Study On Hardening Behavior And Mechanism Of Bearing Rings In Ultrasonic Rolling Extrusion

Bearing is an important part of mechanical equipment,its fatigue life is often related to the service life of mechanical products,and the quality of bearing ring working surface is one of the decisive factors affecting the service life of bearings.It has been proved that the ultrasonic rolling extrusion strengthening process is one of the effective methods to improve the quality of bearing ring working face.Work hardening is one of the important indexes affecting the surface quality in the process of ultrasonic rolling extrusion.Therefore,it is important to study the hardening behavior and mechanism of ultrasonic rolling extrusion to improve the fatigue strength and life of bearing rings.In this paper,the stress-strain relationship of quenched 42 CrMo bearing steel in cold plastic deformation is studied by means of indentation hardness measurement and numerical calculation.Based on the analysis of stress-strain relationship,a Z-A modified constitutive model is established,and a macro model for studying work hardening of materials is established by comparing the characteristics of single crystal hardening curves.Based on the model,the characteristics of work hardening behavior are analyzed,and the relationship between strain and hardness was explored.The results show that the flow stress is sensitive to strain and strain rate,and the strain hardening curve is parabolic.The strain hardening rate decreases rapidly with the increase of strain,and then decreases slowly.There is a positive correlation between strain and Vickers hardness.Based on the analysis of the microscopic influencing factors of strain hardening,the mechanism and law of the influence of dislocations on strain hardening are explored.And the microscopic model of work hardening of 42 CrMo was established.The results show that dislocation and its movement are the main factors causing work hardening.The main factors hindering the movement of dislocations include the multiplication,intersection,accumulation and interaction of dislocations.For 42 CrMo,the peierls stress produced by its lattice structure cannot be neglected.The larger the strain,the higher the dislocation density.Based on the strengthening process of ultrasonic rolling extrusion,the process parameters affecting work hardening were analyzed.Based on Hertz contact theory,the effects of ultrasonic vibration and static pressure on equivalent strain are analyzed.The effects of workpiece rotation speed and feed speed are analyzed by defining contact density,and the theoretical analysis results are verified by single point impact finite element simulation.The effect of process parameters on the degree of work hardening was studied by orthogonal experiment of ultrasonic rolling extrusion,and the distribution characteristics of hardness were analyzed.The results show that the strain on the surface of workpiece is caused by ultrasonic impact and static load.Feed speed and workpiece rotation speed can adjust the contact density of the machined surface.The influence of ultrasonic amplitude on work hardening degree is the most significant,followed by static pressure and feed speed.Hardness increases first and then decreases with the increase of depth.The prediction performance of BP neural network and stepwise regression model is compared and analyzed based on cross-validation method,and the mathematical model of surface work hardening degree was established.The applicability of the process parameters selected in the experiment was evaluated based on genetic algorithm.The results show that the stepwise regression model has smaller deviation distribution and better prediction performance.The model of work hardening degree based on stepwise regression has strong overall significance and good reliability.The range of process parameters selected in the experiment can ensure that the degree of work hardening is within a reasonable range.