Effect of annealing and UNSM treatment on the hydrogen embrittlement resistance of LPBF-printed 316L steel

The laser powder bed fusion (LPBF) method enables the fabrication of complex-shaped 316L steel parts; however, a cellular structure with a high density of dislo­cations and macrodefects is formed in it, thereby increasing its susceptibility to hydrogen embrittlement (HE). The influence of annealing (900°C) and ultra­sonic nanocrystal surface modification (UNSM) on the resistance of LPBF-manufactured 316L steel to HE is investigated. In the as-printed state, the steel absorbs a significant amount of hydro­gen (8.9 ppm after 115 h electrochemical hydrogenation), resulting in a 4.9–14% reduc­tion in its mechanical properties. Post-LPBF processing (annealing at 900°C, UNSM, in various sequences) substantially reduces hydrogen absorption and increases resistance to hydrogen embrittlement by destroying the cellular structure (during annealing) or by den­sifying the surface and forming a hardened nanostructured layer (during UNSM). A corre­lation between the hydrogen embrittlement coefficient and hydrogen concen­tration in the samples was revealed: noticeable HE manifestations are observed at hydrogen concen­trations > 6–6.5 ppm. Combined processing (annealing + UNSM + final annealing) forms fine recrystallized grains in the surface layer, ensures the minimum hydrogen concentra­tion (3.4 ppm) in the steel, and provides its complete insensitivity to HE. The results confirm the promising potential of integrated post-processing for improving the reliability of additively manufactured 316L steel in hydrogen-containing environments.