Mitigating hydrogen embrittlement via film-like retained austenite in 2 GPa direct-quenched and partitioned martensitic steels

Advanced (ultra)high-strength steels that utilise bcc-fcc microstructures are appealing solutions for producing a combination of high strength and deformability. However, they are also susceptible to hydrogen embrittlement (HE). As larger less stable retained austenite (RA) can impair mechanical performance, its size and morphology are critical factors for achieving and maintaining the desired properties. Here, we present a combined experimental–density functional theory (DFT) study on HE with medium-carbon direct-quenched and partitioned (DQ&P) martensitic steels with varying vol% and film-thickness of RA, showing significantly improved HE resistance as a function of bcc-enrichment and increasing RA film-thickness. DFT reveals low attraction of hydrogen in bcc-Fe with Al, implying a stronger push towards fcc. Furthermore, the solubility of hydrogen increases dramatically with the carbon-enrichment of fcc-Fe when hydrogen and carbon are second neighbours. The theoretical results explain the observed differences in hydrogen diffusion, trap density, and the consecutively lower HE in Al-DQ&P over Si-DQ&P. Our combined experimental and theoretical study thus highlights the important interplay of bcc and fcc phases and H-uptake within austenite-containing carbon steels. •Improving HE resistance as a function of RA film-thickness and bcc-alloying with Al.•Insights into the interplay of bcc/fcc and H-uptake for austenite-containing carbon-steels.•Outstanding performance of Al-DQ&P with thickest RA-films under in situ hydrogen-charged SSRT tests.•DFT reveals H-solubility in bcc-Fe lower with Al-alloying over Si-alloying.•H-solubility in fcc increases significantly when H and C are second neighbours.