Strain-Controlled Low-Cycle Fatigue Testing and Seismic Assessment of X65 Pipeline Steel Under Cathodic Hydrogen Charging for Hydrogen Transport

Abstract This study investigated the strain-controlled low-cycle fatigue properties of X65 pipeline steel under cathodic hydrogen charging to assess the seismic resistance of hydrogen pipelines. In Japan, natural gas pipelines are designed to withstand a strain of 3% during earthquakes, a requirement based on the ASME fatigue design curve for steel and seismic wave velocity. To evaluate the seismic resistance of hydrogen pipelines according to these design criteria, low-cycle fatigue testing in a hydrogen environment is essential to determine whether the number of cycles to failure at a strain amplitude of 3% exceeds the design life based on the ASME fatigue design curve. However, the low-cycle fatigue properties of pipeline steels in hydrogen environments have not been fully elucidated. This study aimed to determine whether the allowable strain for natural gas pipelines under seismic conditions is also applicable to hydrogen pipelines. Pd-plated X65 pipeline steel specimens were exposed to hydrogen gas at a pressure of 7 MPa. Hydrogen content was measured using thermal desorption analysis. Cathodic hydrogen charging was performed under conditions where the hydrogen content was higher than that of the specimens exposed to hydrogen gas at a pressure of 7 MPa, and fatigue testing was conducted during ongoing charging. The results revealed that hydrogen adversely affects the low-cycle fatigue life of X65 steel. However, at a strain amplitude of 3% and a strain rate of 0.06 %·s−1 under cathodic hydrogen charging, the number of cycles to failure exceeded the design life based on the ASME fatigue design curve. Additionally, the influence of strain rate on fatigue life was examined, showing that the number of cycles to failure decreased as strain rate decreased, both in air and under cathodic hydrogen charging. This finding indicates that a strain rate of 0.06 %·s−1, which is lower than the strain rate expected during an earthquake, represents a conservative condition for low-cycle fatigue testing. From these results, the allowable strain (3%) for seismic resistance in natural gas pipelines, established using the ASME fatigue design curve and seismic waveforms, is also applicable to hydrogen pipelines made of X65 steel with an internal pressure of at least 7 MPa. This study contributes to the safety assessment of hydrogen pipelines subjected to seismic motion.