Influence of ageing treatment on microstructural and mechanical properties of a solution treated UNS S32750/EN 1.4410/F53 Super Duplex Stainless Steel (SDSS) alloy

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In this present study, the influence of isothermal ageing temperature and ageing duration on microstructural 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) microscopy, tensile and, impact testing techniques. Isothermal ageing treatments, performed at temperatures between 400°C and 600°C and, treatment durations of 3h and, 120h, were applied to a commercial UNS S32750 SDSS alloy. Microstructural characteristics, such as: distribution and morphology of constituent phases, distribution maps of grain's modal orientation (MO) and, obtained mechanical properties were analyzed. The applied isothermal ageing treatments did not induce any significant changes in relation to phase's morphology, grain-size and, MO distribution. Obtained results show that the microstructure consists of elongated γ-phase grains within δ-phase matrix, showing a grain-size close to (45–48) μm in the case of δ-phase, (22–24) μm in the case of γ-phase and, (1.6–2.1) μm in the case of γ2-phase. Significant changes were noticed in the case of mechanical properties (ultimate tensile strength, 0.2 yield strength and, elongation to fracture) due to precipitation of secondary phases. The most influential secondary phase is the R-phase, observed in the case of isothermal ageing treatment performed at 600°C with a treatment duration of 120h (A3.2 state). The A3.2 state, in comparison with solution treated (ST) state, showed a reduction in elongation to fracture of approx. 62% and, an increase in ultimate tensile strength and, respectively, 0.2 yield strength of approx. 24% and, respectively, approx. 16%.