This paper describes the fracture toughness and microstructure relations for ultra-high-strength, low-alloy (UHSLA) steels corresponding to AISI 4340 composition. The steels were either cast or forged, heat treated at austenitizing temperatures from 840 to 1100°C, oil quenched and tempered at 180 and 650°C. Fracture toughness, tensile, and Charpy impact toughness tests, and testing of apparent fracture toughness under the influence of notch root radius, were performed simultaneously. Investigation of microstructures and corresponding mechanical properties, as well as fractographic examinations and evaluation of chemical heterogeneity, made it possible to elucidate the alteration of fracture behavior.

It was found that cast low-tempered steel has higher fracture toughness (K_Ic) than forged steels, though its conventional mechanical properties are lower. Increasing K_Ic caused by the higher austenitizing temperature of forged steel was also observed. The coarsened prior austenite grains and decreased grain boundary cohesive strength are mainly responsible for the intergranular crack initiation and the related modification of the crack path, the consequence of which may be a change in the effective stress intensity factor.

The proposed model, based on assumed average probable crack path angle deflection controlled by the grain boundary, has yielded satisfactory agreement with experimental results. The mechanical properties of the more heterogeneous cast steel can be elucidated in the same sense. It has been concluded that there exists some uncertainty in the determination of “valid” plane strain fracture toughness, particularly in the case of ultra-high-strength steels, along with their more pronounced macro/microstructure heterogeneity. Due to this effect, the recommended procedure of K_Ic measurement can provide undesirable overestimation of the material quality being tested.