Impact of microstructural variations on hydrogen permeation into duplex steel

Hydrogen embrittlement remains a significant challenge in steel applications with its underlying mechanisms still not fully understood. This study examines the influence of microstructural variations in duplex steel on hydrogen uptake during electrolytic charging over a duration of four hours. To address this, three distinct microstructural states are analyzed: initial (coarse-grained), high-pressure torsion (HPT) as-processed, and heat-treated HPT states, with hydrogen penetration affecting depths of approximately 100 µm. In-situ synchrotron cross-sectional X-ray micro-diffraction reveals that, in the nanocrystalline HPT as-processed sample, austenite and ferrite exhibit lattice parameter expansions of 0.015 and 0.003 Å, respectively. In contrast, the initial (coarse-grained) sample shows a 0.005 Å increase in austenite, while no detectable change is observed in ferrite. The pronounced lattice swelling in both phases of the nanocrystalline microstructure is accompanied by an increase in compressive in-plane stresses of 200 MPa in austenite and 850 MPa in ferrite. Furthermore, thermal desorption spectroscopy indicates a hydrogen uptake of 16 ppm in the HPT as-processed state, exceeding the coarse-grained condition by 4 ppm. Subsequent heat treatment reduces hydrogen uptake to 4 ppm, yielding a fivefold decrease in the variation of the austenite lattice parameter while preserving the ferrite response observed in the as-processed HPT sample. The distinct responses of austenite and ferrite to hydrogen charging are attributed to their respective microstructural characteristics, as revealed by electron microscopy analyses. These findings provide new insights into the microstructural control of hydrogen transport in duplex steels, with important implications for the design and development of hydrogen-resistant materials.