σ-Phase Formation in Super Austenitic Stainless Steel During Directional Solidification and Subsequent Phase Transformations

The solidification path and the σ-phase precipitation mechanism in the S31254 (UNS designation) steel are investigated thanks to Quenching during Directional Solidification (QDS) experiments accompanied by scanning electron microscopy observations and electron backscattered diffraction (EBSD) analysis. Considering experimental conditions, the γ-austenite is found to be the primary solidifying phase (1430 °C), followed by δ-ferrite (1400 °C, ≈ 87 pct solid fraction). The σ-phase appears in the solid-state through the eutectoid decomposition of the δ-ferrite: δ → σ + γ2 (1210 °C), whereas the σ-phase is predicted to form from the austenite at 1096 °C in equilibrium conditions. The resulting temperatures of solidification path and phase transformation are compared with Gulliver–Scheil model and equilibrium calculations predicted using Thermo-Calc© software. It is shown that the thermodynamics calculations agree with experimental results of solidification path. The EBSD analysis show that the δ-ferrite has δNW2 ORs with the σ-phase