Gigacycle fatigue of high-strength steel caused by MnS inclusions

Gigacycle fatigue tests were conducted on high-strength steels that contained large amounts of MnS and which had been subjected to different working ratios. Anisotropy was also investigated by fatigue testing in both the longitudinal and transverse directions. The fatigue strengths were positively correlated with higher working ratios, revealing that high working ratios improved fatigue strength. Anisotropy was also observed, indicating that the fatigue strengths in the longitudinal direction were higher than those in the transverse direction. The fracture mode was internal fractures, mostly originating from MnS inclusions. The MnS showed long slender shapes under high working ratios. The inclusion sizes were inversely correlated with working ratios, with small inclusion sizes seen under high working ratios. The inclusion sizes were smaller in the longitudinal direction than in the transverse direction. These results were analyzed by using a prediction derived in the previous study. The results showed the effect of working ratios to be explained by inclusion size. There was little difference between MnS and oxides in that both inclusions revealed almost identical effects when the inclusion sizes were comparable. Anisotropically, some other factor than inclusion size slightly affected the fatigue properties, reducing the fatigue strength in the transverse direction.