The effect of microstructure on the fracture toughness of 300 and 350 grade maraging steels

The fracture toughness of 300 and 350 grade maraging steel that has been given simple heat treatments has been studied. By heating into the austenite region at well-controlled rates and holding for short times, the toughness can be increased significantly above that which is obtained for the standard treatment. It is shown that the toughness increase is due to plastic deformation of the austenite for single-cycled material. When multiple cycles are used, the 300 grade steel undergoes a strain-induced transformation at the crack tip that contributes significantly to the toughness. When singly cycled, 300 grade material is tested at ?196°C, its properties approach those obtained for conventionally treated material at room temperature. Models for the toughness improvements are developed that take crack tip phenomena into account, and they are in good agreement with experiment.