Effect of solid-solution temperature on the microstructure and properties of ultra-high-strength ferrium S53® steel

Ferrium S53® is a new secondary hardening ultra-high-strength (UHS) stainless steel, with more Cr than other Co-Ni UHS steels (UHSSs). The solid-solution step is an important step in the heat treatment of Ferrium S53®. This study investigated the effect of solid-solution temperature on the microstructure and mechanical properties of Ferrium S53®. Steel samples were solid-solution-treated at six different temperatures between 935 °C and 1185 °C for one hour. The resulting microstructures were characterized by X-ray diffraction (XRD), optical microscopy, field emission scanning electron microscopy, and transmission electron microscopy. The tensile strength and impact toughness at different solid-solution temperatures were also evaluated, and their relationship with the microstructure was discussed. The secondary phase of Ferrium S53® steel precipitated during stress annealing was M23C6, mostly around 100 nm, having C, Cr, Mo, and W as main components. After one hour of solid-solution treatment at 1035 °C, the steel was entirely austenitized, but up to 1085 °C, the secondary phase was completely dissolved. The increase of the solid-solution temperature increased the retained austenite content and the toughness and led to the dissolution of the secondary phase. The last one is the main reason for the change of strength. The experimental results showed that the most appropriate solid-solution temperature for Ferrium S53® is 1085 °C.