Microstructure Evolution During the ΔΣΓ Phase Transformation of the SUS 309lSI Stainless Steel After Aging

by Dong Yih Lin, Hung Chih Hsieh, Tao Chih Chang

Publisher - I-Shou University, Kaohsiung, Taiwan

Category - General Novel

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 positions 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 the σ-phase in the 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 observations also exhibited the phase transformations among δ ferrite, σ-phase and γ-phase were very different under various conditions.

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