Considering the mechanisms causing reduction of fretting fatigue strength by hydrogen (2012)

The fretting fatigue test of austenitic stainless steels, JIS SUS304 and SUS316, was carried out in 0.12MPa hydrogen and air. The fretting fatigue strength of both materials was reduced by hydrogen. One of the possible causes was adhesion between the fretting surfaces which was predominant in hydrogen. The effect of specimen finishing on adhesion was also verified. For this purpose, two surface roughness were prepared with Ra = 0.420m and 0.008m. During the fretting fatigue test of these specimens in air, adhesion occurred in the smoother surface specimen but did not occur in the rougher surface specimen. As a result, the fretting fatigue strength decreased when adhesion occurred. Therefore, it can be considered that adhesion resulted in the reduction of the fretting fatigue strength in smoother specimens in air and in 0.12MPa hydrogen. Strain-induced martensite was found in the region of the adhered part, possibly due to the severe cyclic strain occurred locally at the adhered region. Introduction Fretting is one the most important issues in the design of mechanical components involving contact because fretting can cause a significant reduction in the fatigue strength of the contact part. In machines using hydrogen, the importance of fretting is even higher because hydrogen can influence both the fatigue and friction. The authors have reported a significant reduction in the fretting fatigue strength of several kinds of materials due to hydrogen [1-4]. The objectives of this study are: to evaluate the effect of hydrogen on the fretting fatigue strength of austenitic stainless steels and to understand the mechanism of the reduced fretting fatigue strength in hydrogen. The effect of surface finishing roughness on fretting fatigue strength is also verified. Effect of Environmental Hydrogen on Fretting Fatigue Strength of SUS304 Test procedure. The test material was the austenitic stainless steel 304 type which is designated by the JIS as SUS304. The specimen and contact pad were made of 30% pre-strained SUS304 material. The pre-strain was applied to the material by uniaxial tension, and then the specimen was taken from the pre-strained material so that the specimen axis coincided with the tensile load axis. The Vickers hardness was HV311. The microstructure of the material is shown in Fig. 1(a). Strain-induced martensite was present. (a) 30% pre-strained SUS304 (b) Solution heat-treated SUS316 Fig.1. Microstructure of test materials Fretting fatigue test. The fretting fatigue test set-up used in the experiment is shown in Fig. 2. Two contact pads were pressed onto the front and back side surfaces of the fatigue test specimen at a contact pressure of 100MPa. Fretting was induced at the contact part by applying a bending fatigue load. The reduction in the contact pressure during the fretting fatigue test due to fretting wear was within 5% of the initial value. The fretting fatigue test was carried out with a stress ratio of R = -1 at a test frequency of f = 18.7Hz at ambient temperature. The test frequency may be one of the factors affecting the fretting fatigue properties in hydrogen similarly to fatigue [5, 6]. The effect of the test frequency on the fretting fatigue in hydrogen remains to be characterized. The configurations of the specimen and contact pad are shown in Fig. 3. The contact surfaces of the specimen and the contact pad were finished by #400 emery paper in the direction of the fretting motion. A bridge-type contact pad was used to measure the tangential force acting on the contacting surfaces. As shown in Fig. 2, a strain gage was sticked to the center of the recessed part of the contact pad to measure the tangential force. The tangential force coefficient () was defined as the ratio of the tangential force (Ft) to the contact force (Fc). The test environments were 0.12MPa hydrogen and air. The hydrogen purity was 99.9999%. Effect of hydrogen on fretting fatigue strength of SUS304. The result of the fretting fatigue test is shown in Fig. 4. The fretting fatigue test was carried out to 3×10cycles if no failure occurred. The fretting fatigue failure definitely occurred around 10 7 cycles in hydrogen, while no failure occurred at the same stress level in air. Thus, the reduction of the fretting fatigue limit in hydrogen was clearly observed. It can be considered that the reduction in fretting fatigue strength is associated with some specific mechanisms of fretting fatigue other than the mechanism that causes a reduction in the fatigue strength, since there are reports that the high-cycle fatigue strength of the same kind of austenitic stainless steel in hydrogen is not changed [7] or rather increased [8]. Tangential force. Figure 5 shows the relationship between the tangential force coefficient and stress amplitude. The tangential force value was higher in hydrogen than in air due to adhesion between the contacting surfaces. Thus, an increase in the tangential force causes a decrease in the fretting fatigue 20m Strain-induced martensite Cyclic bending moment Strain gage (nominal stress measurement) Strain gage (tangential force measurement) Contact load Contact pad Contact pad