Effect of Methane-Hydrogen Mixed Gas on Fatigue Crack Growth Characteristics of API X65 Linepipe Steels

Abstract The pipeline system is positioned as a major infrastructure which is indispensable for mass transportation of gaseous hydrogen. As an initial challenge for full utilization of hydrogen, a trial was conducted to examine the control of carbon dioxide emissions in the existing natural gas infrastructure using a mixed gas containing 5 % to 20 % added hydrogen gas. The amount of hydrogen uptake in steel from a high-pressure hydrogen gas environment is expressed by a predetermined relationship with the gas pressure according to Sievert’s law; that is, the amount of hydrogen uptake increases as the gas pressure increases. Based on this relationship, the effect of hydrogen embrittlement is considered to vary with the partial pressure of hydrogen gas. In order to examine the application of steel to hydrogen gas or mixed gas infrastructure, it is important to accurately grasp the effects of each factor and obtain experimental data under the actual environment. However, the influence of hydrogen in mixed gas environment has not been clarified from the viewpoint of material structure and gas pressure. In this report, the fatigue crack growth characteristics of steel in hydrogen environments were obtained by changing the gas pressure up to 25 MPa in a mixed gas environment of methane and 20 % hydrogen using an actual X65 linepipe steel. Under high gas pressure condition and high ΔK region, pure hydrogen gas and mixed gas give almost the same results. On the other hand, it was found that the effect of hydrogen was more severe in the mixed gas environment under low gas pressure condition and low ΔK region.