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

摘要

The solidification path and the sigma-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 gamma-austenite is found to be the primary solidifying phase (1430 degrees C), followed by delta-ferrite (1400 degrees C, approximate to 87 pct solid fraction). The sigma-phase appears in the solid-state through the eutectoid decomposition of the delta-ferrite: delta -> sigma + gamma(2) (1210 degrees C), whereas the sigma-phase is predicted to form from the austenite at 1096 degrees 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 (c) software. It is shown that the thermodynamics calculations agree with experimental results of solidification path. The EBSD analysis show that the delta-ferrite has delta NW2 ORs with the sigma-phase.