Investigation of secondary phase effect on 2205 DSS fracture toughness

The fracture toughness properties of duplex stainless steels (DSS) are strongly reduced by intermetallic phases, precipitating at 600-1000°C. In this investigation, the embrittlement effect induced by a low content of these phases has been studied in a 2205 DSS, by means of instrumented Charpy V impact testing. Different contents of intermetallic phases have been produced by isothermal treatments at 800-950°C. The microstructure has been analysed by scanning electron microscope/backscattered electron images. The impact test results, the examination of the fracture surfaces and of the cross-sections close to the notch root allows us to elucidate the role of secondary phases contents in reducing impact fracture toughness. When the quantity of secondary phases is <0•5%, these precipitates are rare and insignificant, reducing the absorbed energy by facilitating a ductile fracture. When the content is >1%, the fracture is completely brittle. The plastic deformation at the notch root and in the resisting section provokes a secondary phase breakage with a high density microcrack formation, inducing an easy nucleation and propagation of the final fracture crack. When the content of secondary phases is between 0•5 and 1%, fracture is mixed. Both the sigma σ and chi χ phases have been found to be responsible for the embrittling of the 2205 DSS, and fracture toughness mainly depends on their density and dimension. © 2010 Institute of Materials, Minerals and Mining.