Impact of δ-ferrite formed during CMT-WAAM on the hydrogen embrittlement behavior of 316L stainless steel

Cold metal transfer wire arc additive manufacturing (CMT-WAAM) can offer a practical route for fabricating large and complex 316L stainless steel components. Compared with powder-based processes such as laser powder bed fusion and laser directed energy deposition, CMT-WAAM not only provides better control over deposition defects but also produces coarse grains with low dislocation density due to its slower cooling rate. While the reduction in defects limits available hydrogen trapping sites, the slower cooling also promotes the formation of δ-ferrite, a BCC phase theoretically associated with a higher risk of hydrogen embrittlement (HE). In this study, the HE resistance of CMT-WAAM 316L stainless steel was evaluated by tensile testing after gaseous hydrogen charging (300 °C, 15 MPa, 72 h) in both the as-built condition and after annealing at 1200 °C to remove δ-ferrite. The results showed that hydrogen charging significantly increased the strength of both conditions while reducing ductility to different degrees. The hydrogen embrittlement index (HEI) was approximately 20 for the as-built and 15 for the annealed samples. The lower HEI of the annealed specimen is attributed to its coarse, fully austenitic grain structure, giving it an HE sensitivity comparable to conventionally processed 316L. Notably, although the as-built specimen contained about ∼7% δ-ferrite, its HEI was only slightly higher than that of the annealed sample, still indicating high resistance to HE. This behavior can be explained by the uniform distribution of δ-ferrite within large austenite grains, which mitigates its potential adverse effect and helps preserve ductility after hydrogen charging.