ppm レベルの酸素添加による水素中フレッティング疲労強度低下の機構

抄録

Fretting fatigue test was carried out in high-purity hydrogen, an oxygen-hydrogen mixture and vacuum. The material was austenitic stainless steel SUS304. The fretting fatigue strength in the oxygen-hydrogen mixture was significantly lower than that in the high-purity hydrogen. The fretting wear was not observed in the high-purity hydrogen. On the other hand, the fretting wear was developed in the oxygen-hydrogen mixture. Based on the X-ray photoelectron spectroscopy (XPS) analysis, it was found that an original oxide layer of the stainless steel was removed by the fretting in the high-purity hydrogen. However, the addition of ppm-level oxygen produced an oxide layer on the fretted surface by overcoming the removal effect of oxide layer by the fretting. The production of the oxide layer by the addition of ppm-levels of oxygen developed the fretting wear. For the shape of the contact part in this study, a high concentration of the contact pressure occurs at the contact edge. This concentration of the contact pressure produces high compressive stress field on the contacting surface of the specimen. This compressive stress suppressed the crack growth. However, the compressive stress was relieved in the oxygen-hydrogen mixture because of the fretting wear. This is one of the reasons for the reduction in the fretting fatigue strength in hydrogen by the addition of ppm-levels of oxygen.