晶粒尺寸对42CrMoVNb钢超高周疲劳性能的影响

EFFECT OF GRAIN SIZE ON ULTRA-HIGH-CYCLE FATIGUE PROPERTIES OF 42CrMoVNb STEEL

研究了不同热处理制度下得到的3种具有不同晶粒尺寸的42CrMoVNb高强度钢的超高周疲劳性能.结果表明,超高周疲劳强度和疲劳强度比并不随晶粒尺寸的减小而单调提高,中等晶粒尺寸的试样具有最高的疲劳强度和疲劳强度比.SEM断口观察表明,绝大部分试样的疲劳裂纹起源于夹杂物.随着疲劳断口裂纹源夹杂物处应力强度因子幅△K_(inc)的减小,疲劳寿命N_f增加;而在夹杂物周围的粗糙粒状区域(GBF)的应力强度因子幅△K_(GBF)并不随N_f变化而变化,基本为一常数,且粗晶粒试样的△K_(GBF)高于细晶粒试样.这表明,细化晶粒对高强度钢的超高周疲劳性能有着复杂的影响,存在一个合理的细化晶粒范围.

For low and medium strength steels, grain size has significant effects on their fatigue properties, whereas nonmetallic 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 × 10^6. Further investigation shows that the stress intensity factor range at crack initiation site of inclusion △Kinc trends to decrease gradually with increasing the fatigue life Nf, while the stress intensity factor range at GBF boundary △KGBF keeps almost constant with varying Nf. △KGBF 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.