On the Precipitation of Sigma and Chi Phases in a Cast Super Duplex Stainless Steel

Super duplex stainless steels are prone to the formation of undesirable secondary phases, particularly when exposed to slow cooling rates. In the current study, the mechanisms of formation of secondary phases have been characterized using a combination of optical microscopy, scanning electron microscopy, and transmission electron microscopy, on samples aged at temperatures ranging between 700 °C and 1000 °C. Among the possible secondary phases, χ-phase and σ-phase were detected and identified as the main phases responsible for the destabilization of the duplex microstructure. The kinetics of both phases were assessed. In particular, the χ-phase forms prior to σ-phase, until it starts to convert into σ-phase. Finally, the TTT curves for the stable σ-phase were calculated. We found that the precipitation of the σ-phase starts as early as 30 seconds at 950 °C.