PSI - Issue 68

Jan Kec et al. / Procedia Structural Integrity 68 (2025) 1091–1097 Jan Kec / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction Hydrogen is considered the fuel of the future due to its potential role in transporting and storing energy from renewable sources, Kevin Topolski et al. (2022). Owing to the relatively wide-range of natural gas pipeline infrastructure, it is likely that these already existing networks would be used to transport hydrogen. Hydrogen is assumed to be blended into natural gas initially before a full transition to hydrogen takes place. It is generally known that hydrogen causes a deterioration of mechanical properties called hydrogen embrittlement (HE), so it is desirable to quantify the degradation of mechanical properties in the hydrogen/natural gas mixture. For certification of piping for hydrogen service, ASME B31.12 requires in-situ testing in hydrogen gas. The required tests are FT, fatigue crack growth rate (FCGR), or determination of the threshold stress intensity factor (SIF) in hydrogen. May L. Martin et al. (2022) investigated FT and FCGR for X70 steel in 10 MPa pure hydrogen. A significant decrease in the FT parameter J Q from the original 675±20 kJ/m 2 to 38±6 kJ/m 2 was found, on the other hand even this decrease still satisfies the minimum required value of K IH ≥ 55 MPa.m 0,5 . The fracture surfaces after FT are characterized by a change in mechanism from microvoid coalescence to transgranular brittle failure. G. Golish et al. (2022) verified the application of pipe and bends by determining threshold SIF in hydrogen, slow strain rate test in hydrogen and CVN test on pre-charged specimens. The decrease in CVN energy at -20°C was approximately 100 J, from the original value of 303 J to 204 J after pre-charging in hydrogen. Some laboratories are able to test low cycle fatigue in hydrogen gas, which is very interesting in terms of the effect of hydrogen on the mechanical properties of piping steels, P.E. Bradley et al. (2023). It is generally known that the in-situ tests recommended by ASME B31.12 are the most reliable and give the most conservative results, as discussed above. Another option is pre-charging specimens electrochemically or pre-charging in hydrogen gas, followed by testing in atmospheric air. A. Zafra et al. (2023) compared FCGR for ex-situ and in-situ testing in hydrogen on 42CrMo4 steel and found that FCGR is faster for samples tested in-situ, even at low SIF range (beginning of the test) values where the hydrogen content should be highest in ex-situ specimens. For ex-situ tests, there is also a decrease in FCGR at higher SIF range values due to a reduction in hydrogen content as a result of desorption. A. Zafra et al. (2020) also studied the change in mechanical properties in ex-situ TT of 42CrMo4 steel at different strain rates. The change in mechanical properties was very slight for cylindrical specimens. On the other hand, a significant decrease in mechanical properties was observed for notched specimens, especially for the low crosshead speed tests. No changes in fracture mechanism were observed in fractographic analyses, only coalescence of microvoids. This paper aims to investigate the mechanical properties of X70 steel and in particular at long pre-charge times. Most of the presented articles in the literature also pre-charge in pure hydrogen, but in the submitted article the pre charging is carried out in a hydrogen and methane blend.

Nomenclature FT

fracture toughness CVN Charpy V-notch TT tensile test SMYS specified minimum yield strength a crack depth t pipe thickness c half-length of crack HE hydrogen embrittlement COD crack opening displacement FCGR fatigue crack growth rate SIF stress intensity factor LSAW longitudinally submerged arc welded pipe OD outer diameter of pipe WT wall thickness of pipe T-L circumferential orientation of specimens