Research On Fatigue Behavior And Surface Residual Stress Evolution Of 18CrNiMo7-6 Alloy Steel

The fatigue performance of materials is of great significance to the design of mechanical parts.Fatigue fracture is one of the main failure modes of mechanical components in practical engineering application.There are many factors affecting the fatigue performance of materials.When testing the fatigue performance,the test conditions greatly affect the results of the fatigue performance,such as specimen size,loading frequency,stress ratio,surface state,etc.Residual stress is an important parameter that affects the fatigue performance of materials.During the service process of mechanical components,the residual stress on the surface is not static.Under the action of mechanical driving force,the residual stress will be released.If the effect of residual stress is ignored,it will lead to inaccurate fatigue strength prediction.18 CrNiMo7-6 alloy steel is widely used in gears,shafts,bearings and other mechanical parts due to its excellent mechanical properties.In this paper,the factors affecting the fatigue performance of 18 CrNiMo7-6 alloy steel was analyzed,and the evolution of surface residual stress in the fatigue process is studied.The main work was as follows:(1)Taking 18 CrNiMo7-6 alloy steel as the research object,through rotating bending and axial fatigue tests,the effects of specimen size,loading frequency and stress ratio on the fatigue performance of 18 CrNiMo7-6 alloy steel were studied respectively.The S-N curves under different conditions were obtained,and the fracture surfaces of the specimens after the fatigue tests were observed by scanning electron microscopy(SEM).According to the results of the fatigue test,the influence mechanism of different test conditions on the fatigue performance of 18 CrNiMo7-6alloy steel was analyzed,and the S-N curve obtained from the test was verified by the finite element method.The results showed that the fatigue strength of 18 CrNiMo7-6alloy steel increases with the increase of specimen size and loading frequency.When the maximum stress is constant,the fatigue strength increases with the increase of stress ratio.The main factors affecting the size effect were residual stress,stress gradient,and crack growth rate.The main factors affecting frequency effects were strain rate and crystal structure.The results of fracture analysis showed that smaller specimens were more prone to brittle fracture,and there was no significant difference in the fracture mechanism of specimens with different loading frequencies and stress ratios.(2)Taking the 18 CrNiMo7-6 alloy steel carburized layer as the research object,the specimens with different carburized layer depths were subjected to intermittent fatigue tests,and the evolution law of the residual stress on the surface of the specimen during the fatigue process was studied and the evolution mechanism of the residual stress was analyzed.The results showed that the residual stress on the surface of the specimen was released during the fatigue process,and the release was most obvious in the first cycle.With the increase of the number of cycles,the residual stress tends to stabilize.The stress level will significantly affect the degree of residual stress release.There is a critical value,when the applied stress level is less than the critical value,the residual stress does not release.(3)The effect of surface residual stress on the fatigue life of 18 CrNiMo7-6alloy steel was studied by finite element analysis,and combined with the residual stress release phenomenon during the test,the fatigue life after the release of residual stress was calculated.The results showed that the residual stress will significantly affect the fatigue strength,and the effect was small when the stress amplitude was high.After considering the release of the residual stress,the fatigue life of the18 CrNiMo7-6 alloy steel was significantly reduced.