Fatigue strength and fracture mechanisms in severely drawn eutectoid steel wires

通过试验，得到了以下结果，阐明了各种因素对共析钢丝疲劳强度的影响.在拉伸应变范围为100 ~ 3.8时，疲劳强度随拉伸应变的增加而单调增加，最高可达800 MPa，与铁素体钢相同，其增加程度较低。然而，在某些情况下，取决于制造条件，它在λ =2.5处具有峰值。它随金属丝直径的减小而增大，与片层间距（0.01-0.02 μ m）无关.与高密度位错和在TEM中观察到的窄片层间距相关联，在表面层中的硬度和拉伸残余应力随着Hv的增加而增加，最高可达Hv=520和1000 MPa。随着发蓝温度的升高，拉伸残余应力单调下降，在500 ℃时逐渐减小，而300 ℃发蓝硬度达到峰值Hv=650。400 ℃发蓝疲劳极限达到峰值900 MPa，这是应变时效、回复和残余应力松弛的综合作用。通过喷丸处理，残余应力从1000 MPa降至-300 MPa，最高疲劳极限为1200 MPa 4。疲劳裂纹起源于表层的夹杂物或铁素体层，在珠光体团边界处改变方向。随着ε 5的增加，疲劳裂纹萌生持续时间在疲劳寿命中所占的比例增加。随着粗糙度的增加，<th>由于珠光体团尺寸的减小，*K_通过粗糙度引起的裂纹闭合水平的减小而减小。巴黎区的抗疲劳裂纹性也有所提高，“m值”有所降低。

Following results clarifying the effects of various factors on the fatigue strength of eutectoid steel wires were obtained through the experiments.1. Within the drawing strain range of epsilon 0 to 3.8, fatigue strength increased monotonically up to 800MPa with increase in epsilon , as in the case of ferritc steel in which the degree of increase is low. In some cases, however it had a peak value at epsilon=2.5 depending on the fabrication condition. It increased with decrease in the wire diameter and did not depend on the lamellar spacing (0.01-0.02 mu m).2. Associated with high density dislocations and the narrow lamellar spacing observed in TEM, the hardness and tensile residual stress in the surface layer increased with increase in epsilon up to Hv=520 and 1000 MPa.3. While the tensile residual stress monotonically decreased with increase in bluing temperature and diminished at 500ﾟC, the hardness increased to a peak value of Hv=650 by bluing at 300ﾟC The fatigue limit increased to a peak of 900MPa by bluing at 400ﾟC due to combined effects of strain aging, recovery and relaxation of residual stress. By shot peeing, the residual stress decreased from 1000 to -300 MPa and the highest fatigue limit of 1200 MPa was obtained4. Fatigue crack initiated from inclusions or the ferrite layer in the surface region and changed its direction at the boundary of pearlite colony. The ratio of the duration for fatigue crack initiation in the fatigue life-time increased with increase in epsilon5. With increase in epsilon , the *K_<th> decreased through decrease in the roughness induced crack closure level due to decrease in the pearlite colony size. The fatigue crack resistance in Paris' region also increased and the "m-value" decreased.