Revealing the activation energies for H desorption from spherical semi-coherent and incoherent V4C3 precipitates in martensitic medium-Mn steel

Abstract

We investigated H desorption activation energies (E_H) associated with semi-coherent and incoherent V carbides in martensitic medium-Mn steels, as well as the effect of these precipitates on hydrogen embrittlement (HE) resistance. The V-added specimens contained numerous spherical V₄C₃ precipitates with an average C/V ratio of 0.6. These precipitates exhibited size-dependent interface characteristics: fine precipitates smaller than 30 nm formed semi-coherent interfaces with a misfit strain of 0.07, following the Baker-Nutting orientation relationship, whereas larger precipitates exceeding 30 nm exhibited incoherent interfaces. Thermal desorption analysis (TDA) combined with ab initio simulations identified four distinct H desorption peaks. Peak 1 (75 °C) was associated with coherent V₄C₃ interfaces and other reversible trap sites. Peak 2 (100 °C, E_H = 46.4 kJ/mol) was attributed to H trapped in elastic fields surrounding semi-coherent V₄C₃ interfaces, while Peak 3 (520 °C, E_H = 67.7 kJ/mol) originated from elastic fields around incoherent V₄C₃ interfaces. Peak 4 (631 °C, E_H = 124.3 kJ/mol) corresponds to C vacancies within V₄C₃ precipitates. Using Oriani’s equilibrium theory applied to a spherical precipitate model considering precipitate volume fraction and H binding energy, the calculated H trapping fraction in incoherent precipitates was approximately twice that in semi-coherent precipitates. This trend is consistent with the TDA results, where the H content associated with Peak 3 was about twice that of Peak 2. The V-added specimens exhibited ductile fracture, whereas the V-free specimen showed brittle fracture, indicating that V₄C₃ precipitates act as effective H trapping sites and enhance HE resistance in martensitic steel.