Characterization and Control of the Compromise Between Tensile Properties and Fracture Toughness in a Quenched and Partitioned Steel

The enhancement of the fracture toughness is essential for opening the possible range of applications of advanced high-strength steels, while the focus in the literature is primarily on the strength–ductility compromise. A high fracture toughness is indeed needed for energy absorbing components as well as to limit edge cracking sensitivity during part forming. This study investigates the tensile properties and the fracture toughness of various quenched and partitioned microstructures. The fracture resistance is evaluated using double-edge notched tension tests. While the uniform elongation continuously increases with the retained austenite (RA) fraction, the fracture toughness shows a maximum at intermediate RA content. For the highest amount of RA, the relatively low fracture toughness is mainly attributed to the formation of brittle necklace of fresh blocky martensite in the fracture process zone due to a high stress triaxiality, inducing an intergranular fracture mode. For intermediate RA fraction, the RA morphology evolves from blocky to film type, leading to a transition from intergranular to ductile fracture mode, and the RA-to-martensite transformation contributes to a higher total work of fracture compared to tempered martensitic steel. A proper control of both the amount and morphology of RA during microstructure design is thus essential to generate the best compromise between tensile properties and fracture toughness.