<p>The effect of temperature-time parameters of isothermal hardening in the bainitic transformation range on the thermal stability of retained austenite and impact toughness of sparingly alloyed structural steels 38KhS (38CrSi) and 60S2 (60Si2) is studied. The amount of retained austenite in the steels is determined by a magnetometric method. The fine structure of foils is investigated. Impact bending tests of samples are carried out. It is shown that the thermal stability of the thin foils of retained austenite at liquid nitrogen temperature correlates with the impact toughness of the steels after the isothermal hardening. A magnetic method for assessing the impact toughness of an isothermally hardened structural steel is suggested.</p>