加工硬化層と湿度に依存した高強度鋼の長寿命疲労特性

抄録

This paper presents the fatigue response of high strength steel, SNCM 439, depending on work hardened layer and humidity. Tests were conducted in laboratory air and in dry air using cantilever type rotating bending fatigue testing machines. The dew point of dry air is -60°C. Specimens with two different surface conditions were prepared, i.e., buff finished and electro polished specimens. In the buff finished specimens, a step wise S N curve was seen in both environments, but the horizontal part of the S N curve below which subsurface crack initiation occurred shifted to a higher stress level in dry air. In the electro polished specimens, a step wise S N curve was not observed in laboratory air, but seen in dry air. At stress levels where cracks initiated at the surface, the fatigue lives of the electro polished specimens decreased compared to the buff-finished. On the contrary, at stress levels where subsurface cracks were generated, the fatigue lives of both specimens were the same, indicating no influence of surface finishing condition and environment.