Fatigue strength and fracture mechanisms in severely drawn eutectoid steel wires

通过试验得到了不同因素对共晶钢丝疲劳强度的影响。在拉伸应变epsilon 0 ~ 3.8范围内，随着epsilon的增大，疲劳强度单调增加至800MPa，铁素体钢的疲劳强度单调增加程度较低。然而，在某些情况下，根据制造条件，它在epsilon=2.5处具有峰值。它随线径的减小而增大，与片层间距（0.01 ~ 0.02 μ m）无关。在TEM中观察到高密度位错和窄片层间距，随着epsilon的增加，硬度和表层的拉伸残余应力增加，达到Hv=520和1000 mpa。拉伸残余应力随蓝化温度的升高而单调降低，在500ºC时逐渐减小，300ºC蓝化使硬度达到峰值Hv=650, 400ºC蓝化使疲劳极限达到峰值900MPa，这是由于残余应力的应变时效、恢复和松弛的共同作用。通过喷焊，残余应力从1000 MPa降至-300 MPa，最高疲劳极限为1200 MPa 4。疲劳裂纹由表面区域的夹杂物或铁素体层引发，并在珠光体群体边界处改变方向。随着epsilon5的增大，疲劳裂纹萌生时间占疲劳寿命的比例增大。随着epsilon的增大，*K_<th>减小，这是由于珠光体集落尺寸减小，粗糙度致裂纹闭合程度减小所致。巴黎地区的抗疲劳开裂性能也有所提高，“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.