PSI - Issue 42

Aleksander Omholt Myhre et al. / Procedia Structural Integrity 42 (2022) 935–942 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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and hydrogen environment are presented in Figure 5a) and b) for the base metals of Material B and Material C, respectively. In air, both materials displayed a typical cup-and-cone-type fracture, where the degree of necking is more prominent in Material C. In contrast to the dimpled fracture surface observed in air, the fracture surface from tests in hydrogen reveal the facets representing the typical quasi-cleavage (QC) fracture features on the outer edge of the specimens, while the remaining centre areas were dominated by dimples like those in air, as shown in the magnified SEM images of the fracture surfaces in Figure 5b.

a

hydrogen

air

b

air

hydrogen

Figure 5: Lateral and top view of the fracture surfaces obtained from SSRT testing in air and under in-situ electrochemical hydrogen charging of the base metals extracted from position 2 of a) Material B and b) Material C. An evident difference between Material B and Material C lies in the cross-section shape after testing in air. While the latter still present a perfectly circular fracture surface, the former clearly feature an oval fracture surface shape. This is indicative of an inhomogeneous deformation and stress flow, which can be related to the presence of elongated grains and pearlite bands appearing in the rolling direction. This difference seems to have mostly disappeared as a result of hydrogen, as can be seen in the subfigures marked in blue in Figure 5. 4. Discussion As seen in Figure 3 and Table 2, there is no significant, observable effect of electrochemical hydrogen charging on the materials tensile properties until the onset of localised plasticity, in particular for Materials B and C, which were tested in both air and hydrogen at the same strain rate. This is consistent with the previous research (Moro et al. 2010; Lee et al. 2011). Measured EI values indicate that position 2 was the overall most negatively impacted by hydrogen uptake. This can be associated to the comparatively larger grain size featured the mid-section of the pipe, as reported in Figure 1. On the other hand, the grain size refinement has been associated with the increased resistance to hydrogen embrittlement (Park, Kang, and Liu 2017; Takasawa et al. 2012). Coherently to the aforementioned observations, Material C exhibits a higher resistance to hydrogen embrittlement than the vintage material, and Material B performed the overall worst. Since Material C represents a more recent steel variant, its higher cleanliness and the lower amount of inclusions and other impurities, which may act as preferred crack initiation sites and/or constitute stress concentrators causing a