Hydrogen effects on ultrafine-grained steels processed by high-pressure torsion

This article reviews the interactions between hydrogen and lattice defects produced by high-pressure torsion (HPT) process in ferritic iron and austenitic steels, and it shows the direction of development of hydrogen-resistant high-strength steels. In ferritic iron, grain boundaries as well as dislocations participate in hydrogen trapping. For austenitic steels, the contribution of grain boundaries to hydrogen trapping is negligible, while short-circuit diffusion occurs through them. Hydrogen trapping on precipitates caused by annealing after HPT processing depends on the affinity of hydrogen with constituting elements. In metastable austenitic steels, the presence of hydrogen suppresses deformationinduced martensitic transformation through enhanced slip planarity. If hydrogen-containing austenite transforms to martensite, excess hydrogen corresponding to a difference in the solubility between both phases is generated and diffuses out of the martensite, resulting in hydrogen concentration at the interphase. Deformation-induced martensite generated from ultrafine-grained austenite is used to simultaneously enhance strength and ductility without suffering from hydrogen embrittlement.