炭素鋼軟窒化処理材の疲労限度に対するき裂状表面欠陥の影響

抄録

Nitrocarburizing is a surface-hardening process that has been widely used to increase the fatigue limit of vehicle parts. It has the advantage of producing small thermal deformations during the hardening process. However, if the treatment is applied to long shafts, such as crankshafts, an undesired bending deformation exceeding the tolerable values can occur. Therefore, in manufacturing, it is essential to perform bending correction within the bending correction limit. If bending correction is performed when the limit value has exceeded, small cracks initiate from the surface. To expand the range of bending correction, it is necessary to determine whether the presence of small cracks is acceptable. This study aimed at clarifying the influence of the crack depth on the bending fatigue limit of nitrocarburized medium carbon steel. To simulate the presence of small cracks, semi-circular slits with depths of 0.040, 0.075, and 0.100 mm were introduced on the surface of specimens used for fatigue tests under bending. From the fatigue tests, it was found that the fatigue limit decreases as the depth of the slit increases. The acceptable crack size was evaluated based on the relation between the stress intensity factor range of a semi-circular crack and the threshold stress intensity factor range of the tested material. Consequently, it was found that cracks with a depth smaller than 0.033 mm did not decrease the fatigue limit.