There is a gradually increasing awareness of the deleterious effects of grain-refining precipitates on the Charpy V-notch impact toughness of high-strength steels, and recognition of these effects resulted in the development of a general process that provides substantial improvements in the ductile fracture resistance of tempered mar-tensite. The results of this investigation indicate that the processing-induced refinement of grain-refining precipitates also provides significant improvements in the lower-shelf and transition toughness of 3310, 8219 and 4340 steels tempered at 180‡C and 4340 steel tempered at 593‡C. In contrast to the upper-shelf regime, where the application of the process produces a change in either the mode or character of unstable fracture at constant strength and austenite grain size, the refinement of grain-refining precipitates is associated with an increased amount of ductile fracture and an increased resistance to the initiation of unstable crack propagation at any given temperature in the lower-shelf regime.