Evaluation of Fracture Toughness of Welded Line Pipe Steels for Hydrogen Gas Transportation Based on ASME B31.12 and Alternative Test Method Development

Abstract A high-strength line pipe steel grade, namely API X65, has been developed and qualified for its use under pressurized hydrogen gas following the ASME B31.12 code. The bolt-load compact specimens taken from the base metal, heat-affected zone, and weld seam of the electric resistance welded (ERW) pipes were tested using a constant displacement method in 100% hydrogen at pressure up to 13.8 MPa (2000 psi) for 1000 hours. All fracture toughness testing was conducted at RINA Consulting – Centro Sviluppo Materiali SpA and EWI Hydrogen Environment Testing Lab, using approximately 50 specimens from different heats produced at Evraz North America (ENA). At the fracture surface of the tested specimens, no significant crack extension was observed; the crack extension beyond the fatigue pre-crack was smaller than 0.25 mm after preloading and hydrogen exposure. Scanning electron microscopy (SEM) fractographs revealed a dimpled fracture surface in front of the fatigue pre-crack, i.e. no hydrogen-associated brittle fracture feature was noted. Based on these results, the minimum value of the threshold stress intensity factor, KIH, was calculated to be 70–80 MPa√m for all the specimens, i.e., fulfilling the qualification criteria (KIH > 55 MPa√m) described in the code ASME B31.12 Option B. The lessons learned during the qualification testing, ongoing efforts to develop alternative testing methods, including elastic-plastic fracture testing (i.e., ASTM E1820) and implications for the future will be discussed.