The effect of hydrogen content and yield strength on the distribution of hydrogen in steel: a diffusion coupled micromechanical FEM study

In this study, we investigate the effect of the heterogeneous micromechanical stress fields resulting from the grain-scale anisotropy on the redistribution of hydrogen using a diffusion coupled crystal plasticity model. A representative volume element with periodic boundary conditions was used to model a synthetic microstructure. The effect of tensile loading, initial hydrogen content and yield strength on the redistribution of lattice (C_L) and dislocation trapped (C_x) hydrogen was studied. It was found that the heterogeneous micromechanical stress fields resulted in the accumulation of both populations primarily at the grain boundaries. This shows that in addition to the well-known grain boundary trapping, the interplay of the heterogeneous micromechanical hydrostatic stresses and plastic strains contribute to the accumulation of hydrogen at the grain boundaries. Higher yield strength reduced the amount of C_x due to the resulting lower plastic deformation levels. On the other side, the resulting higher hydrostatic stresses increased the depletion of C_L from the compressive regions and its diffusion toward the tensile ones. These regions with increased C_L are expected to be potential damage initiation zones. This aligns with the observations that high-strength steels are more susceptible to hydrogen embrittlement than those with lower-strength.