Experimental Investigation on Effects of Various Factors on Very High Cycle Fatigue Property for Spring Steels

摘要

In order to investigate the effects of hardness,residual stress and loading type on the fatigue property for spring steel(SUP7),two kinds of tempering temperature,three kinds of surface finishing (grinding,electrolytic polishing and shot-peening) and two kinds of fatigue test (rotating bending and axial loading) were accepted.Specimens in harder (lower tempering temperature) series showed higher fatigue limit; however,those in shot-peened series showed approximately same fatigue limit as grinding finished series,although the former has compressive residual stress higher than that in the latter.In the case of same tempering temperature and surface finishing,specimens in rotating bending series showed higher fatigue limit comparing with the axial loading.This fact can be attributed to the difference of critical volume participating in the crack initiation.Fracture surfaces of all the failed specimens were carefully observed by a scanning electron microscope(SEM) in order to examine the fracture mode.Two types of interior initiated fracture,with and without inclusion,were found.The latter was found only in axial loading and had granular structure whose angle of slope to the loading direction was 60 degree.In the case of specimens with residual stress (grinding and shot-peening),the depth of interior fracture origin in rotating bending was shallower than that in axial loading.In both grinding and shot-peening series,the crack has initiated beneath the surface layer with compressive residual stress,and,the fatigue limits of these series were thus almost same to each other.