Hydrogen enhanced fatigue crack growth rates in a vintage and a modern X65 pipeline steel

• Hydrogen accelerated fatigue crack growth is shown for modern and vintage X65 steel. • Modern seamless steel exceeded the BS 7910 design FCGR curve at low ΔK in hydrogen. • Proportion of quasi cleavage fracture increased with hydrogen induced acceleration. • ECCI confirmed plastic deformation even in the highly accelerated FCGR regime. Given the potential of hydrogen as an energy carrier in achieving carbon neutrality, assessing the fatigue crack growth rate behaviour of new and vintage pipeline materials exposed to hydrogen for new or repurposing of existing infrastructure is vital. Fatigue crack growth rate (FCGR) curves were established under in-situ electrochemical hydrogen charging at 1 Hz, observing up to 10 times acceleration compared to air. Constant Δ K testing at 11 MPa·m 0.5 and 18 MPa·m 0.5 under varying frequencies (1 Hz, 0.1 Hz, and 0.01 Hz) was performed to investigate the frequency dependence of hydrogen accelerated FCGR. The acceleration factors (AF) were observed to vary significantly, with the modern steel exhibiting an AF of 6.3 at 0.01 Hz and Δ K of 11 MPa·m 0.5 and 22.9 at Δ K of 18 MPa·m 0.5 . The vintage material showed AFs of 1.5 and 30 under the same conditions, respectively. The increase in AF was associated with a larger fraction of quasi-cleavage fracture. Electron channelling contrast imaging revealed evidence of plastic deformation even in the accelerated regime. The findings indicate notable differences in fatigue behaviour influenced by the microstructure and hydrogen environment, consistent with previous research and providing insights into the feasibility of repurposing existing pipelines for hydrogen transport considering existing FCGR design curves.