The structure and mechanical properties of low- and medium-carbon steels quenched and tempered with different isothermal holding times at temperatures ranging from 250 to 650 °C are studied. It is shown that, for the steels under study, three stages can be distinguished on the curves of hardness as a function of isothermal holding at different temperatures. Hardness testing is used to determine tempering conditions leading to a comparable level of hardness. To calculate the tempering parameter, the constant C in the Hollomon-Jaffe equation is determined for the steels in question.