Abstract
An in-situ small punch (SP) test method has recently been developed as a simple screening technique for evaluating the properties of metallic materials used in high-pressure hydrogen environments. With this method, the test conditions including temperature and gas pressure can easily be adjusted to those used in practice. In this study, specimens of STS316L steel and 18 wt% Mn steel were prepared at two different surface roughness, fabricated using wire-cutting and mechanical polishing. Their effects on hydrogen embrittlement (HE) were evaluated using in-situ SP testing at both room temperature and a lower temperature where HE was shown to occur under 10 MPa hydrogen. Both steels were evaluated using two variables obtained from in-situ SP testing, the SP energy, and the relative reduction of thickness (RRT), to quantitatively determine the effect of specimen surface roughness on HE susceptibility. Their fracture characteristics due to HE under 10 MPa hydrogen showed little difference with surface finish. Surface roughness had a negligible influence on these quantitative factors describing HE, indicating that it is not a dominant factor to be considered in in-situ SP testing when it is used to screen for HE compatibility in steels used in high-pressure hydrogen environments.
Highlights
Hydrogen energy is drawing attention in application fields such as mobilities wherein global warming and microparticle emissions are being addressed
This is consistent with the results reported for 9% Cr steel, wherein surface roughness had a minimal effect on deformation behaviors and fractures [26]
Because relative reduction of thickness (RRT) describes a loss of ductility due to hydrogen embrittlement (HE), the effect of surface roughness on HE susceptibility is minimal for these two steels, considering that failure occurred after some amount of plastic deformation due to HE during the in-situ small punch (SP) tests
Summary
Hydrogen energy is drawing attention in application fields such as mobilities wherein global warming and microparticle emissions are being addressed. HE behaviors of materials under high-pressure conditions have primarily been evaluated in either external hydrogen conditions using slow strain rate tensile testing (SSRT), where a specimen is typically placed in a large autoclave with explosion-proof capabilities, or in internal hydrogen conditions using hydrogen-precharged specimens in ambient air [4,5,6]. When SSRT tests take place in high-pressure hydrogen environments in particular, extensive equipment and facilities for creating and maintaining external hydrogen test conditions are required [3,7,8] as well as large amounts of cost and manpower for installing, operating, and maintaining autoclave equipment. It is not feasible to perform SSRTs under high-pressure hydrogen to obtain varieties of HE compatibility data for materials used in devices exposed to actual hydrogen environments, given the breadth of chemical composition and microstructures and the number of tests required. This type of specimen has been used in high-pressure cryogenic environments and tested using SSRT
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