The Simulation And Optimization Study Of Surface Nanocrystallization Technology Of 42 CrMo Steel For Automobile Shafts Via High Energy Shot Peening

As a surface strengthening method for workpieces,shot peening(SP)is widely used in the industry.It is a treatment method that can effectively improve the fatigue properties of materials.High energy shot peening(HESP)is a new type of SP.HESP can not only introduce compressive residual stresses into the layer near the treated surface of the target,but also produce nanocrystalline layer near the treated surface,achieving fine grain strengthening.In order to obtain the best shot peening effect and cost saving,it is necessary to optimize the HESP parameters.Among them,finite element simulation(FEM)is an effective research method.In this thesis,the 42 CrMo steel commonly used in the rear axle of the automobile was selected as the research object.The FE HESP model was established to study the universality of the influence of mesh convergence on the accuracy of predicting the SNC results.And then,the work was to investigate the influence of HESP parameters,including coverage,shot velocity and diameter on the SI and grain refinement degree of the workpiece via combining the FEM and experimental verification.The optimal HESP parameters were solved by Response Surface Method.The improvement of fatigue performance was analyzed by considering the grain refinement and residual stress.The equivalent plastic deformation(PEEQ)is an important parameter for determining the degree of grain refinement in FEM.By establishing a HESP model with different impacting cases,it is confirmed that the PEEQ value increases with the refinement of the shot and workpiece element size,especially the shot element size can’t be ignored in FEM of HESP prediction surface nanocrystallization.The linear relationship between the maximum PEEQ value and the shot or target element size and the planar relationship between the maximum PEEQ value and their combination can be obtained by fitting in all HESP condition,improving the efficiency of FEM.These characteristics are universal in all HESP simulations.It is found that the increase of coverage,shot diameter and velocity is beneficial to the formation of SNC and work hardening of the workpiece via the mesh optimal HESP model.The values of PEEQ,micro-hardness and full width at half maximum(FWHM)on surface increase with these parameters increasing.Compared with shot diameter and shot velocity,increasing surface coverage leads to significant increment of these three values on surface.However,increasing shot diameter leads to significant increment of influence depth of HESP.High coverage,large velocity and diameter are effective ways to realize SNC in HESP treatment while causing different extent effect on surface quality,such as surface roughness,stress concentration and surface cracks.According to the optimial goal of response value of treated target,the Response Surface Method(RSM)was used to optimize the shot peening parameters and the optimal combination of shot peening parameters were V = 100 m/s,d = 0.6 mm,C= 1000%.The unpeened group,the different HESP parameters groups and the optimal HESP parameters group were selected for comparison via fatigue test.The results show that the optimal HESP parameters group is the most beneficial to improve the fatigue life.The formed NC gradient structure of the treated workpiece after HESP can effectively improve the fatigue stability and compensate for the decline in fatigue resistance due to the compressive residual stresses relaxation.