パーライト鋼の疲労き裂進展挙動に及ぼす初析セメンタイトの影響

抄録

The influence of pro-eutectoid cementite on fatigue crack growth behavior was investigated using various pearlitic steels containing from 0.64 to 1.21 mass%C. The fatigue crack growth rates of the hypo-eutectoid and eutectoid pearlitic steels hardly changed, while that of the 1.21 mass%C steel having a large amount of pro-eutectoid cementite (θ) was accelerated especially in the high stress intensity factor region. Scanning electron microscope (SEM) observation revealed that fatigue fracture surface in the 1.21 mass%C steel more frequently contained islanded brittle fracture surfaces than that in other steels. In the 1.21 mass%C steel, the total area fraction of brittle fracture surface was notably increased with an enhancement in maximum stress intensity factor (K_max) due to crack extension. More detailed SEM fractographies were performed comparing between before and after etching in order to identify microstructures beneath the brittle fracture appearances on the fatigue fracture surface of the 1.21 mass%C steel. As a results, it was suggested that the pro-eutectoid θ was involved in the formation of brittle fracture. Based on these investigations, the accelerated fatigue crack growth behavior of hyper-eutectoid steel was discussed in terms of static brittle fracture induced by pro-eutectoid θ near the crack tip.