Effects of microstructure banding on hydrogen assisted fatigue crack growth in X65 pipeline steels

•Banded X65 pipeline steel was tested under fatigue in high pressure hydrogen gas.•Fatigue crack growth rates were accelerated in hydrogen as compared to tests in air.•Orientation-dependent fatigue crack growth rates observed in air and hydrogen gas.•Cracks grown perpendicular to banded microstructure exhibited slower growth rates. Banded ferrite–pearlite X65 pipeline steel was tested in high pressure hydrogen gas to evaluate the effects of oriented pearlite on hydrogen assisted fatigue crack growth. Test specimens were oriented in the steel pipe such that cracks propagated either parallel or perpendicular to the banded pearlite. The ferrite–pearlite microstructure exhibited orientation dependent behavior in which fatigue crack growth rates were significantly lower for cracks oriented perpendicular to the banded pearlite compared to cracks oriented parallel to the bands. The reduction of hydrogen assisted fatigue crack growth across the banded pearlite is attributed to a combination of crack-tip branching and impeded hydrogen diffusion across the banded pearlite.