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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2.2. Methods SSRT tests were conducted in laboratory air and under in-situ electrochemical charging conditions at room temperature in a 0.1 M Na2SO4 electrolyte with a constant potential of -1050 mV SCE and with the constant cross head speeds with g nominal strain rates of 2.5x10 -4 1/s and 1x10 -6 1/s, respectively. Fractographic examinations were performed on all ruptured specimens using a Quanta 650 ESEM (Thermo Fisher Scientific., USA) scanning electron microscope (SEM) operated at an acceleration voltage of 15-20 kV. 3. Results 3.1. Slow Strain Rate Tensile Testing Results from tensile testing on all positions for all pipeline steels carried out both in air and under in-situ electrochemical hydrogen charging are reported in in Table 2. Overall, the stress-strain response obtained from tests in air shows a notable decrease in elongation-to-failure with decreasing strain rate for the two steels tested at the additional strain rate. In all steels, the yield and maximum stress have not been particularly affected by the exposure to hydrogen. Also, the Lüders bands, which has been observed in Materials A and C, does not seem to be particularly affected by hydrogen. On the other hand, the elongation-to-failure showed a remarkable reduction, albeit to different degrees. The susceptibility of different materials to hydrogen has been quantitatively assessed through two parameters: the Embrittlement Index (EI) (Takasawa et al. 2012; Moro et al. 2010) is defined as: 100 air H air RA RA EI RA − =  where the RA air and RA H are the reduced area at fracture from tests in air and in hydrogen environment. For comparison, the EI values are reported for all four pipeline steels in Figure 4. The results show that the samples from position 2 systematically have the highest EI values for each steel; of all, Materials B and C stand out as the two materials revealing the overall highest and lowest hydrogen embrittlement susceptibility based on the EI value. For this reason, two more tests in air were carried out at the nominal strain rate of 1x10 -6 1/s - the same as it was in electrochemically charged hydrogen - for Materials B and C specimens extracted from position 2. For these materials, nominal stress vs. strain plots are summarized in Figure 3. In Table 2, two EI values are reported for the tests performed on the specimens extracted from Materials B and C pipelines; here the values calculated by accounting for the reduction of area obtained from the test in air at 1x10 -6 1/s are marked in blue.

Figure 3: Nominal stress vs. strain curves obtained in air at conventional and slow strain rate in air and in hydrogen from specimens extracted from position 2 from a) Material B and b) Material C.