Study On The Evolution Of Residual Stress During Fatigue Of Induction Quenched 42CrMo Steel

Crankshaft is the core component of heavy duty engine.How to deal with the reinforced layer on the surface of waste crankshaft is still a problem to be solved in remanufacturing process.At present,during the remanufacturing process,it is often all removed for the crankshafts with nitriding layer,however because of the deep hardening layer of the induction hardening crankshafts,if all removed,the size of the parts changes greatly,which brings great difficulty to remanufacturing.As one of the important parameters of surface integrity,residual stress has the same meaning as external load,which will change the average load of the part and have a great influence on the service condition of the part.Therefore,it is necessary to grasp the historical service status of crankshaft remanufacturing old parts.Based on this,42 CrMo steel was taken as the research object in this paper,and the surface of the sample was treated with the ultrasonic GMT-1200 vertical induction quenching machine.The interpolation method was used to check the hardened depth of the induced quenching sample.After that,the three-point bending fatigue test was carried out,and the fatigue samples with different cycles were obtained.The X-350 A stress tester was used to detect the residual stress.Finally,the fracture of the sample was observed by SEM,the conclusions of the study are as follows:The final calculation results showed that the depth was 0.51 mm and 1.6 mm,respectively.The structure of the sample can be divided into three layers: hardened layer,transition zone and matrix.In the hardened layer,the fine martensite is the main part.The transition zone is dominated by pearlite and ferrite.It was found that the maximum compressive residual stress appeared in the hardened layer.As the depth continued to increase,the absolute value of residual compressive stress gradually decreased.The result showed that the sample of the hardening layer on surface residual stress relaxation in the fatigue process can be roughly divided into three stages,initial residual stress relaxation of fast,stable residual stress relaxation,and final sample after fracture,residual stress around the crack moment release.Through data processing,it is showed that the relation between residual stress after relaxation and lg N can be expressed by two models,one is linear model,the other is exponential model.By comparing the residual stress relaxation of the two samples at each stage,it is found that the f actors that have a greater impact on the residual stress relaxation are the depth of the hardened layer and the maximum stress applied.The relaxation of residual stress in static relaxation,which has a great influence on the fatigue life of the sample,is inversely proportional to the fatigue life of the sample.It provides an idea for predicting the fatigue life of hardened laminated parts.In addition,the fracture study of the sample shows that the crack initiation of the sample is in the hardened layer or on the surface.For the sample with the same depth of the hardened layer,the larger the fatigue load is,the closer the crack initiation position of the sample is to the surface,and the shorter the fatigue life is.Many micro-cracks were found in the crack initiation site of the sample,and they were mainly concentrated in the surface of the sample and the crack initiation site.Therefore,for remanufacturable old parts,its internal crack will not propagate too long,and for remanufactured products,when it starts a new round of service,its internal micro-cracks will become a weak point,more prone to nucleation,which have important impact for remanufactured products.Based on this,this paper proposes for the first time the treatment method of the surface hardened layer of the remanufactured blank,and the crack initiation of the remanufactured blank should be firstly removed.