PSI - Issue 19

Masanobu Kubota et al. / Procedia Structural Integrity 19 (2019) 520–527 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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as 3.5 or 4 by the design code and standards for high-pressure vessels [ASME Boiler & Pressure Vessel Code. Section VIII (2001), JIS B 8265 (2003) and JIS B 8267 (2008)]. Thus, the allowable stress is UTS / 3.5 or 4. This means that the design is considered to be safe against fatigue failure, since it is well-known that the fatigue limit of steel is approximately 0.4 [Yamada and Kobayashi (2012)] or 0.5 [Furuya et al. (2005)] times the UTS. Suresh (1991) also showed that the ratio of the fatigue limits of the steels to the UTS ranged from 0.45 to 0.53 except for the very high strength steels. However, if the high-pressure medium is hydrogen gas, the effect of hydrogen on the fatigue limit should be determined to avoid any dangerous situation. Figure 1 shows the relationship between the fatigue limit and allowable stress provided by the design-by-rule approach. If the fatigue limit is reduced by hydrogen, the margin between the fatigue limit and allowable stress is reduced. When considering the large variation in the fatigue limit as seen in the studies by Nishijima (1980) and Schneider and Maddox (2003), the reduction of the fatigue limit due to hydrogen may cause the risk of fatigue failure. To ensure the safety of high-pressure hydrogen containment systems, the effect of hydrogen on the fatigue limit must be studied. The objective of this study was to gain a basic understanding of the effect of hydrogen on the fatigue limit. Therefore, the material was high-strength steel that suffered from severe hydrogen embrittlement during the slow strain rate testing. The pressure of the hydrogen gas was 0.1 MPa in gauge pressure. Of course, characterization of the fatigue limit in the high-pressure hydrogen gas is required, but it was regarded as the next step in this study because the high pressure experiment requires special experimental facilities as seen in the studies by Yamabe et al. (2016), Miyamoto et al. (2012), Nakamura et al. (2010) and Kubota et al. (2014).

Fig. 1. Need to determine the effect of hydrogen on the fatigue limit.

2. Experimental procedure

2.1. Material

The test material was JIS SCM435 Cr-Mo steel. Table 1 shows the chemical composition of the material. According to the studies on the effect of hydrogen on the high-cycle fatigue strength by Matsunaga et al. (2015), Ogawa et al. (2017) and Kubota and Kawakami (2014), there was no significant effect of hydrogen on the fatigue limit. Therefore, this study used a material modified to be sensitive to hydrogen embrittlement by heat treatment. The heat treatment was tempering at 443 K for 2 hours following heating at 1143 K for 2 hours and oil quenching. Consequently, the Vickers hardness of the material was HV600. In addition, the use of the high strength steel has another purpose. There are two possible situations when the fatigue limit is reached, i.e., the presence and absent of non-propagating cracks in the unbroken specimen at the fatigue limit. Generally, for a smooth specimen of high-strength steel, non-propagating cracks are rarely observed except when the crack origin is a small defect such as non-metallic inclusion. This means that the fatigue limit was determined by crack initiation. On the other hand, for a deep-notched specimen, non-propagating cracks are usually found at the notch root of the unbroken specimen at the fatigue limit. This means that the fatigue limit was determined by the threshold for crack propagation. This study tried to examine the effect of hydrogen on the fatigue limits achieved by both mechanisms using the same material.