Mechanism of Long Life Fatigue Fracture Induced by Interior Inclusion for Bearing Steel in Rotating Bending
Noriyasu OGUMA, Hisashi HARADA and Tatsuo SAKAI
Abstract:Mechanism of interior inclusion induced fracture of bearing steel in long life region was experimentally examined under cantilever-type rotating bending, and discussed from microscopic observations by SEM, TEM and SIM, respectively. Fish-eye was usually formed on the fracture surface in the interior inclusion induced fracture, and an inclusion was always found at the center of the fish-eye. Furthermore a characteristic rough surface area was necessarily observed in the vicinity around the inclusion. Based on TEM observation of longitudinal section of fracture surface at the rough surface area, it was found that structural changes were caused around the inclusion. Fine grains were caused around the inclusion in the long sequence of the cyclic loading. The sizes of these grains were within 40nm. It was finally found that the fatigue crack in such a long life region took place through the boundary disadjoinings of the above fine grains. Thus the characteristic surface area around the inclusion was well interpreted as the fine granular area (FGA) produced by the above boundary separations. Key Words:Bearing steel, Inclusion, Structural changes, Rotating bending, Duplex S-N curve, Fine granular area