Research On Ultrasonic Surface Rolling Process And Process Parameter Optimization Of Bearing Ring Raceway

During bearing operation,frequent contact,relative motion,and friction between the bearing ring raceway and the rolling elements may cause fatigue failure,wear failure,and failure of the raceway surface strength,affecting the bearing’s service performance.The integrity of the raceway surface of the bearing ring primarily determines the service performance of the bearing.Using surface modification technologies to improve the surface integrity of bearing raceways has become a hot topic in the field of bearing manufacturing.As a simple and efficient new surface modification technology,ultrasonic surface rolling process(USRP)can significantly improve the surface integrity,fatigue resistance,corrosion resistance,and wear resistance of materials.Currently,research on this technology is focused on alloy materials with low hardness,such as titanium and aluminum,while there are minor studies on materials with greater strength and hardness,such as bearing steel.The object of study is mainly a sample,such as a bar or a disk,whose surface is planar or cylindrical.There are no related reports on the research of the concave surfaces of the bearing raceway.Exploring the impact of ultrasonic surface rolling on the surface integrity of the bearing raceway is therefore of significant theoretical guidance and engineering application value for improving the fatigue life of bearings using this technology,and will advance the real-world application of this technology to bearing processing and manufacturing.In this thesis,we develop a finite element simulation model for ultrasonic surface rolling by subjecting the inner of a 7224 AC angular contact ball bearing raceway to ultrasonic surface rolling processes.The simulation results are compared with the experimental results to verify the accuracy of the finite element simulation model for ultrasonic surface rolling.Orthogonal experimental parameter combinations were designed to extract and resolve residual stress peaks,equivalent plastic strains,and surface roughness values for different combinations of process parameters using simulation models.Based on Taguchi gray correlation analysis,the ultrasonic surface rolling process parameters were optimized,and the significance and influence rules of the process parameters on surface integrity and various evaluation parameters were analyzed.The influence of ultrasonic surface rolling on the positional tolerance of the bearing inner race raceways is discussed.In order to verify the impact of optimized process parameters on the surface integrity of the raceway,based on the finite element simulation results,the effects and mechanisms of grinding(Before-USRP),ultrasonic surface rolling process with initial parameters(Initial-USRP)and ultrasonic surface rolling process with optimized parameters(Optimized-USRP)on surface integrity were compared and analyzed.The main results and conclusions of the study are as follows:(1)Comparing simulation results with experimental results,it is concluded that:the simulation results of the distribution of residual stress along the depth direction agree with the experimental results,and the maximum error of the stress value is about3.8%.The simulation calculation errors for the surface roughness of the raceway bottom and the raceway edge are 7.3% and 7.2%,respectively.The equivalent plastic strain value(PEEQ value)of the simulation model coincides with the variation of micro-hardness with layer depth measured experimentally,verifying the rationality of evaluating work hardening degree using equivalent plastic strain.This indicates the accuracy of the results of the well-established finite element simulation model for ultrasonic surface rolling.(2)Within the allowable range of processing conditions,the optimum process parameters for ultrasonic surface rolling process of 7224 AC bearing inner race raceways are obtained,with the amplitude of 10μm,static pressure 2400 N,spindle speed 45rad/min,rolling 7 times.The results of the multi-objective optimization show that the static pressure and amplitude have the most significant impact on the surface integrity of the bearing raceway,followed by the number of machining operations and the minimum impact of the spindle speed.(3)After ultrasonic surface rolling process,the shape tolerance and position tolerance of bearing inner race raceway have changed.The application of the designed bearing fixtures ensures that the bearing raceway position tolerance changes only extremely slightly,which improves the variation of the bearing tolerance by about 60%,but still has a large effect on the bearing raceway shape tolerance.This process should be scheduled after the rough grinding of the bearing raceway,with a certain machining allowance reserved so that the tolerance for line position changes caused by the ultrasonic surface rolling can be adjusted during the fine grinding of the bearing raceway.The plastic flow of the martensite causes the dissolution and fracture of the coarse carbide particles,resulting in the refinement of the carbide particles.The degree of refinement of the carbide gradually decreases along the depth of the surface.(4)After process parameter optimization,1200 MPa and 0.84 mm deep residual pressure stress are introduced on bearing raceway surface,and the depth of hardening layer is about 750μm,surface roughness about 0.16μm.The residual compressive stress at the bottom of the bearing raceway is larger than that at the edge of the raceway on both sides,ensuring that the center point of the rolling head is constantly in contact with the channel surface,which will make the residual compressive stress in the raceway uniformly distributed along the axial direction,which is also the direction of research and development of ultrasonic rolling actuators in the future.Under rolling pressure and tangential shear forces,adjacent tempered martensitic flat noodles merge and deflect at an angle along the rolling direction,and the grains stack in a directed manner to form a flocculent plastic deformation layer.Due to the poor plastic deformation during cold working of GCr15 bearing steel,the thickness of the plastic deformation layer after ultrasonic rolling is only about 4.5μm.The plastic flow of the martensite causes the dissolution and fracture of the coarse carbide particles,resulting in the refinement of the carbide particles.The degree of refinement of the carbide gradually decreases along the depth direction of the surface.The micro-hardness of the bearing raceway surface increases from 760 HV to 840 HV and slowly decreases along the depth direction under the dual effect of fine grain strengthening and processing hardening.It is difficult to further increase the hardness after reaching a certain depth.