PSI - Issue 68

Birhan Sefer et al. / Procedia Structural Integrity 68 (2025) 1129–1139 Sefer et al. / Structural Integrity Procedia 00 (2025) 000–000

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In contrast, the specimens tested at high temperature (Figure 3b) for both gases, present sponge-like fracture appearance. This observation indicates that the SiMo51 material has experienced tremendous change at high temperature which also affected notably the properties (see Table 3). Interestingly, for the specimen at high temperature in H 2 vertical cracks were also identified. As previously mentioned, these cracks are considered to have been developed because of hoop stress due to pressurising the hole with 200 bar hydrogen. In addition, it is also considered that the sponge-like structure seen for the specimens tested in H 2 at high temperature might also be accompanied with high temperature hydrogen attack, where the carbon has reacted with hydrogen gas and resulted in methane gas formation Poorhaydari (2021). Further study is required to investigate this hypothesis. Overall, the fractographic analysis of the SiMo51 material at room and high temperature confirmed that this material is sensitive to hydrogen embrittlement under the testing conditions. Figure 4 shows SEM-micrographs taken on the fractured surfaces of the CGI specimens in similar manner as those presented for the SiMo51 in Figure 3. The micrographs at room temperature (Figure 4a) show that both specimens tested in Ar and H 2 follow similar type of brittle fracture mode including mixture of cleavage and quasi-cleavage decohesion. In contrary to this, at high temperature the CGI material exhibited ductile type of fracture surface characterized with dimples as result of micro void coalescence (Figure 4b) for both specimens tested in Ar and H 2 , respectively. This observation agrees with the higher relative elongation to leakage estimated for the SSRT tests at high temperature as compared to the one at room temperature.

Figure 4. Representative fracture surfaces taken from SSRT specimens tested in argon (left) and hydrogen (right) at a) room and b) high temperature for CGI material. Higher magnification micrographs taken in the area marked with numbers in the white boxes are given below the overview micrographs.