Hydrogen diffusion and trapping mechanisms have been revisited in martensitic steels with a large variability of microstructural parameters (γ-phase, inclusions, precipitate state, dislocation density, solute content, stress states…). An accurate electrochemical permeation set-up associated with thermal desorption spectroscopy and elastic theoretical calculations was methodically used to find a relationship between physical parameters associated with diffusion and trapping and microstructure evolutions. The different site of hydrogen and associated energy were identified and discussed in relation with microstructural features. Additionally, we used the microstructural variability of the tempered martensitic steels studied to illustrate and discuss the different trapping mechanisms in term of the relative hydrogen concentration. Using FEM calculations and a new design of permeation testing under tensile loading until fracture we revisit the question of hydrogen embrittlement (HE) of martensitic steel. More precisely, we explore the impact of mobile and trapped hydrogen on ductile and brittle fracture of martensitic steel using local approach of fracture and a specific analyse of the defect evolution under hydrogen flux (vacancies and dislocations). We discussed damage conditions in relation with mechanical state using tensile samples with different notch for two conditions: hydrogen precharging samples and sample strengthening under hydrogen flux. We highlight that in first conditions, trapped hydrogen promotes ductile fracture and in opposite, the second condition demonstrates that the mobile hydrogen leads to a quasi-cleavage fracture mode at the scale of martensitic laths. Finally, the contribution of plasticity and hydrostatic state on damage is discussed using local approach of fracture (SEM localization of damage and FEM calculation of hydrogen and mechanical states) in term of hydrostatic stress versus Von Mises equivalent plastic strain diagram. Both contributions are finally questioned in relation with the different HE models.
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