Study of phase transformations in cast UNS S32760 SDDS and their effects on corrosion and mechanical properties.

Due to their exceptional strength and corrosion resistance, with a PREN greater than 40, super duplex stainless steels (SDSS) are widely used to manufacture large valves and pipelines in offshore and desalination installations. In such applications, long-lasting and low-intensity cast metallic components are considered a viable option. However, in the temperature range where the precipitation of harmful phases is critical, cast components are particularly susceptible to slow cooling rates. This study investigates the mechanisms and kinetics of secondary phase formation in UNS S32760 super duplex stainless steel subjected to isothermal ageing at temperatures ranging between 850 °C and 950 °C for 3 to 210 minutes. Among the possible secondary phases, the sigma (σ) and chi (χ) phases were identified as the main contributors to the instability of the duplex microstructure. Increasing the ageing time led to a higher percentage of secondary phase precipitation. The kinetics and morphology of the secondary phases are strongly influenced by variations in isothermal annealing temperature. Because these aspects have a significant impact on corrosion properties, it was found that the finer the structure of the intermetallic phases, the larger the sensitized surface area. Consequently, at the same volume fraction of the secondary phases, a different degree of sensitization can be observed. This behavior was confirmed through DLEPR (Double Loop Electrochemical Potentiodynamic Reactivation) , CPT (Critical Pitting Temperature) and PDP (Potentiodynamic Polarization) corrosion tests. In addition, the detrimental influence of these secondary phases on mechanical integrity was assessed together with micro-, macro-, and nano-indentation measurements and charpy impact testing. All isothermally aged samples were characterized using SEM-BSE, while a selection of representative samples was further examined using SEM-EBSD and SEM-EDS for detailed phase & compositional analysis and quantification of phases was also done using ImageJ software.