イオン窒化した低炭素鋼S15CKの疲労破壊機構

抄録

The fatigue strength and the fracture mechanism of ion-nitrided steel S 15 CK were investigated by using three types of the specimen, that is, the annealed specimen, the nitrided specimen and the specimen that the surface layer was removed. The last specimen is called the surface removed specimen and its surface layer was removed 0.1 mm in radius after nitriding. The fatigue limit of the nitrided specimen was higher than that of the annealed specimen, but was almost the same as that of the surface removed specimen. This is related to the hardness distribution of the specimen. In the case of the nitrided specimen, the hardness is recorded maximum at the surface layer and decreased in the internal section. The estimated strength from the hardness of that specimen is higher than the measured fatigue limit within the 0.1 mm thickness layer from the surface. At the internal section deeper than 0.1 mm from the surface, the estimated strength is expected to be lower than the fatigue limit. Thus the fatigue limit and strength of the nitrided specimen are determined by the material properties of internal site. So, the fatigue limit and strength of the nitrided specimen are not decreased even if the surface layer is removed. In the case of the nitrided specimen, the crack was difficult to be observed from the surface because the crack initiation occurred from the internal site. To remove the surface layer makes ease to observe the crack on the surface. In the case of the surface removed specimen, many cracks were initiated and propagate along the needle shaped participate Fe₄N and finaly these cracks made coalescence each other. The similar mechanism is expected to be operated at the internal site of the nitrided specimen from the observation of the fracture surfaces.