MICROSTRUCTURAL EVOLUTION AND MECHANICAL PROPERTY─FRACTAL BEHAVIOR RELATIONS OF AN AGED SUPER DUPLEX STAINLESS STEEL

Abstract

We studied the relation between the mechanical properties and the fractal dimensional increment for 40 V–notch Charpy and 16 round tensile samples of SAF 2507 super duplex stainless steel, aged between 0 and 288 h at 475 °C, and broken at room temperature. A variety of techniques such as optical microscopy, scanning electron microscopy and image analysis were used to study the microstructure and the nature of the fracture surfaces, whereas the values of the fractal dimensional increment [(DT)* for tension and (DI)* for impact], were determined using the slit island method. The relation between impact toughness I, and (DI)* was of a linear positive type and the impact fracture surfaces showed a transition from ductile to brittle behavior for increased aging times. As the impact fracture surfaces become brittle, numerous cleavage cracks nucleate in the embrittled ferrite, but their propagation is controlled by the austenite, which remains ductile. Tension tests reveal that the strength of the material increases as the time of aging increases while ductility slightly decreases. Tension fracture surfaces were of the ductile type, and it was impossible to distinguish one from the other on a qualitative basis. Then, specification of (DT)* can be successfully used to discriminate between very similar fracture surface morphologies which correspond to different values of strength and ductility, both, in SAF 2507 as well as in many other metallic alloys.