Microstructural evolution during the δ/σ/γ phase transformation of the SUS 309LSi stainless steel after aging under various nitrogen atmospheric ratios

This microstructural evolution of the δ/σ/γ phase transformations in SUS 309LSi stainless steel has been investigated as aged under different nitrogen atmospheric ratios from 0% up to 80%. The results showed that the increment of the nitrogen/air ratio accelerated the δ → γ phase transformation and that of δ → σ was inhibited. There are four precipitation sites of σ-phase at 800 °C, which were δ/γ interface boundary, triple junction, corner, and cellular. On the other hand, the δ/σ phase transformation was the dominant factor that caused the precipitation of σ-phase. The mapping analysis indicated that the δ-ferrite and σ-phase in the SUS 309 stainless steel were Si-rich and Cr-rich phases, respectively. It leaded to the decrease of enrichment of Cr when the nitrogen/air ratio increased. The microstructural observation also exhibited the phase transformations among δ-ferrite, σ-phase and γ-phase were very different under various conditions.