Influence of isochronal treatments on microstructure and mechanical properties of solution treated UNS S32750 SDSS alloy specimens

In this study, the influence of isochronal treatments performed at temperatures between 700 °C and 1060 °C and, with a treatment duration of 15 min, on microstructure and mechanical properties of a solution treated UNS S32750 Super Duplex Stainless Steel (SDSS) alloy was investigated by SEM-EBSD (Scanning Electron Microscopy-Electron Backscatter Diffraction), tensile testing and, impact testing techniques. Microstructural characteristics, such as: constituent phases, weight fraction, grain-size, morphology and, obtained mechanical properties, such as: absorbed energy, ultimate tensile strength, yield strength and, elongation to fracture were analysed. The isochronal treatments induced changes in relation to alloy’s constituent phases morphology, grain-size and, precipitation of secondary phases. Obtained results show that the microstructure consists of elongated γ-phase grains, due to prior hot-deformation, in a δ-phase matrix. Also, within the δ-phase matrix, secondary phases can precipitate during isochronal treatments. The following secondary phases were identified: γ2-phase, σ-phase and, χ-phase. Changes were observed, also, in recorded mechanical properties, due to precipitation of deleterious secondary phases. The most influential precipitated secondary phase is the σ-phase, which was observed in samples treated at temperatures between 780 °C and 1000 °C and, induced decrease in alloy’s absorbed energy and elongation to fracture, both reaching minimum values, close to 3.3 [J] and, respectively 9 [%].