The evolution of microstructures and mechanical properties with tempering temperature of a novel 2.5 GPa grade ultra-high strength steel with synergistic precipitation strengthening was investigated. With increasing tempering temperature, the experimental steel initially progressed from ε-carbides to M₃C and then to M₂C, followed by further coarsening of the M₂C carbides and β-NiAl. Concurrently, the martensite matrix gradually decomposed and austenitized. The ultimate tensile strength and yield strength initially increased and subsequently decreased with rising tempering temperature, reaching peak value at 460 and 470 °C, respectively. Conversely, the ductility and toughness initially decreased and then increased with rising tempering temperature, reaching a minimum at 440 °C. The increase in strength was attributed to the secondary hardening effects resulting from carbide evolution and the precipitation of β-NiAl. The subsequent decrease in strength was due to the recovery of martensite and coarsening of precipitates. The decrease in ductility and toughness was linked to the precipitation of M₃C, while their subsequent increase was primarily attributed to the dissolution of M₃C and an increase in the volume fraction of reverted austenite. The high dislocation density of martensite, the film of reverted austenite, nanoscale M₂C carbides, and ultrafine β-NiAl obtained during tempering at 480 °C resulted in the optimal mechanical properties of the experimental steel. The strength contributions from M₂C carbides and β-NiAl were 1081 and 597 MPa, respectively.

Synergistic strengthening of nano−scaled M₂C and β−NiAl has become a new route to develop ultra−high secondary−hardening steel. At present, the effect of Co on the synergistic precipitation behavior of duplex phases of M₂C and β−NiAl has been rarely reported. This paper revealed the effects of Co on the mechanical properties and duplex precipitates of M₂C and β−NiAl in a novel 2.5 GPa ultra−high strength secondary−hardening steel. The tensile tests indicated that a 10% Co−alloy steel achieved a much stronger secondary−hardening effects compared to a Co−free steel during aging process, especially in the early−aging state. Needle−shaped M₂C and spherical β−NiAl particles were observed in both Co−alloy and Co−free steels. However, the number density, and volume fraction of M₂C were significantly enhanced in the 10% Co−alloy steel. The Mo contents in M₂C carbide and α−Fe after aging treatment were both analyzed through experimental determination and thermodynamic calculation, and the results indicated that Co decreased the solubility of Mo in α−Fe, thus promoting the precipitation of Mo−rich carbides.