Abstract: Hydrogen traps influence hydrogen embrittlement (HE) in steel by regulating hydrogen diffusion and enrichment. This study provides an in-depth examination of the HE mechanism in bainitic steel with varying aluminum content by designing and optimizing grain size, dislocation density, and retained austenite (RA) stability. Hydrogen charging increases dislocation density, leading to a higher local hydrogen concentration. Simultaneously, it reduces the stability of RA, making it more susceptible to martensitic transformation. The interaction of these factors significantly increases the HE susceptibility of the steel, transforming the fracture morphology from ductile fracture to a combination of intergranular and quasi-cleavage fractures. The HE susceptibility of the steel decreases with increasing Al concentration, which is attributed to grain refinement, reduced dislocation density, and enhanced RA stability. The refinement of prior austenite grains and the reduction in bainitic ferrite lath thickness significantly increase the density of phase boundaries, facilitating uniform hydrogen trapping and suppressing grain boundary hydrogen enrichment. Meanwhile, the reduction in dislocation density decreases the temporary retention of hydrogen in reversible traps, preventing its re-release and further diffusion. Additionally, highly stable RA effectively mitigates hydrogen redistribution caused by stress-induced phase transformation. These combined effects reduce the hydrogen diffusion coefficient of Al-0.6 steel by approximately 41 % compared to Al-0 steel, improving HE resistance by about 24 % and resulting in a more tortuous crack propagation path. However, at low temperatures, the sharp decline in RA stability weakens this advantage, leading to only a slight 2 % improvement in the HE resistance of Al-0.6 steel. In summary, compared to optimizing dislocation density and grain size, RA stability is the key factor in regulating HE sensitivity.

Key words: Bainitic steel, Hydrogen traps, Hydrogen embrittlement, Crack propagation path