Vol.8 No.2 2015
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Research paper : Development of material testing equipment in high pressure gaseous hydrogen and international collaborative work of a testing method for a hydrogen society (T. IIJIMA et al.)−62−Synthesiology - English edition Vol.8 No.2 (2015) the hydrogen atoms diffuse in the metal lattice, and the material property of the metal declines. This is called hydrogen embrittlement. Specifically, when tensile tests for metallic materials are conducted in high-pressure gaseous hydrogen environment, or when tensile tests for metallic materials that are hydrogen-charged by exposure in testing chambers of hydrogen environment are conducted in atmosphere (in inert gas), the strength properties such as yield stress and tensile strength or the ductilities such as breaking elongation and reduction of area are reduced. Due to the word “embrittlement,” it may present the impression that “hydrogen embrittlement” is a breakage within the elastic range of metallic materials where no elongation takes place. Of course, some materials may break within the elastic range in the hydrogen atmosphere, but most materials show plastic deformation. Therefore, Murakami et al. described hydrogen embrittlement as “ductile fracture that is accompanied by microscopic plastic deformation.”[5]Up to the present, so many research works have been performed for the effect of hydrogen on the strengths and ductilities of various materials. As a result, it became clear that while there is no metallic material that does not show some degree of hydrogen embrittlement, the materials can be roughly categorized as follows: (1) materials that cannot be used due to large effects of hydrogen embrittlement such as fractures occurring in the elastic range, (2) materials that may be used in certain conditions although ductility such as elongation and reduction of area may decrease due to the effect of hydrogen embrittlement, and (3) materials that receive little effect of hydrogen embrittlement under limited conditions. The materials categorized in (3) include austenitic stainless steel with high nickel content and aluminum alloys. One of the materials categorized in (2) is low-alloy steel. Low-alloy steel is a material used widely as structural material in various fields such as chemical plants, and it is characterized by having higher material strength and being less expensive than austenitic stainless steel.2.2 Standards for qualifying the materials compatibility of high-pressure gaseous hydrogen equipmentDetermination and review of the standards for FCV on-board containers and hydrogen station vessels are being conducted around the world. Characteristically, since the hydrogen filling stations are installed domestically compared to FCVs that will be distributed widely around the world, the domestic considerations are reflected strongly in hydrogen filling stations.For on-board containers, it is designated by the “Exemplified Standard for Container Inspections, etc.” (2013), which is the technical standard set by the Safety Regulations for Containers of the High-Pressure Gas Safety Law in Japan, that the maximum fill pressure of the compressed hydrogen FCV on-board container shall be 70 MPa, and the materials that can be used for such containers are austenitic stainless steel (SUS316L) containing specific nickel content (nickel equivalent) and aluminum alloys (6061-T6).[6] In the USA, the 6061 aluminum alloys and high nickel SUS316 are designated as materials that can be used for on-board containers for 70 MPa compressed hydrogen FCV in the annex of SAE J2579 (2009) of the Society of Automotive Engineers (SAE). If any other materials are to be used, they must be subject to designated material tests: (1) slow strain rate tensile tests in hydrogen or of hydrogen-charged material, (2) fatigue tests in gaseous hydrogen, and (3) crack growth tests in gaseous hydrogen condition.[7] The standard for 70 MPa on-board containers in Europe used to follow the ISO/TS 15869 (2009) “Gaseous Hydrogen Blends & Hydrogen Fuels: Land Vehicle Fuel Tanks.” However, as the review of the global standard was started by the United Nations, as will be explained later, the review by the ISO Technical Committee (TC197/WG18) has started from 2013.[8] In the World Forum for Harmonization of Vehicle Regulations (WP29) of the United Nations Economic Commission for Europe (UNECE), the need to promote international mutual recognition of global standards with international harmonization was recognized to diffuse automobiles with excellent safety and environmental performance. Therefore, the creation of the “Global Technical Regulation for Hydrogen and Fuel Cell Vehicles (HFCV global technical regulations)” was started from 2007, and gtr Phase 1 was adopted in 2013. In accordance to this, the items of the Safety Regulations for Containers were revised in June 2014 in Japan.[9] However, the deliberation for the materials compatibility of on-board containers will be continued in gtr Phase 2.For the vessels, Japan designates stainless steel (SUS316, SUS316L) as the compatible material for the compressed hydrogen vessels and the pipes through which compressed hydrogen passes, and designates the chemical composition (nickel equivalent) at normal operation pressure (82 MPa) and normal operation temperature (−40~250 ºC), in the Exemplified Standard for Security Regulation for General High-Pressure Gas Safety Regulations (2014) of the High-Pressure Gas Safety Laws. It also allows the steel for machine structural use (SCM435) to be used for vessels at normal operation pressure of 40 MPa or less.[10] In the USA, alloy steels such as SA-372 and SA-723, stainless steels such as SA-336 and Gr.F316, and aluminum alloys such as 6061-T6 are indicated as compatible materials in high-pressure gaseous hydrogen up to 103 MPa, according to Article KD-10 in Division 3: Special Requirement for Vessels in Hydrogen Service (2010) of the American Society of Mechanical Engineers (ASME). For actual use, it requires evaluations of the following: (1) plane strain fracture toughness value KIC by rising load and rising displacement in atmosphere (crack-initiation threshold test in accordance to ASTM E399 or E1820), (2) fracture toughness value KIH by constant load or constant displacement in gaseous hydrogen (crack-arrest

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