Effect of Pro-eutectoid Cementite on Fatigue Crack Growth Behavior of Pearlitic Steels

: 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 hy-po-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 en-hancement in maximum stress intensity factor ( K max ) due to crack extension. More detailed SEM fractog-raphies 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 re-sults, 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.