Phase Characterization and Formation Behavior in 6 wt% Si High-silicon Austenitic Stainless Steel during Isothermal Aging

The precipitation behavior of different phases in a high-silicon stainless steel (6 wt% Si) during aging at 600–1050 °C for 24 h was investigated. The morphology, crystal structure and composition of various precipitates were detailly characterized using optical microscopy, scanning electron microscopy and transmission electron microscopy. Four phases were mainly identified: χ -phase, M 6 C carbides, σ phase and a new type of fcc -phase. During aging at 600–900 °C, the main precipitate was (Cr, Mo and Si)-rich χ -phase which was directly precipitated from γ matrix. The χ -phase was calibrated as bcc structure with a lattice parameter of 8.90 Å. The peak temperature for the precipitation of χ -phase was 800 °C, and it was dissolved when aging at temperatures above 1000 °C. The σ -phase was observed only at 700 °C and grew next to χ -phase. Above 700 °C, a new fcc -phase was found to be precipitated along with χ -phase, with a space group of Fd3c and a lattice parameter of 12.56 Å. The M 6 C carbides started to be precipitated at 700 °C in the vicinity of χ -phase. And its amount basically increased with the increasing of temperature. An orientation relationship between M 6 C/ γ was found: [100]c//[100] γ , (001)c//(001) γ , i.e., the cube-on-cube relationship.