The effect of step quenching on the microstructure and fracture toughness of aisi 4340 steel

The microstructure and fracture toughness of AISI 4340 steel in the direct and in the step quenched and tempered condition has been studied. Austenitizing temperatures of 1473 K followed by step quenching to either 1373 or 1143 K prior to oil quenching have been employed. A consistent drop in the fracture toughness values was observed as the intermediate holding temperature decreased or the holding time at this temperature in-creased. A concurrent increase in the amount of twinning was seen without any change in the amount and/or distribution of retained austenite. While direct evidence for segre-gation has not been found, the observed facts are consistent with segregation effects during the austenitizing treatment.