Ultrafine-grained dual-phase maraging steel with high strength and excellent cryogenic toughness

A novel heat treatment route consisting of a low-temperature solution followed by an over-aging treatment at 500 degrees C is proposed to develop a high-strength, high-cryogenic-toughness maraging steel by forming an ultrafine-grained martensite (alpha') and austenite (gamma) dual-phase microstructure. Compared to the same maraging steel subjected to the conventional heat treatment with a mostly martensitic microstructure, the present dual-phase microstructure offers a remarkable increase by 12 times of cryogenic impact energy (similar to 140 J at 77 K), while the yield strength is not reduced obviously. A large amount of ultrafine-grained austenite (about 50% volume fraction) are formed in the present steel due to the reversed transformation of martensite to austenite during the over-aging process at 500 degrees C. It is surprised that the present steel with such a high fraction of austenite still possesses a high yield strength comparable to the conventional maraging steel with a mostly martensitic microstructure. Intensive nanoprecipitates are found not only in martensite but also in austenite phases, indicating both phases have high strength. This is confirmed by nanoindentation test, showing similar hardness values in both martensite and austenite phases. Such intensive nanoprecipitates in both phases ensure the high yield strength of the present steel. The excellent cryogenic toughness of the present steel mainly origins from: (i) the pronounced amount of reversed austenite that are intrinsically tough due to their face-centered cubic (fcc) structure; (ii) transformation-induced plasticity (TRIP) toughening as some austenite grains transforming to marten-site during impact test; (iii) the ultrafine-grained structure of both martensite and austenite phases. The present heat treatment route offers a potential solution for processing large engineering components for cryogenic application that require a long heat treatment duration to achieving uniform mechanical properties in the components. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

A novel heat treatment route is proposed to develop a high-strength, high-cryogenic-toughness maraging steel with an ultrafine-grained martensite and austenite dual-phase microstructure. This dual-phase microstructure significantly increases the cryogenic impact energy while maintaining high yield strength. The steel with a large volume fraction of ultrafine-grained austenite exhibits comparable yield strength to conventional maraging steel, thanks to intensive nanoprecipitates in both martensite and austenite phases.