Precipitation mechanism of σ phase in S32654 super austenitic stainless steel

Abstract

By utilizing High-Temperature Confocal Laser Scanning Microscopy (HT-CLSM), the microstructure evolution of S32654 super austenitic stainless steel was observed in situ during both solidification and solid-state phase transition. Moreover, the predominant focus was on investigating the precipitation mechanism of the σ phase. The findings revealed that the initial stages of the solidification process involved the nucleation and growth of austenite, followed by a gradual reduction in the residual liquid phase between dendrites until complete solidification was achieved. Furthermore, as the temperature decreased, the composition of the austenite formed from the residual liquid phase differed from the initially formed austenite, ultimately leading to the formation of granular austenite through extrusion. Below the critical temperature, the excessive amounts of Cr and Mo could not be dissolved in the supersaturated austenite between dendrites, thereby resulting in the instantaneous eutectoid decomposition of austenite, namely γ → γ2 + σ.