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 delta/sigma/gamma 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 delta -> gamma phase transformation and that of delta -> sigma was inhibited. There are four precipitation sites of sigma-phase at 800 degrees C, which were delta/gamma interface boundary, triple junction, corner, and cellular. On the other hand, the delta/sigma phase transformation was the dominant factor that caused the precipitation of a-phase. The mapping analysis indicated that the delta-ferrite and sigma-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 delta-ferrite, sigma-phase and gamma-phase were very different under various conditions. (c) 2007 Elsevier B.V. All rights reserved.