Issue 51

M. Cauwels et alii, Frattura ed Integrità Strutturale, 51 (2020) 449-458; DOI: 10.3221/IGF-ESIS.51.33 452 Hydrogen induced mechanical degradation Fig. 3 shows the engineering stress-strain curves for the samples tested in air (full lines) and for those tested in hydrogen charged condition (dotted lines). Although some experimental variation can be detected on the mechanical data, trends are still reproducible and valid for further evaluation. A possible explanation for the experimental spread is the microstructural nature of heat treated DSS material, while the tested samples were only between 0.600 and 0.700 mm thick, increasing the possibility of having different amounts of austenite at the edges where hydrogen is entering the material. Additionally, because of the quenching procedure after the heat treatment, the tensile specimens were geometrically distorted, influencing the reproducibility of the results. Nevertheless, the averaged results are still reliable with clear differences between both material conditions. Figure 3 : Stress-strain curves of samples tested in air (full lines) and with in-situ hydrogen charging after 1 day of hydrogen pre-charging (dotted line) for both heat treated materials (HT 1190 and HT 1110) For the tensile tests performed in air, the elongation at fracture increased with an increased austenite phase fraction. This is to be expected since austenite is more ductile compared to ferrite. The ultimate tensile strength of both heat treated materials was similar. Hydrogen charging has a clear effect on the tensile behaviour of the material. For both heat treatments, there is an evident loss of ductility, seen in the reduction of the total elongation at fracture. Based on the elongation at fracture in air and in hydrogen charged condition, an embrittlement index (EI) was calculated according to the following formula [25]: elongation in air - elongation when charged EI =  elongation in air (1) This resulted in a value of 43.3% for HT 1190 and 36.5% for HT 1110. This is in agreement with other findings in literature since ferrite is considered more susceptible to hydrogen embrittlement than austenite [18,26–28]. Moreover, Örnek et al. [29] found that DSS microstructures with a large austenite spacing were more sensitive to HE. The spacing of the austenite ribbons was larger in the HT 1990 sample, both on account of it having a lower austenite fraction and a slightly larger grain size, as mentioned in the previous section and as seen in Fig. 2. HT 1190 also had a higher hydrogen content than HT 1110 after one day of hydrogen charging and Zakrocymski et al. [26] found that the severity of HE in DSS increased with higher hydrogen concentrations. Additionally, the diffusion of hydrogen was faster in HT 1190 samples which contributed to its higher sensitivity to hydrogen [30]. The decrease in ductility is significant, even though the sample was not saturated with hydrogen after one day of hydrogen pre-charging and thus hydrogen is not present throughout the entire thickness of the specimen. Additionally, the yield stress of the material increased in hydrogen charged condition. This effect was more pronounced for HT 1190. The increase of the yield strength after hydrogen charging was also observed in other works, such as in [26, 31] for DSS, and in [32, 33] for austenitic stainless steels. The strengthening effect of hydrogen can be attributed