Elucidating the role of secondary cryogenic treatment on mechanical properties of a martensitic ultra-high strength stainless steel

Cryogenic treatment (CT) has been proved to be a critical process of the iron-based alloys. A unique series of processes including two steps of CTs are utilized for the treatment of Ferrium S53 ultra-high strength stainless steel. In the current work, the role of the secondary cryogenic treatment on the mechanical properties and corresponding microstructure of Ferrium S53 ultra-high strength stainless steel was investigated. The results indicate that, compared with the single cryogenically treated secondary aged samples (SC-SAT), a superior combination of strength and impact toughness is achieved in double cryogenically treated secondary aged samples (DC-SAT). More specifically, the yield strength significantly increases from 1315 +/- 3.7 MPa to 1569 +/- 5.0 MPa with the improvement of both ultimate strength and impact toughness. The synergistic enhancement of strength and toughness of DC-SAT samples via secondary cryogenic treatment is primarily ascribed to the refinement of martensitic hierarchical structures and M2C precipitates as well as elimination of metastable reversed austenite. The recovery and annihilation of dislocations are remedied by secondary cryogenic treatment, which provides more preferential nucleation sites for the secondary strengthening particles. Additionally, stronger precipitation hardening effect contributes to the superior yield strength of DC-SAT samples.