Three magnetic hard alloys 22Kh15K, 25Kh15KYuBF and 30Kh5K of the Fe-Cr-Co system are used to study the kinetics of σ phase formation after quenching from 1300–1100°C and tempering in the temperature range 700–800°C by measuring HV300 hardness, X-ray diffraction, and metallographic analysis. The main σ-forming element in these alloys is shown to be cobalt. The rate of formation of σ-phase during tempering is maximal at 750°C.