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

Abstract

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 [%].