Effect of Removing Surface Hardened Layer on the Fatigue Strength of Bearing Steel SUJ2 Ground Specimen in the Long Life Field
Masahiro Goto, Takaei Yamamoto, Hironobu Nisitani, Tatsuo Sakai and Norio Kawagoishi
Abstract: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. Key Words:Ultra high-cycle fatigue, High strength steel, Fish-eye, Rotating bending, S-N characteristics, Grinding, Residual stresses, Work-hardening, SEM, Surface roughness