Research On Nanocrystallization Mechanism And Comprehensive Properties Of 40Cr Steel Treated By High Frequency Impacting And Rolling

40Cr steel has been widely used in the machinery manufacturing industry.After proper heat treatment,it can obtain good comprehensive mechanical properties.It is commonly used in the manufacture of shafts,gears,and medium-sized plastic molds and other mechanical parts.However,in the course of work,serious failures are caused by wear,fatigue damage,and corrosion.In this paper,Surface gradient nanostructure of 40Cr steel was treatmented by using high frequency impacting and rolling.The hardness change and residual compressive stress change from different depth after high frequency impacting and rolling(HFIR)treatment were measured.The surface roughness before and after treatment and the microstruct ure of 40Cr steel after HFIR were characterized by means of X-ray diffracto meter(XRD)and transmission electron microscopy(TEM)to analyze the surface nanocrystalline mechanism of 40Cr.The wear,friction coefficient,wear morphology and wear mechanism of 40Cr steel were analyzed.Finally,the fatigue strength of HFIR treatment and as-received specimens corresponding to 3 million cycles was studied.The main conclusions are as follows:(1)A hardened layer of approximately 100?m was generated after HFIR;the maximum hardness after treat ment was 328 HV and 185 HV compared with the surface of the as-received sample,and the surface hardness was increased by 77.3%.After HFIR,the maximum residual compressive stress value of the surface layer is-861 MPa,its effective depth is about 1.58 mm,and the surface roughness(R_a)is reduced from 0.8to 0.16.The phenomenon of grain refinement caused by dislocation tangles,the transformation of subgrain to nanocrystals and the destruction of pearlite lamination constitute the surface nanocrystallization mechanism of40Cr steel after HFIR.(2)After HFIR,the wear mass loss of 40Cr steel is obviously reduced,especially when the loading load is 50 N,The wear mass loss of 40Cr samples were 0.7 mg and 4.7 mg,respectively.The friction and wear mechanism of as-received 40Cr steel was changed from abrasive wear to fatigue wear,while fatigue wear of gradient nanostructured 40Cr steel was not presented as the increase of applied load.The results show that HFIR can improve the wear properties of the material.(3)Fatigue fracture analysis shows that:corresponding to the high frequency impact rolling 3×10~6 cycles of specimens compared with the 40Cr untreated sample processing to enhance the fatigue strength of 18.4%.It was observed that fatigue cracks were initiated on the surface of the specimen,but the number of crack sources in the specimen after HFIR was less than that of the untreated specimen.