## 3 Results and Discussion
### Hydrogen Embrittlement of 316L Stainless Steel Under Different Hydrogen Storage Conditions
#### 3.2.1 Tensile Properties Under Different Hydrogen Storage Conditions  
Effect of Hydrogen Charging and Testing at Room Temperature  
The tensile properties of 316L stainless steel in its as-rolled, as-built, HT1, and HT2 conditions were evaluated at room temperature (testing condition 1). Figure 9 presents the engineering tensile stress-strain curves, while Figure 10 summarizes the tensile property values for each processing condition at room temperature. As observed from the results, the as-built specimen exhibits lower ultimate tensile strength (UTS) and elongation compared to the as-rolled specimen, but it demonstrates a 30.15% higher yield strength. This difference is primarily attributed to the higher strain hardening in as-rolled specimens due to stress-induced martensitic transformation, which occurs more significantly in wrought materials than in additively manufactured (AM) specimens [9]. Figure 9: Engineering stress–strain curves of 316L stainless steel under different processing conditions at testing condition 1. [PERSON] et al. [9] reported that AM 316L stainless steel exhibits greater resistance to phase transformation than wrought 316L stainless steel due to its high dislocation density, which increases the stress required for martensite formation. The higher yield strength observed in AM materials is mainly due to their increased dislocation density and the formation of a dislocation substructure [9; 21]. As a result, AM 316L stainless steel is well-suited for high-stress and high-pressure applications, as it can sustain higher loads without undergoing plastic deformation. Post-processing heat treatment (PPHT) results in relatively higher work hardening during deformation compared to the as-built material [9]. The HT1 treatment positively impacts elongation, as annealing reduces residual stresses in the as-built condition while also lowering strength due to a decrease in dislocation density. The reduction in dislocation density allows dislocations to nucleate and migrate more freely during deformation, improving ductility [9; 21]. Conversely, the HT2 treatment has a minimal effect on the mechanical properties of the as-built material. This limited impact may be attributed to partial recrystallization and associated grain refinement during the short holding time at 1050 \({}^{\circ}\)C; moreover, the quenching effect may counterbalance the high cooling rates experienced during the additive manufacturing process, which contributes to a high dislocation density. The engineering tensile stress-strain curves and tensile properties of the hydrogen-charged specimens tested at room temperature (testing condition 2) are presented in Figures 11 and 12, respectively. As observed, hydrogen charging has a consistent and significant effect on the tensile properties of 316L stainless steel across all material conditions. In all cases, hydrogen-charged specimens exhibit higher strength and lower elongation compared to those tested under Condition 1 (without hydrogen charging). This behavior occurs because hydrogen reduces plastic deformation, thereby inhibiting strain-induced martensite transformation. Furthermore, post-processing heat treatments mitigate the impact of hydrogen on tensile properties compared to the as-built material. These findings align with previous studies [7; 9; 23]. Figure 10: Tensile properties of 316L stainless steel under different processing conditions at testing condition 1. Influence of Hydrogen Charging and Testing at Cryogenic Temperature  
To assess the effect of cryogenic temperature (\(-196\)\({}^{\circ}\)C) on the tensile properties of 316L stainless steel, specimens were tested in a liquid nitrogen environment (testing condition 3). The engineering tensile stress-strain curves for Condition 3 are shown in Figure 13, while the corresponding tensile property values are presented in Figure 14.  
Compared to Condition 1 (room temperature testing), cryogenic testing significantly increases the strength of all specimens.