Abstract: Influence of Co addition on the microstructure and mechanical properties of maraging stainless steel was studied by using numerical simulation, microstructure characterization and performance test. According to the thermodynamic and kinetic calculation with Thermo-Calc software, the results show that with addition of 6wt%Co,the precipitation temperature of ε-Cu phase rises and the incubation time becomes longer, and under the same aging condition the average radius of ε-Cu precipitates decreases. The optical microscopy (OM) and transmission electron microscopy (TEM) analysis results show that for the Co-containing tested steel after solution treatment and aging, the ultra-low carbon lath martensite is refined obviously, the dispersed ε-Cu phase size becomes smaller and the precipitation amount increases gradually, and the film-like inverted austenite increases in amount and in size. Owing to the dislocation strengthening of lath martensite, the precipitation strengthening of ε-Cu phase and the toughening effect of the inverted austenite phase, the strength, hardness, elongation and impact absorbed energy of the tested Co-containing steel reach 1350 MPa, 433 HV0.5, 16% and 75 J, respectively.

Key words: maraging stainless steel, cobalt alloying, Thermo-Calc software, ε-Cu phase