Influence Of Ultrasonic Rolling On Rolling Contact Fatigue Properties Of Bearing Steel

With the rapid progress of large machinery and equipment in China,the concern of rolling contact fatigue of bearings is becoming increasingly prominent.As bearings are vital and indispensable elements within mechanical frameworks,the quality of the entire equipment is largely influenced by bearing performance.Further,the lifespan of mechanical components and operational safety are affected.Consequently,delving into the mechanics of contact fatigue,advancing technologies to enhance the rolling of the bearing contact fatigue lifespan,and achieving fatigue-resistant manufacturing are endeavors of profound significance,both in theoretical exploration and practical implementation.In this study,surface ultrasonic rolling processing(SURP)was employed to treat a sample of carburized bearing steel and examine the effect of various process parameters on vital properties,such as surface roughness,morphology,microstructure,surface hardness,and residual compressive stress was focused.An in-depth examination of different process conditions was conducted,shedding light on the intricate contact fatigue failure process.To counteract the issue of small microstructure and complexities of characterization,a coarse martensitic microstructure was successfully obtained via high-temperature quenching.Further,the microstructural transformations of larger lamellar martensitic structures during contact fatigue and SURP were investigated.In addition,the thermal stability mechanism that enhances the effects of SURP under specific operational conditions was explored.Through microstructure characterization and mechanical model calculations,the mechanism underlying the effect of SURP on the fatigue resistance of bearing steel was revealed.The results can be summarized as follows:(1)The rolling contact fatigue life of G20Cr2Ni4 A bearing steel is substantially enhanced by SURP,and its capability to decrease surface roughness,refine surface microstructure,and concurrently enhance surface hardness while inducing residual compressive stress are the primary advantages.Additionally,the buildup of fatigue-induced damage is considerably reduced by this process,effectively countering crack initiation.This is accomplished by reinforcing residual compressive stress and surface hardness across the depth of the material,which relocates the initial point of cracks from the surface to a specific depth below it.Consequently,the propagation rate of cracks is mitigated,substantially improving the overall contact fatigue life.(2)When subjected to contact fatigue loading,superficial cracks frequently develop between the finely ground grain layer on the sample surface and matrix,with these shallow cracks subsequently transitioning into fatigue-induced cracks that reduce fatigue life.This problem is effectively tackled in SURP by causing the finely ground grain layer to peel off,reducing the influence of grinding defects and lowering the likelihood of hidden fatigue crack initiation.Furthermore,surface roughness is substantially decreased by this process,resulting in a notable reduction in stress concentration on the surface.(3)During the rolling contact fatigue process,a phenomenon known as work hardening occurs on the surface of GCr15 Si Mn bearing steel.This is evinced by an increase in the maximum residual compressive stress and peak hardness of the surface layer,a trend that becomes more pronounced with higher cyclic loads and extended operation cycles.However,the density of geometrically necessary dislocations within the microstructure is augmented by SURP.This effect plays a crucial role in the substantial retardation of the onset of work hardening in the rolling contact fatigue process,thereby decelerating the grain refinement process.The fatigue life is significantly extended by these effects.(4)During the rolling contact fatigue experiment on GCr15 Si Mn bearing steel,the surface lamellar martensite undergoes significant plastic deformation that drives microstructural changes.These changes can be primarily categorized into three stages.The first stage involves the proliferation of the internal sections of the lamellar martensite,which increases the density of geometrically necessary dislocation.The second stage involves the preferential formation of small-angle grain boundaries ranging from 2° to 5° at the grain boundaries of lamellar martensite.As the degree of deformation degree increases,the number of these small-angle boundaries and angle differences between subgrains increases.In the third stage,a gradual realignment of the slight-angle grain boundaries within the flake martensite is triggered by the accumulated strain,leading to the development of wide-angle grain boundaries with angles ≥10°.Thus,the original coarse martensite structure is refined,and the density of geometrically necessary dislocations within the grain bodies is reduced by this process.(5)The microstructure,hardness,and residual compressive stress demonstrate remarkable thermal stability following ultrasonic rolling.For GCr15 Si Mn steel,after undergoing regular quenching and tempering at 260℃,the ultrasonic rolling process resulted in a substantial increase in the density of geometrically necessary dislocations within the martensitic structure.A modest decline in the geometrically necessary dislocation count is induced after annealing at 260℃,allowing the retention of higher levels of hardness and residual compressive.