PSI - Issue 14

Author name / Structural Integrity Procedia 00 (2018) 000 – 000

2

Takuya Yoshimoto et al. / Procedia Structural Integrity 14 (2019) 18–25 Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers.

19

Keywords: Hydrogen embrittlement; Ductile cast iron; Tensile test; Graphite size; Hydrogen content

1. Introduction

Hydrogen equipment, such as a hydrogen fuel cell vehicle and a hydrogen station, are being developed to solve increasing global warming and energy resource problem. One of the obstacle that has to be overcome for safety of hydrogen equipment is a hydrogen embrittlement (i.e. the degradation of strength properties of metallic material due to hydrogen penetration) (Nagumo (2008); Murakami et al. (2012)). At present, to avoid this problem, the material used for hydrogen equipment is limited to only a few materials such as austenitic stainless steel and aluminum alloy, because these material is less susceptible to hydrogen embrittlement. However, these materials are relatively expensive and it has cause-and-effect relationship with increasing the cost of hydrogen equipment. In order to realize a hydrogen society, it is necessary to expand the range of available material for hydrogen equipment to include common and inexpensive material, and to establish a guideline of safety design for such a common material even though it is modestly susceptible to hydrogen. Therefore, from the viewpoints of safety and economy, it is essential to investigate the hydrogen embrittlement behavior of common and inexpensive material. Ductile cast iron (DCI) has good mechanical properties such as strength, toughness and wear resistance and it is used for various parts in the wide range of industry. In addition, casting manufacturing provide parts of complex shape easily and at low-cost, compared to conventional machining process. If it is possible to use of DCI in hydrogen environment, it can contribute to cost reduction. Therefore, we focused on DCI as a prospective material for hydrogen equipment in this study. However, there are only a few studies on the hydrogen embrittlement of DCI (Ogawa et al. (2015); Matsuo (2017)). It is well known that mechanical properties of DCI strongly depend on the graphite size, grain number of graphite, matrix structure and so on. Thus, it is important to find out an optimal microstructural condition that is less susceptible to hydrogen embrittlement by appropriate controlling of microstructural factors. As a first step of this project, the graphite size was focused and the role for graphite size on hydrogen embrittlement in DCI was considered. In this study, the tensile tests were carried out using hydrogen-charged specimen of ferritic DCIs (JIS-FCD400) with a different graphite diameter of approximately 10 µm - 30 µm in air at room temperature. The effect of graphite size on the hydrogen-induced ductility loss and hydrogen absorption capability were investigated. 2. Experimental procedures Three kinds of casting Y-block of ferritic DCIs with different mean graphite diameters of approximately 10, 20 and 30 µm were used in this study, as shown in Fig. 1 and Fig. 2. The graphite grows larger during cooling in casting, and so the graphite diameter is dependent on the cooling speed. Furthermore, the cooling speed is higher at the block size becomes smaller. Consequently, in the case of block A, small graphite nodules were distributed, because there was no much time to grow the graphite. On the other hands, large graphite nodules were distributed in the block C. These materials were hereafter called Material S , Material M and Material L respectively. Microstructures of these DCIs are shown in Fig. 3. The tensile specimen shown in Fig. 4 was machined from as-cast blocks and then polished by a #2000 emery paper and paste of alumina particles with a diameter of 1 µm. Regarding Material S , there is relatively large difference in graphite diameter, depending on the location (i.e., relatively small graphite nodules are located at the lower side of casting block that is close to casting surface and relatively large graphite nodules are on the upper side of block.). Therefore, two kinds of tensile specimens were machined from Material S . Hereafter, the specimen machined from upper side of block is called Material S U , which contains relatively large graphite, and the specimen machined from lower side of block is called Material S L , which contains relatively small graphite. The chemical compositions are listed in Table 1. 2.1 Material and specimen