The processes of formation of carbide-free bainite and the effect of temperature-time parameters of isothermal hardening in the range of bainite transformation on the structure and impact strength of low alloyed structural steels have been studied. Based on the identified patterns of changes in the properties of retained austenite and the impact resistance of steels, a method for non-destructive testing of the impact toughness of medium-carbon steels heat-treated by isothermal hardening has been developed.