Hydrogen embrittlement resistance of rolled and additively manufactured SUS 316L under in-situ hydrogen charging

Ensuring high resistance to hydrogen embrittlement (HE) in structural materials used in hydrogen environments is crucial because hydrogen diffusion can degrade mechanical properties and promote premature failure. This study compares the HE behavior of SUS 316L from a manufacturing-process perspective. Specimens fabricated via as-received rolling (AR) and additive manufacturing (AM) processes, including directed energy deposition (DED) and powder bed fusion (PBF), were evaluated using slow strain rate testing (SSRT) under in-situ electrochemical hydrogen charging conditions. HE indices derived from SSRT-based elongation and area reduction revealed that PBF exhibited the highest HE resistance, followed by DED and AR. Under in-situ hydrogen charging, AR exhibited reduced γ-austenite stability with α′-martensite formation, which provided preferential hydrogen diffusion paths and promoted hydrogen accumulation at crack tips, causing brittle fracture and premature failure. Conversely, DED and PBF specimens showed no significant α′-martensite transformation, and the underlying reasons for this behavior and the differences between the AM processes were investigated. PBF formed a finer sub-grain structure that facilitated more homogeneous hydrogen distribution through reversible trapping, as reflected by a dominant low-temperature desorption peak in TDS. In contrast, DED exhibited relatively coarser sub-grains, with microvoids acting as irreversible trapping sites, as reflected by a relatively more pronounced high-temperature desorption peak.