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

The influence of pro-eutectoid cementite (θ) 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 θ was accelerated, especially in the high stress intensity factor region. Scanning electron microscope (SEM) observation revealed that the fatigue fracture surface of 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 surfaces was notably increased with an enhancement in maximum stress intensity factor (Kmax) due to crack extension. More detailed SEM fractographies were performed comparing between before and after etching were performed in order to identify microstructures beneath the brittle fracture appearances on the fatigue fracture surface of the 1.21 mass% C steel. As a result, it was suggested that 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.