Research On Shot Peening Parameters Optimization And Fatigue Properties Of 42CrMo Steel For Automobile Shafts

Shot peening?SP?is a simple and effective surface treatment method that can significantly improve the fatigue performance of components.In industrial area the determination of SP parameters is usually according to practical experience,which requires considerable time and money.In order to reduce the cost of preliminary SP test and to get a better understanding of SP process,SP simulation has become the method of choice in the last four dacades with the development of computational science.42CrMo steel is an ultra-high strength quenched and tempered steel,which has been widely used in the rear axle of automobiles due to its excellent properties.In this paper,42CrMo steel was selected as the research object.The method combined with finite element analysis and experimental verification was used to optimize the SP model by considering mesh density,friction coefficient and shot hardness.The effects of shot velocity,diameter,impact angle and coverage ratio on residual stress distribution were examined and discussed via the optimized SP model.The Response Surface Methodology?RSM?was used to optimize the parameters of 42CrMo steel in SP model.Finally,the optimal SP parameters were verified through experiments and the improvement of fatigue performance with different SP parameters was analyzed and compared.The SP calibration result shows that adjusting the size of the surface elements to1/16^th of the dimple diameter leads to an optimal mesh generation and residual stress data convergence when the velocity and diameter of shots were fixed.It's very important for the accuracy of the residual stress distribution evaluation to make sure the friction coefficient greater than 0.2 and the hardness difference between the shot and the target plate greater than 200 HV.In summary,appropriately increasing shot velocity,diameter and coverage ratio while maintaining shot impact angle of 90°can improve the residual stress distribution in the near surface region of 42CrMo steel during SP process,which is favorable for fatigue performance.However,the improvement does not always increase with these parameters increasing.RSM can accurately predict the value of response parameters after SP and optimization calculation.Different weight ratios were assigned to three response parameters,i.e.the maximum compressive residual stress,the depth of compressive residual stress and the stress concentration factor.The weight ratios in order assigned to 5:4:3 respectively:?1??_ma^RS _x,?₀,K;?2??₀,?_ma^RS _x,K;?3?K,?_ma^RS _x,?₀.The three groups of optimal SP parameters were:?1?v=88 m/s,d=0.8 mm,cr=170%;?2?v=85 m/s,d=1.0 mm,cr=150%;?3?v=92 m/s,d=0.8 mm,cr=130%.In the experimental verification,SP can effectively improve the surface hardness,surface micro-morphology and residual stress distribution of the specimen.The fatigue test results showed that the average fatigue life of the 42CrMo steel specimens without SP was 116773,and the average fatigue life after SP using the above three groups of optimal SP parameters were 238444,231766 and 211286 respectively.Comparing with the samples without SP,the enhancement ratio of fatigue life after SP were 1.04,0.98 and 0.81,respectively.It can be obtained from the analysis of fatigue test that the maximum compressive residual stress value and compressive residual stress field depth should be given priority,and the stress concentration factor followed,when selecting the SP parameters.In summary,the optimal SP parameter for 42CrMo steel specimens were shot velocity v=88 m/s,shot diameter d=0.8 mm,and coverage cr=170%.