Numerical analysis of microstructural characteristics on the hydrogen susceptibility of pipeline steels

Hydrogen-induced degradation challenges the safe operation of pipeline steels. However, the quantitative influence of complex microstructural features on hydrogen transport and mechanical response is not well understood. This study presents a two-way coupled crystal plasticity–hydrogen diffusion framework for analyzing the hydrogen susceptibility of two API X70 pipeline steel grades with different microstructures: ferritic–pearlitic (=X70-T14) and predominantly bainitic (=X70-T19). Statistically representative three-dimensional microstructure models were generated using EBSD-based data enhanced by a Wasserstein generative adversarial network to capture realistic morphological and crystallographic distributions. The models were calibrated using macroscopic tensile tests and phase-specific macro-indentation data. The hydrogen transport, trapping, and degradation were evaluated under various stress states using a fracture indicator parameter that accounts for HELP (hydrogen-enhanced localized plasticity) and HEDE (hydrogen-enhanced decohesion) mechanisms. The results demonstrate that bainitic X70-T19 steel accumulates higher plastic strain, stress triaxiality, and hydrogen concentration. This leads to increased hydrogen susceptibility under uniaxial loading. In contrast, X70-T14 steel is more vulnerable under plane strain and biaxial tension. Additional parametric case studies reveal that non-metallic inclusions, voids, grain size, phase fraction, and banded pearlite-structures significantly influence hydrogen kinetics and local damage initiation. The proposed framework establishes a data-driven foundation for microstructure design toward hydrogen-resistant pipeline steels. • At low stress triaxiality, both X70 grades exhibit similar hydrogen resistance. • Ferritic-pearlitic X70 shows higher FIP values at elevated stress triaxialities. • Inclusions, voids and phase fraction control hydrogen localization. • HELP/HEDE coupling accelerates hydrogen-assisted damage evolution.