Optimized Design of Ultra-High Strength Stainless Steel Based on Thermo-Calc Calculation

The thermodynamic calculation software Thermo-Calc was used to study the influences of alloy elements Cr, Co, and Mo on the precipitated phases of a S280 ultrahigh strength stainless steel at the temperatures ranging from 400 °C to 1200 °C. The results showed that the precipitated phases in the steel were mainly composed of M23C6 carbide, M6C carbide, intermetallic compound of Laves phase and σ phase. The temperature and the content change of alloying elements Cr, Co and Mo have little effect on the precipitation of M23C6 carbide, while have great effect on the precipitation of Laves phase and σ phase. By lowering the element content and adjusting the heat treatment temperature, the precipitation of σ phase in the alloy can be reduced．According to the results of thermodynamic simulation, after optimizing the chemical composition, the Ф300mm bar was trial-produced. The chemical composition, microstructure and mechanical properties of the alloy were analyzed and tested. The results show that, after tempering at 550 °C, the main precipitated phases in the S280 alloy were M23C6 carbide and intermetallic compound of Laves phase. The characterized nano-precipitates have good agreement with simulation results. The optimized design of S280 steel reached a tensile strength over 1930 MPa and a fracture toughness of 90 MPa•m1/2.