corrosion environment reduces the fatigue life of EA4T steel mainly because the corrosion environment reduces the threshold value of crack propagation and promotes the crack initiation and short crack propagation,while the corrosion environment has no effect on the crack propagation when the crack reaches a certain length.(2)The surface of EA4T axle steel samples were treated by ultrasonic rolling.The surface properties of the treated samples were tested.The changes of three-dimensional surface morphology,roughness,hardness,residual stress,full width half maximum and grain size were analyzed.Then the fatigue test and corrosion fatigue test of EA4T axle steel samples were carried out by rotating bending fatigue testing machine.The fatigue properties,corrosion fatigue properties and crack propagation behavior of ultrasonic rolling treatment EA4T axle steel were studied.Ultrasonic rolling can significantly reduce the surface roughness,improve the surface hardness,refine the surface grain,and introduce residual stress layer;The fatigue limit of ultrasonic rolling samples was 455MPa in air,which was28%higher than polishing samples.However,the fatigue strength of SURP treated samples in corrosion environment was increased to 390 MPa,which was 59.2%higher than polished samples;ultrasonic rolling in both air and corrosive environment can significantly improve the surface crack propagation threshold of EA4T sample,slow down the crack initiation and short crack propagation,therefore,its fatigue performance and corrosion fatigue performance was improved.(3)By observing of corrosion fatigue process,the corrosion fatigue process can be simplified into two main stages:corrosion pit growth and crack propagation.The exponential model and Faraday model were used to predict the corrosion pit growth stage,the modified Hobson model+Paris formula and equivalent initial flaw size method were used to predict the crack propagation stage.The applicability of different models to corrosion fatigue life prediction of EA4T axle steel were compared.The result show that the exponential model was better for predicting the life of EA4T axle steel in corrosion pit growth stage,and the equivalent initial flaw size method was better for predicting the life of EA4T axle steel in crack propagation stage.