Abstract
High cycle fatigue properties and failure mode in high strength steel tempered at low temperature were studied. The material used was Ni-Cr-Mo steel, JIS SNCM439, which was tempered at 453 K. After heat treatment, round bar specimens with hourglass shape were machined. Fatigue tests were performed using two kinds of specimens with two surface conditions, i.e., as-ground specimen and buff-finished specimen. Tests were conducted under rotating bending and axial loading. Under rotating bending, in as-ground specimens, fatigue failure took place in high cycle region above 107 cycles and failure mode was surface-related. On the other hand, buff-finished specimens exhibited the step-wise S-N curve, i.e., the subsurface fracture with a fish-eye was observed. However, under axial loading, even in buff-finished specimens the subsurface fracture didn’t occur due to small nonmetallic inclusions. Based on the results, the fatigue limit was estimated by the statistics of extremes. The prediction gave the conservative values.
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