PSI - Issue 68

Akihiko Fukunaga et al. / Procedia Structural Integrity 68 (2025) 1059–1065 Akihiko Fukunaga / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction The use of high pressure gaseous hydrogen is essential for achieving zero greenhouse gas emissions. In Japan, more than 150 hydrogen refueling stations with a pressure of 82 MPa have been built, mainly in the four major metropolitan areas, to promote the spread of fuel cell vehicles and fuel cell buses. A hydrogen refueling station consists of a compressor, high pressure vessels, and a dispenser [1]. The temperature around the compressor rises due to the increase in pressure, so the operating temperature is between − 10 and 180 ℃ . Compressor components are made of high strength low-alloy steel and A286 (UNS. S66286, AMS5732, JIS SUH 660). For the high pressure vessel, high strength low-alloy steel or carbon fiber reinforced plastic composites with aluminum liner are used. In the dispenser, the operating temperature is between − 40 and 80 ℃ , so austenitic stainless steel with a high nickel content and A286 are used. A286 is a precipitation (γ' phase (Ni3 (Al, Ti))) hardening type heat-resistance alloy with more than 1000 MPa strength at room temperature [2]. The alloy is also used for low temperature applications requiring a ductile ranging from room temperature to − 253 °C. The microstructure of A286 is similar to that of austenitic stainless steel. Therefore, in order to use high pressure gaseous hydrogen safely, it is important to clarify the hydrogen embrittlement behavior of A286 in hydrogen environments. The compatibility of metallic materials to hydrogen embrittlement can be determined by the relative reduction on area (RRA: (reduction area of specimen in hydrogen / reduction area of specimen in air or inert gas) × 100) value obtained from the slow strain rate tensile (SSRT) test using smooth surface specimen, as shown in the NASA safety standard database [3]. In other words, materials with RRA values close to 100% are hardly affected by hydrogen, while materials with low RRA values are extremely susceptible to hydrogen. In particular, it is known that materials with tensile strengths of 900 MPa or higher are susceptible to hydrogen. There are two types of SSRT test: one is to conduct SSRT testing in an environment of high pressure gaseous hydrogen around the specimen, and the other is to use a specimen that has been charged with hydrogen beforehand. The former method can maintain the specimen surface in high pressure gaseous hydrogen during test, but equipment is required to maintain the high pressure. The latter method can maintain a constant internal hydrogen content during test, but it is necessary to determine the appropriate hydrogen content before testing. Researchers use each method according to their objectives and equipment. While, hydrogen embrittlement mechanisms of metallic materials can be classified into three types of mechanisms. One is dissolved hydrogen directly weakens the bonding strength of base metal atoms (hydrogen-enhanced decohesion (HEDE) [4]) and others are during plastic deformation hydrogen promotes dislocation migration (hydrogen enhanced local plasticity (HELP) [5]) and hydrogen accelerates vacancy formation (hydrogen enhanced strain-induced vacancies (HESIV) [6]). HEDE is thought to occur at relatively high hydrogen content because it is caused by hydrogen diffusing in the crystal lattice and weakening metallic bonds. On the other hand, HELP and HESIV are thought to be caused by localized concentration of hydrogen due to dislocation migration and other factors, leading to localized plastic deformation, so embrittlement is thought to occur even with small amounts of hydrogen content. Recently combined mechanisms (HEDE + HELP) have been deeply discussed based on fracture surface morphology [7]. In cases where the theories are mixed, it is believed that hydrogen weakens bonds in the crystal lattice, while local hydrogen accumulation occurs due to dislocations and other movements, which also affect the deformation mechanisms around defects, leading to progressive fracture. In the previous study, it was clarified that the dominant mechanism of hydrogen embrittlement differs between the elastic deformation region and the plastic deformation region [8,9]. However, the relationship between the hydrogen environment and hydrogen embrittlement behavior during SSRT test is still unclear. In this study, we will compare the hydrogen embrittlement behavior of A286 in high pressure gaseous hydrogen and the hydrogen embrittlement behavior of A286 that has been pre-charged with hydrogen by closely analyzing vicinity of fracture surfaces using electron backscatter diffraction [EBSD] etc., and clarify the hydrogen embrittlement behavior in each hydrogen environment.