We present the results of a micromechanical model of the influence of hydrogen on fracture in martensitic steels. In this model we take hydrogen diffusion explicitly into account, such that local influences of the hydrogen concentration on plasticity and fracture can be studied. The martensitic microstructure is represented by prior austenite grains that are subdivided into packets. This model is applied to different boundary conditions, mimicking the standardized NACE-A and NACE-D tests, respectively.
Based on these models, we can predict the influences of microstructure and mechanical and chemical loading conditions on the macroscopic flow behavior and quantities like strength and fracture toughness. The agreement of the model results with experimental data is favorable, such that the model is exploited to reveal the mechanisms of hydrogen embrittlement in the studied microstructures. It seems important to understand that in such heterogeneous microstructures local stresses and hydrogen concentrations can exceed their macroscopic average values by one order of magnitude. Hence, local processes under the most severe conditions will dominate the damage and failure behavior on the material. In this context the detrimental effect of hydrogen on the cohesive properties of grain boundaries appears to be the dominant mechanism of hydrogen embrittlement in martensitic steels. Based on these findings of our numerical simulations and their comparison to experimental results published in the literature, we propose a model for hydrogen embrittlement in martensitic and ferritic steels that describes the influence of hydrogen on plastic flow behavior and fracture.