Effects of MnS on Gigacycle Fatigue Properties of SCM440 Steel

Gigacycle fatigue tests were conducted on high-strength steels whose Mn and S contents were so high as to be close to upper limits of the JIS standard. Three types of materials were prepared with different working ratios. The fatigue strengths were in order of the working ratios, indicating that the high working ratios improved the fatigue strength. All specimens ended in internal fractures mostly originating from MnS. The MnS observed on the fracture surface was largely elongated in the cases of the materials with high wording ratios. ODAs (Optically Dark Areas) were then observed at around the center of the elongated MnS, meaning that small internal cracks were formed there. Although the shapes of the MnS were very complicated, the sizes were approximated by using ellipses which covered the whole MnS. When the MnS was largely elongated, the length l was cut off at four times of the width W, i.e., l = 4W, based on the results of Makino's fracture mechanics analysis. The measured MnS sizes were applied to analysis by using a previously derived prediction, comparing the effects of MnS with those of oxides. As the result, it was suggested that the MnS was more harmful to the fatigue strength than the oxides. This was probably attributable to the properties of the MnS, while too large number of the MnS possibly reduced the fatigue strength.