On the sensitivity of fracture mechanism to stress concentration configuration in a two-step quenching and partitioning steel

The effect of stress concentration geometry on the fracture behavior and fracture mechanisms of a two-step quenching and partitioning steel consisting of tempered martensite and retained austenite (RA) is investigated using double edge notched tension (DENT) specimens with different notch root radii. The initial notch opening is varied from a fatigue pre-crack to 15 mm radius. The fracture surface consists in flat triangular regions and slant zones. The fracture mode evolves from intergranular brittle fracture to ductile fracture with dimples. The ductile fracture process further changes from internal necking coalescence to a void sheet shear type coalescence when increasing the notch root radius. A similar transition of the fracture behavior is observed along the ligament of the DENT specimen. This change in the fracture mode results from a decrease of the stress triaxiality. The essential work of fracture is equal to 40kJ/m2. This is small when looking at the large value of the product of ultimate tensile strength and total elongation (∼25,000MPa·%). The small fracture toughness is ascribed to an intergranular fracture mode, which results from the distribution of blocky RA islands along the boundaries of martensitic packets and from the large stress concentration near the notch tip. 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, and to avoid the relatively low toughness and more brittle failure observed when stress triaxiality increases.