## 3 Results and Discussion
### Hydrogen Embrittlement of 316L Stainless Steel Under Different Hydrogen Storage Conditions
#### 3.2.2 Impact Toughness and Fractography at Different Hydrogen Storage Conditions
Figures 20-25 present low- and high-magnification SEM micrographs of the fracture surfaces of as-built and HT2 specimens tested under different conditions: uncharged at room temperature (Testing Condition 1), hydrogen-charged and tested at room temperature (Testing Condition 2), simulating hydrogen gas storage, and hydrogen-charged and tested at cryogenic temperature (Testing Condition 4), simulating the storage conditions of liquid and cryo-compressed hydrogen. Figure 19: Hydrogen embrittlement factor (HEF) based on the loss of Charpy impact toughness of 316L stainless steel under different processing conditions. Figure 21: Fracture morphologies of the as-built specimen charged with hydrogen and tested at room temperature (testing condition 2) at different magnifications: (**a**) low magnification and (**b**) high magnification. Figure 22: Fracture morphologies of the as-built specimen charged with hydrogen and tested in a liquid nitrogen environment (testing condition 4) at different magnifications: (**a**) low magnification and (**b**) high magnification. The fracture morphologies of the as-built, uncharged specimen (testing condition 1) are shown in Figure 20. At low magnification, a few isolated cleavage facets are apparent, and the fracture morphologies of the HT2 specimen charged with hydrogen and tested in a liquid nitrogen environment (testing condition 4) at different magnifications: (**a**) low magnification and (**b**) high magnification. Figure 23: Fracture morphologies of the HT2 uncharged specimen tested at room temperature (testing condition 1) at different magnifications: (**a**) low magnification and (**b**) high magnification. Figure 17:0 Fracture morphologies of the HT2 specimen charged with hydrogen and tested at room temperature (testing condition 2) at different magnifications: (**a**) low magnification and (**b**) high magnification. Figure 17:1 Fracture morphologies of the HT2 specimen charged with hydrogen and tested in a liquid nitrogen environment (testing condition 4) at different magnifications: (**a**) low magnification and (**b**) high magnification. ent (Figure 20a). The fracture surface exhibits a ductile fracture with homogeneously distributed fine dimples (Figure 20b). The fracture surface of the as-built specimen charged with hydrogen (testing condition 2) also exhibits a ductile fracture but with a lower fraction of dimples and some isolated cleavage facets, as shown in Figure 21b. At low magnification, transgranular cracks are evident and marked by white arrows (Figure 21a). Similar results have been reported in a previous study [30]. Figure 22 presents the fracture surface of the as-built specimen charged with hydrogen and tested in a liquid nitrogen environment (testing condition 4). Despite a loss of ductility, most of the observed area retains a ductile nature, though it exhibits the lowest fraction of fine dimples compared to Conditions 1 and 2. Additionally, some transgranular cracks and unmelted powder are observed, as indicated by white arrows (Figure 22a). This explains the reduction in impact toughness at this condition to 7.5 J. The fracture morphology of the uncharged HT2 specimen (testing condition 1) is shown in Figure 17:2 Evidence of ductile fracture is observed, characterized by fine dimples and some isolated cleavage facets. Additionally, ductile microvoid formation is present. In testing condition 2, where the HT2 specimen was hydrogen-charged and tested at room temperature, transgranular cracks and ductile microvoid formation are observed, as shown in Figure 17:3 Despite the presence of hydrogen, the fracture surface largely retains its ductile nature. While hydrogen exposure generally reduces ductility and increases HE susceptibility to fracture, the applied heat treatment mitigates the effects of hydrogen embrittlement. Subjecting AM 316L stainless steel to a holding temperature of 1050 \({}^{\circ}\)C followed by quenching enhances its resistance to hydrogen embrittlement.