Fracture Mode Transitions in Aged Ultra-High Strength 18Ni (350) Maraging Steel

The influence of test temperature on the blunt notch and precracked fracture toughness behavior of aged 18Ni (350) maraging steel has been examined. Both toughness measurements exhibit a decrease with decreasing test temperature. The tough-to-brittle transition observed in the precracked fracture toughness samples is similar to that previously reported for the tensile ductility of this alloy and can be associated with a gradual change in fracture mode, from microvoid coalescence at elevated temperatures to quasi-cleavage at low temperatures. This fracture mode transition is suppressed to lower temperatures in the blunt notch samples. Here the fracture behavior is complicated by the presence of a shear failure region immediately below the notch root. It is suggested that it is the development of this shear failure region which controls the fracture resistance of blunt notched aged 18Ni (350) maraging steel. Apparently shear initiation requires the attainment of a critical strain at the surface of the notch root, with this strain decreasing with decreasing test temperature.