Research On The Evolution Law And Control Method Of Friction Properties Of M50 Bearing Steel By Ultrasonic Shot Peening And Electromagnetic Shock

M50 bearing steel is widely used in aerospace bearings because of its excellent strength and temperature-bearing capacity,but because of the lack of suitable surface strengthening technology,wear failure is still one of its major forms of failure.Ultrasonic shot peening(USP)is a new surface peening technology that can effectively promote surface grain refinement and form nano-scale grains with superior mechanical properties.Electromagnetic shock treatment(EST),as an external field treatment technology,can achieve solid-state phase change,electro-plasticity,stress relief and damage repair in materials with the help of electron wind and Joule heat.To cope with the challenges of surface peening technology for aerospace bearings,this paper proposes to combine USP technology with EST technology,in order to obtain gradient nanostructured M50 bearing steel with stronger wear resistance.In this paper,the effects of USP,EST and USP-EST on the surface quality,surface microstructure,surface mechanical properties and wear resistance of M50 bearing steel have been studied,and the evolution law and influence mechanism of frictional wear behavior have been investigated.The main research results are as follows.(1)M50 bearing steel with ultra-strong gradient nanostructured surface layer has been successfully fabricated by USP technique.The thickness of the gradient nanostructured surface layer exceeds 200 μm,and the width of the most superficial lath martensite has been refined to 20-600 nm.The higher KAM values of the surface layer also demonstrate the presence of high dislocation density in the near-surface layer of USP-M50 steel,and the dislocation density gradually decreases with increasing depth.The Vickers hardness of the surface layer increased significantly,increasing and then decreasing with depth,with the maximum hardness at a depth of about 100 μm from the surface.The dry sliding wear rate of USP-M50 steel is 50.4% lower than that of the untreated sample.The wear mechanism changes from oxidative wear and severe abrasive wear to mild abrasive wear,and the thickness of the plastic deformation layer generated during friction also decreases.(2)The effect of EST treatment on the surface quality of M50 steel is closely related to the current density and energizing time.High energy EST promotes surface oxidation and localized austenitization of the part in contact with the electrode.ESTM50 steel surface Vickers hardness is basically unchanged,but the standard deviation of hardness is significantly reduced,and the consistency of surface hardness is improved.The electromagnetic impact energy effectively contributes to the improvement of the wear resistance of M50 steel,resulting in a reduction of the average friction coefficient as well as the wear rate during dry sliding friction wear,as well as a relative reduction in the standard deviation of the wear rate.After the electromagnetic shock treatment,the wear mechanism is dominated by oxidative wear,thanks to the formation of an oxide film on the wear surface adhering to the wear surface during the wear process.(3)EST treatment has a blurring effect on the surface martensite texture of USPM50 steel.The USP-EST combination treatment can deepen the surface submicron grain region and obtain a more uniform and stable surface layer organization.EST treatment also has a releasing effect on the residual compressive stresses introduced by the USP treatment by promoting dislocation movement,resulting in a reduction in dislocation density,a reduction in the content of LABs,and further grain refinement.Although the Vickers hardness is slightly reduced,the USP-EST treatment is still sufficient to further enhance the wear resistance by promoting the formation of oxide films during wear.(4)A new surface treatment process—M50 steel USP-EST intermittent treatment was proposed.The USP-EST intermittent treatment can effectively reduce the surface microscopic defects caused by USP and modulate the microstructure.The wear resistance is enhanced by USP,and the microstructure is modulated by EST to improve the stability of friction properties.This new USP-EST intermittent treatment method provides a new process idea and theoretical guidance for the surface strengthening method of M50 bearing steel.The USP-EST intermittent treatment method provides a new process idea and theoretical guidance for the surface strengthening method of M50 bearing steel.