Modeling of local hydrogen concentration on microscopic scale to characterize the influence of stress states and non-metallic inclusions in pipeline steels

Abstract

Hydrogen as an energy carrier plays an important role in achieving the ambitious climate targets associated with decarbonization. The main challenge is to ensure safe transport of gaseous respectively liquid hydrogen through the already existing natural gas pipelines. Since experimental methods, such as hydrogen-induced-cracking tests, fail to provide quantitative answers to the question of which local effects/mechanisms trigger failure under hydrogen loading, complex numerical multi-scale approaches are needed. In this work, a phenomenological crystal plasticity model based on dislocation slip mechanisms was implemented in Abaqus/Implicit and coupled with a hydrogen diffusion model to investigate the influence of microstructural characteristics in terms of grain size, texture, phase fraction but also lattice defects such as dislocations, grain boundaries, carbides, and non-metallic inclusions on hydrogen-induced-damage behavior. As a driving force for hydrogen diffusion, both hydrostatic stresses and plastic strains associated with mechanically and thermally induced residual stresses under various stress states were investigated. It was found that the impurity degree of the steel in terms of non-metallic inclusions plays an important role in hydrogen susceptibility in two aspects, namely geometrically (notch-effect) and in the generation of residual stresses. The effect of matrix shrinkage on the inclusions and the resulting stress field after cooling leads to the accumulation of hydrogen atoms around the inclusions, resulting in locally high critical concentrations. The simulations performed on microstructures with and without non-metallic inclusions under different stress states demonstrated the notched effect that the presence of non-metallic inclusions increases the local hydrogen concentration up to 28 %.