[en] It is shown that substantial amounts of untransformed austenite may be obtained in low alloy steels by a combination of alloy modification (addition of silicon) and heat treatment. In silicon modified AISI 4330 steel, the amount of untransformed austenite is varied by utilizing isothermal treatments both above and below the M/sub s/. The stability of the untransformed austenite is found to be dependent on the amount of silicon and also on the tempering temperature. A magnetic saturation technique was utilized to monitor the transformation of the austenite under uniaxial tensile loading. An improvement in the plane strain fracture toughness (K/sub Ic/) was observed when the retained austenite transformed with respect to strain. This improvement was optimized with particular combinations of stability and volume fractions of retained austenite. The addition of silicon resulted in an increase of both the strength and the fracture toughness (K/sub Ic/) of the quenched and tempered AISI 4330 steel. Yield strengths in the range of 200-220 ksi and K/sub Ic/ values as high as 107 ksi√in were obtained. The microstructure was characterized using both optical and electron microscopy and is correlated with the mechanical properties