Microstructure and Mechanical Performance of X120 Linepipe Steel in High-Pressure Hydrogen Gas

Abstract Hydrogen-induced degradation, especially hydrogen-assisted fatigue, and fracture, must be carefully considered to maintain reliability and structural integrity of steel pipelines in gaseous hydrogen service. Additionally, the susceptibility of steels to hydrogen is well documented to be sensitive to the strength of the steel. Therefore, high strength pipeline steels with yield strength greater than 600 MPa, such as X100 and X120, must be evaluated to assess their susceptibility to hydrogen embrittlement. In this work, microstructure and mechanical properties, including the fatigue crack growth rate and fracture toughness in high-pressure hydrogen gas at a pressure of 1000 bar (105 MPa), of an as-received X120 steel were experimentally investigated. The results show the fatigue crack growth rates in gaseous hydrogen of this X120 steel follow the ASME design curves for hydrogen from the Boiler and Pressure Vessel Code Section VIII, Section 3 Code Case 2938-1. The fracture resistance in 1000 bar hydrogen gas is 43 MPa·m1/2, which is higher than approximately 30 MPa·m1/2 reported for Cr-Mo and Ni-Cr-Mo pressure vessel steels with the similar tensile strength level (approximately 950 MPa). Multi-scale metallurgical analyses were conducted to characterize the underlying microstructural features contributing to the higher fracture toughness of X120 compared to the Cr-Mo and Ni-Cr-Mo steels. The characterization shows a fine bainitic microstructure (grain size: 1 μm) and fewer carbides, characteristics created by the thermo-mechanically controlled processing (TMCP) and a lower carbon content, respectively. These features likely provide a higher fracture resistance in gaseous hydrogen compared to the traditional tempered martensitic microstructure of pressure vessel steels with greater carbon contents. A fractography analysis on the tested X120 specimens was also conducted to further reveal its fracture characteristics under high-pressure hydrogen gas. Both fatigue and fracture surfaces exhibit mainly quasi-cleavage cracks. Secondary cracks propagated in a mixed intergranular and transgranular mode. The fracture surface is much coarser and has deeper secondary cracks than that in the fatigue region.