Difference in hydrogen trapping behaviors between epsilon carbide and cementite in steels

The hydrogen trapping sites associated with epsilon carbide (ε-carbide) and cementite precipitates in high‑silicon martensitic steels tempered at different temperatures were investigated by direct observation using atom probe tomography (APT) combined with a deuterium charging method. Charged deuterium was obviously trapped within film-like ε-carbide precipitates in steel tempered at 400 °C. In contrast, no charged deuterium was observed in fine spheroidal cementite precipitates in steel tempered at 500 °C. However, in undeformed pearlitic steel, deuterium was weakly segregated at/near the lamellar cementite/ferrite interface. First-principles calculations have predicted that both ε-carbide and cementite have stable sites for hydrogen within their carbides, but diffusion barriers for hydrogen are too high to reach these sites. APT analysis indicated that the undeformed cementite (Fe₃C) has a stoichiometric carbon composition of 25 at. % without carbon vacancies, whereas ε-carbide has much lower carbon concentrations than the stoichiometric composition (Fe₂C), indicating high concentrations of carbon vacancies in ε-carbide. The high concentrations of carbon vacancies increased stable sites in ε-carbide and facilitated hydrogen diffusion by providing low-barrier diffusion pathways, enhancing its hydrogen-trapping ability. The difference in hydrogen trapping behavior between the two carbides is therefore attributed to the presence or absence of low-barrier diffusion pathways in the carbides, driven by the high concentrations of carbon vacancies.