Control of hydrogen-induced failure in metastable austenite by grain size refinement

To promote the use of metastable austenite for use in hydrogen structures, grain refinement is used. Fe-16Cr-10Ni is a metastable austenitic alloy whose grain size can be controlled by conventional thermo-mechanical processing. With a minimum grain size just below 1 µm, the material was exposed to hydrogen gas at different pressures and tensile tests were performed. SEM/EBSD in-situ tensile tests were also performed. Hydrogen affected all grain sizes, with evidence of intergranular and transgranular fracture at higher hydrogen contents in the larger grains. Smaller grains retained a high yield stress with good elongation, while the largest grains were most embrittled. Cracking was preceded with significant phase transformation and occurred by microvoid coalescence in fine grains and along grain boundaries in coarse grains (with traces of ductility), without indication of failure of austenite-martensite interfaces.