The mechanical properties of X70 steel pre-charged in blend of hydrogen and methan

This paper deals with the mechanical properties of X70 steel pre-charged in blend of hydrogen and methane. Specifically, the fracture toughness (FT) test, Charpy V-notch (CVN) impact test and tensile test (TT) were evaluated. The pre-charging was carried out at room temperature, 100 bar pressure and for 3000 and 6000 hours. The specimens for FT and TT were pre-loaded to the corresponding operational conditions and, on the other hand, the specimens for CVN remained in the unloaded state during pre-charging in the blend. The TT specimens were pre-loaded to 67% of the specified minimum yield strength (SMYS), corresponding to the operating pressure of 100 bar. The CT specimens for FT were initially fatigue-cracked and then bolt-loaded to a stress intensity factor K Iapp = 84,5 MPa.m 0,5 , which corresponds to internal semi-elliptical crack with a depth of a = 10 mm (0,46.t) and surface length of 2c = 60 mm (2,79.t) loaded at 100 bar. The decrease in fracture parameters was noticeable, whether it was the CVN results or the FT test results. The decrease in average CVN energy values was 30 J for circumferential specimens and 20 J for longitudinal specimens. The decrease in the size-independent fracture mechanical parameters J ic and J 0.2 was 50% and 40%, respectively. A decrease of 22% was observed for the size-dependent parameter J u(20) .