A laser-polished subsurface microstructural barrier mitigates hydrogen embrittlement in high-pressure hydrogen environments

Hydrogen embrittlement (HE) is a critical bottleneck that limits the safe transport of hydrogen through existing steel pipelines and hinders the widespread use of hydrogen as a clean energy carrier. This work demonstrates that laser polishing (LP) can significantly improve the HE resistance of X80 pipeline steel in a high-pressure H₂ environment, primarily by constructing a subsurface microstructural barrier rather than merely smoothing the surface. By systematically adjusting the laser power (6–20 W) and the number of processing passes, an optimal heat input window (10 W) was identified, wherein the microstructure of the modified layer was optimized. This resulted in a gradient transition layer consisting of ultrafine nanograins near the surface, lath-shaped nanograins, and an underlying heat-affected zone (HAZ), along with a synergistic increase in dislocation density and interface/trap density. The more uniform grain-size transition caused by multiple passes effectively reduced the risk of interfacial hydrogen embrittlement due to the mismatch in physical properties between the modified layer and substrate. This artificially constructed subsurface barrier effectively reduces hydrogen diffusion and permeation, as indicated by the lowest apparent diffusion coefficient (D_app) observed at 10 W, which further decreases with multiple passes. Slow-strain-rate tensile tests conducted in high-pressure H₂ showed a significant reduction in elongation loss (I_δ) and an increase in absorbed energy, both closely linked to microstructural changes. Fractographic analysis revealed a shift in fracture mode toward a more ductile behavior, with enhanced local plasticity, consistent with the HELP mechanism. Notably, the best HE resistance was achieved even with the highest surface roughness, indicating that HE mitigation depends more on subsurface barrier integrity than surface topography alone. This work offers mechanistic understanding and a practical surface-engineering approach for developing hydrogen-resistant pipeline steels, with direct benefits for safely and economically converting existing infrastructure for hydrogen transport.