Influence of microstructure on the 2507 duplex stainless steel toughness

Duplex stainless steels are often appreciated in high-demanding applications for the excellent combination of mechanical and corrosion properties. Nevertheless, the complexity of its chemical composition requires that the maximum service temperature be lower than about 300°C. Such characteristics can be obtained through a proper manufacturing cycle, including a heat treatment able to prevent the formation of detrimental second phases and give these steels a 50%-50% ferrite–austenite microstructure. Their ferrite matrix makes duplex stainless steels susceptible to brittle fracture. Nevertheless, the presence of austenite islands improves their toughness. The toughening mechanisms make these steels similar to composite materials, whose properties strongly depend on the disposition and the size of the reinforcing particles or fibres. This paper investigates the toughness of a 2507 duplex stainless steel by Charpy V-notched impact and J-Integral tests by varying the samples orientation. Moreover, the effect of the loading rate will be investigated by comparing the transition curves obtained by impact and J-integral tests. The results demonstrated that a high austenite length-to-spacing ratio is beneficial for the material toughness. Regarding the loading rate, no clear effect was observed, even though further tests are necessary to confirm the small differences among the brittle-to-ductile transition curves determined by means of impact and J-Integral tests.