In welded pipeline steels, hydrogen embrittlement (HE) is governed by microstructure and hydrogen trapping, and our results indicate that the microstructural modifications in the heat-affected zone (HAZ), produced by controlled high-frequency induction welding, enhance resistance to HE compared to the base metal. Using advanced microscopy, thermal desorption spectroscopy (TDS), fracture toughness testing in hydrogen, and density functional theory (DFT), we explain the mechanism. Faster cooling in HAZ refines ferrite–bainite laths and generates off-stoichiometric cementite (carbon-deficient), implying carbon vacancies. TDS reveals that, unlike the base metal (BM) which releases most hydrogen at low temperatures, the HAZ releases a larger fraction at higher temperatures (≈850 °C), evidencing deeper traps and slower effective diffusion. DFT shows hydrogen is unfavorable in regular Fe 3 C interstitials but binds strongly at vacancies, with additional stabilization near α-Fe/Fe 3 C interfaces and in vacancy clusters (capacity ≈3–5H per vacancy), explaining the origin of high-temperature TDS peak in HAZ. Fracture-toughness tests confirm the HAZ is more resistant to hydrogen-assisted cracking than the BM; both satisfy ASME B31.12 acceptance criteria
Safyari et al. (Mon,) studied this question.