Microstructure dependence of impact toughness in duplex stainless steels

Abstract

In this study, the microstructure dependence of impact toughness was studied for a 2205 duplex stainless steel in the temperature range of −196 to 25 °C. Three markedly different austenite morphologies (i.e., rolled (R), equiaxed (E) and Widmanstätten (W)) were produced through different thermomechanical routes. It was found that while the room temperature impact toughness of all microstructures were quite similar, the microstructure dependence of impact toughness significantly increased with decreasing testing temperature. At cryogenic temperatures, microstructure R showed significantly higher toughness compared to microstructures E and W. Considering a 40 J criterion, the ductile to brittle transition temperature was estimated to be ∼ −80 °C for microstructures W and E, while microstructure R showed impact toughness values higher than 40 J even at −196 °C. The lamellar character of microstructure R and the termination of ferrite phase on the (100) plane orientation in this microstructure were found to have a positive effect on the toughness. The occurrence of deformation twinning within ferrite at low temperatures, facilitated by significantly coarser grain sizes in microstructures E and W compared to R, appeared to be the main reason behind the observed deterioration of the impact toughness of the former microstructures at these temperatures.