EFFECT OF GRAIN SIZE ON ULTRA-HIGH-CYCLE FATIGUE PROPERTIES OF 42CrMoVNb STEEL
晶粒尺寸对42CrMoVNb钢超高周疲劳性能的影响

For low and medium strength steels, grain size has significant effects on their fatigue properties, whereas non--metallic inclusion has no or little effect. In previous work, the effects of grain size on high--cycle fatigue fracture behaviors of 42CrMoVNb high strength steel were studied and illustrated that grain refining has a complicated influence on its fatigue properties. In this paper, the ultra--high--cycle fatigue properties of 42CrMoVNb high strength steel with three kinds of prior austenite grain sizes produced by different heat treatment procedures were studied. Experimental results show that both fatigue strength and fatigue strength ratio don't increase monotonically with the decrease of gain size, and fairly better fatigue properties could be obtained at a medium grain size of 15 μm. SEM observations of fatigue fracture surface reveal that most of fatigue cracks initiated from inclusions and a granular bright facet (GBF) was found in the vicinity around inclusion at cycles beyond about 1×106. Further investigation shows that the stress intensity factor range at crack initiation site of inclusion ΔKinc trends to decrease gradually with increasing the fatigue life Nf, while the stress intensity factor range at GBF boundary ΔKGBF keeps almost constant with varying Nf. ΔKGBF of coarse grain size is higher than that of fine grain size. It could conclude that the effect of grain size on ultra--high--cycle fatigue properties is rather complicated and an appropriate size of prior austenite might be existed.