Mechanism of reduction of fretting fatigue limit in hydrogen gas in SUS304

Abstract

The authors reported reduction of fretting fatigue strength in hydrogen gas using several kinds of materials such as low alloy steel, heat resistant steel, aluminum alloy, austenitic stainless steels, etc. In this study, mechanism of the reduction was discussed thorough detailed observations of small fretting fatigue cracks and two-step test in which the environment was changed during a test. The material was 30% pre-strained austenitic stainless steel SUS304. Hydrogen gas pressure was 0.12MPa in absolute pressure. Whereas oxidation process was important to develop fretting damage in air, adhesion was dominant in the fretting in hydrogen gas. In hydrogen gas, small cracks were emanated at both ends of the adhered parts. Since the major crack was propagated from one of the small cracks emanating adhered part, the adhesion and formation of small cracks were important determinants of the reduction of fretting fatigue strength in hydrogen gas. In the fretting fatigue test in hydrogen gas following fretting fatigue test in air, no fracture occurred even if the stress amplitude was higher than the fretting fatigue limit in hydrogen gas. The reason was that the oxidized wear film, which was produced in the first air environment, prevented the adhesion and the formation of small cracks. Another two-step test was that small cracks were formed in hydrogen gas and the test environment was then changed to air. As the result, there was no fracture of the specimen even the test was performed at stress amplitude that the fracture of specimen occurred in the single hydrogen gas environment. The result suggested that the stress field in the vicinity of small cracks were severer in hydrogen gas than in air. © 2010 The Society of Materials Science, Japan.