Impact of Microstructural Variations on Hydrogen Permeation into Duplex Steel

Hydrogen embrittlement remains a significant challenge in steel applications with itsunderlying mechanisms still not fully understood. This study examines the influence ofmicrostructural variations in duplex steel on hydrogen uptake during electrolytic chargingover a duration of four hours. To address this, three distinct microstructural states areanalyzed: initial (coarse-grained), high-pressure torsion (HPT) as-processed, and heattreatedHPT states, with hydrogen penetration affecting depths of approximately 100 μm.In-situ synchrotron cross-sectional X-ray micro-diffraction reveals that, in thenanocrystalline HPT as-processed sample, austenite and ferrite exhibit lattice parameterexpansions of 0.015 and 0.003 Å, respectively. In contrast, the initial (coarse-grained) sampleshows a 0.005 Å increase in austenite, while no detectable change is observed in ferrite. Thepronounced lattice swelling in both phases of the nanocrystalline microstructure isaccompanied by an increase in compressive in-plane stresses of 200 MPa in austenite and850 MPa in ferrite. Furthermore, thermal desorption spectroscopy indicates a hydrogenuptake of 16 ppm in the HPT as-processed state, exceeding the coarse-grained condition by4 ppm. Subsequent heat treatment reduces hydrogen uptake to 4 ppm, yielding a fivefolddecrease 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 andferrite to hydrogen charging are attributed to their respective microstructural characteristics,as revealed by electron microscopy analyses. These findings provide new insights into themicrostructural control of hydrogen transport in duplex steels, with important implicationsfor the design and development of hydrogen-resistant materials.