The microstructure, precipitation and mechanical properties of Ferrium S53 steel, a secondary hardening ultrahigh-strength stainless steel with 10% Cr developed by QuesTek Innovations LLC, upon tempering were studied by scanning electron microscopy（SEM）, transmission electron microscopy（TEM）, X-ray diffraction（XRD）, and tensile and impact tests. Based on these results, the influence of the tempering temperature on the microstructure and properties was discussed. The results show that decomposition occurred when the retained austenite was tempered above 440℃ and that the hardening peak at 482℃ was caused by the joint strengthening of the precipitates and martensite transformation. Due to the high Cr content, the trigonal M7C3 carbide precipitated when the steel was tempered at 400℃, and M7C3 and M2C（5–10 nm in size） coexisted when it was tempered at 482℃. When the steel was tempered at 630℃,M2C and M23C6 carbides precipitated, and the sizes were greater than 50 nm and 500 nm, respectively,but no M7C3 carbide formed. When the tempering temperature was above 540℃, austenitization and large-size precipitates were the main factors affecting the strength and toughness.
