Abstract
The fatigue strength and the fracture mechanisms of surface treated specimens were investigated using five types of specimens - annealed, nitrided, surface-removed, shot peened and multi treated specimens with ion-nitriding and shot peening. In the case of surface-removed specimens, 0.1mm of surface layers were removed after nitriding. The shot peening was carried out after or before nitriding. The fatigue limit of the nitrided specimens was higher than that of the annealed specimens, but was almost the same as that of the surface-removed specimens. This is related to the hardness distribution of the specimens. In the case of the nitrided specimens, the estimated strength deduced from the hardness is higher than the measured fatigue limit from surface to a depth of 0.1mm. This shows that fatigue limit and strength of the nitrided specimens are determined by the internal properties of the materials, and thus the fatigue strength of the nitrided specimens are not decreased even if the surface layer is removed. It is also found out that the fatigue strength of nitriding specimen could be improved by the shot peening. The fatigue strength of the nitrided low carbon steel was related to the hardness distributions.
