Effect of removing surface hardened layer on the fatigue strength of bearing steel SUJ2 ground specimen in the long life field

In order to study the fatigue damage of a high carbon chromium bearing steel SUJ2, rotating bending fatigue tests of smooth specimens (Kt = 1.06, stress concentration factor) were carried out. The specimens were finished by the grinding after the heat-treatment. The fatigue strength of EP-0 (ground) specimen exhibited large scatter and it tended to drop again in the long-life field in excess of 107 cycles. In such a case, fish-eye was left on the fracture surface. In order to study the effect of surface hardened layer due to the grinding, two types of EP (electropolished) specimens, EP-15 specimen whose surface hardened layer and grinding flows were almost removed and EP-3 specimen which has the decreased surface roughness and surface hardened layer, were fatigued. The fatigue strength of EP-15 specimen was dramatically decreased when compared to the EP-0 specimen, and no drop in fatigue strength in the long-life field were observed. This indicates that the drop in fatigue strength in the long-life field results from the surface strengthened layer due to the grinding. On the other hand, S-N data of EP-3 specimen exhibit large scatter, but the range of scatter is smaller than that of EP-0 specimen. This narrowed scatter may be related to the decreased surface roughness of EP-3 specimen. Considering the experimental results of EP -15 and -3 specimens, the physical basis of peculiar S-N characteristics of EP-0 specimen was investigated.