Long Life Fatigue Fracture Induced by Interior Inclusions for High Carbon Chromium Bearing Steels Under Rotating Bending

The characteristics of the long life fatigue fractures induced by interior nonmetallic inclusions for two types of high carbon chromium bearing steels were experimentally examined under rotating bending. Specimens were machined from the round bars with different diameters. One type was cut from the central area of a thin bar; the other type was cut from the medium area of the radius of a thick bar. As a result of the fatigue tests, the fatigue life of the latter type specimen was longer in the long life region. Crack initiation sites on fracture surfaces of all the failed specimens were observed by means of a scanning electron microscope. The number of alumina inclusions was decreased, and the average size of titanium nitride inclusions on fracture surfaces was smaller for the latter type specimens. And a characteristic rough surface of fine granular area (FGA) was observed in a circle around the inclusion on the fracture surface. The size of FGA tended to increase with an increase of the fatigue life and a decrease of the stress amplitude at the position of the inclusion. From a view point of fracture mechanics, the stress intensity factor range at the front of FGA, ⊿KFGA, was calculated for each specimen by using √area model. Values of ⊿KFGA of both type specimens were almost constant regardless of the stress amplitude at the site of inclusion, and the average value was about 5 MPa√m. That is, it seems that the fatigue life of the specimens cut from the thick bar was improved by downsizing the inclusion area on the fracture surface.