Enhancement of the strength-toughness balance in a quenched laser-additively-manufactured low alloy mild steel: Effect of grain refinement and nanotwin bundle formation

Abstract

A laser additively manufactured (LAM) ferrite-austenite low-alloy mild 12CrNi2 has a low ultimate tensile strength but a larger elongation. To improve the strength but without compromising the toughness, a concept to martensitize the low alloy mild steel with introducing nanotwin bundles into the martensitic laths is proposed, through deliberately-designed cyclic quenching (CQ). It is found that the prior austenitic grain refinement helps to form a fine-grained martensite (α′) constituted of the laths with the formation of {112}<111> type nanotwin bundles. The steel with such a nanotwinned martensite structure, in comparison to the steel counterpart with a dislocated lath martensitic structure obtained through conventional direct quenching (DQ), is enhanced not only in ultimate tensile strength, but also in elongation. The mechanism for the nanotwin bundles formation is interpreted in connection with the grain-refinement-induced decrease in martensitic transformation starting temperature (Ms).