Objective One of the most efficient and economical approaches for storing and transporting hydrogen energy is to blend hydrogen at a certain ratio into the existing natural gas pipeline network for large-scale long-distance transmission. However, the exposure of pipeline steel to hydrogen environments can lead to hydrogen embrittlement, which undermines its mechanical properties and poses potential risks to pipeline safety. Consequently, it is crucial to evaluate the mechanical properties of pipeline steel used for hydrogen-enriched compressed natural gas (HCNG) transmission and understand the influence rules of hydrogen blending ratios on the steel's susceptibility to hydrogen embrittlement.

Methods Slow strain rate tensile experiments were conducted with in-situ hydrogen charging under high-pressure gas-phase conditions using a self-designed experimental system adhering to the requirements of Standard Test Method for Determination of Susceptibility of Metals to Embrittlement in Hydrogen Containing Environments at High Pressure, High Temperature, or Both (ASTM G142-2016). Additionally, quasi-static fracture toughness experiments with in-situ hydrogen charging under high-pressure gas-phase conditions were performed using an Instron 8801 servo hydraulic fatigue test system. These experiments investigated the hydrogen embrittlement behavior of X65 pipeline steel, considering varying hydrogen pressure conditions. Moreover, the influence of different hydrogen blending ratios on the mechanical properties of the pipeline steel was analyzed, taking into account fracture morphology characteristics.

Results The X65 pipeline steel exhibited a decrease in total percentage elongation after fracture by up to 11.5% when hydrogen was charged at 3%, 5%, and 10% under a total pressure of 9 MPa, compared to hydrogen-free environments. The fracture toughness decreased by 23.5% and 43.1%, respectively, when hydrogen was charged at 3% and 10%. The severity of hydrogen embrittlement increased with higher hydrogen ratios. Concerning fracture morphology, specimens with hydrogen ratios below 10% displayed ductile fractures without typical brittle fracture morphology.

Conclusion To ensure the safe operation of X65 pipelines, particularly those that have been in service for several years, it is recommended to conduct evaluations and analyses of the mechanical properties, focusing on the steel's susceptibility to hydrogen embrittlement before introducing HCNG transmission.