Effects of microstructure and surface roughness on the fatigue strength of high-strength steels

Abstract

Rotating bending fatigue (RBF) tests were conducted on a high-carbon steel 100CrMnMoSi8 (martensite and bainite), and a tough-tempered medium-carbon steel 50CrMo4. The specimens were surface-finished to different conditions: polished surface and ground surface with a range of roughness levels. The experimental results indicate that the hardened high-carbon steel specimens with rough surface failed predominantly by surface crack initiation, whereas the fatigue fracture of the specimens with smoother surface tend to fail by subsurface crack initiation from non-metallic inclusions. Moreover, the fatigue strength of the martensitic specimen is lower than that of the bainitic specimen in the low stress-cycle range in which failure is dominated by surface initiated fatigue fracture, whereas this difference diminishes in the high-cycle fatigue regime where subsurface initiated fatigue prevails. The non-hardened medium-carbon steel samples, however, fail only by surface crack initiation, and the fatigue strength is much less sensitive to surface roughness. A unified model is developed to predict the fatigue strength of the tested samples with different microstructures and surface finish.