The effect of aging on impact toughness and fracture surface fractal dimension in SAF 2507 super duplex stainless steel

The relation between room temperature impact toughness and fractal behavior of the fracture surfaces of SAF 2507 super duplex stainless steel, aged between 0 and 288 h at 475 °C, has been studied. Fractography was performed in a scanning electron microscope and the fractal dimension was determined according to the slit island method. A monotonical decrease in fractal dimension was observed as the time of aging increases from 0 to 288 h. For the entire range of aging times, a complete transition from ductile to brittle behavior was observed as the impact toughness decreases from 284 J (0-24 h) to 43 J (288 h). Ductile fracture occurred by nucleation growth and coalescence of micro-dimples, while the brittle behavior was characterized by the propagation of cleavage cracks in ferrite, controlled by plasticity of the surrounding austenite. Partially based on several previous concepts and relationships well established for brittle materials, a direct linear correlation between impact toughness and fractal dimensional increment has been developed. The concept of critical volume of material is proposed and tentatively related with the unstable crack propagation event, through both, the so called critical length (microstructurally significant distance) and the size of the largest micro-dimple and cleavage facet (structural parameter) developed in ductile and brittle fracture respectively.