Effects of S-content on Gigacycle Fatigue Properties of SCM440 Steel

Abstract

Our previous study clarified the effect of MnS on gigacycle fatigue properties of SCM440 steel by conducting gigacycle fatigue tests on High-S steels whose S-content was so high as to be close to an upper limit of the JIS standard. On the other hand, this study discusses methods to avoid fatigue-failure from MnS by conducting the gigacycle fatigue tests on Low-S steel whose S-content is as low as possible in commercially available processes. Three types of materials were prepared with different working ratios, and the fatigue tests were conducted in transverse directions. The Low-S revealed higher fatigue strengths than the High-S. The fatigue strengths of Low-S with high working ratios were as high as those of a hot-rolled round bar. The Low-S with the lowest working ratio was fractured from shrinkages, resulting in low fatigue strength. The Low-S with the highest working ratio was fractured from oxides and matrix. As the results, MnS caused fatigue failure in a few specimens of the Low-S only with the middle working ratio. These results meant that the MnS-induced fatigue-failure was avoidable by minimizing the S-content. The fatigue test results were analyzed by using a previously derived prediction. The analysis results suggested that the MnS of High-S and the shrinkages of Low-S were more harmful than others. This was probably attributed to the large number of the MnS of High-S and of the shrinkage of Low-S, which accelerated crack propagations by interaction between the MnS inclusions and between the shrinkages.