Secondary hardening is accomplished by the precipitation of fine-scale M₂C alloy carbides in high CoNi steels containing the strong carbide-forming elements Cr and Mo. Thermodynamic stability and coarsening resistance of these carbides depend on the alloy content of these elements. Thermodynamic calculations are performed for the determination of phase diagram, composition of alloying elements M, mole fraction and lattice parameters of M₂C, and volume percent of austenite in AerMet 100 system. And these results are compared with experimental ones and the coarsening theory of rod-type particles.