Sigma Phase 

Preventing σ-Phase Precipitation with Expanite® Treatments. 

Duplex stainless steels contain 2 types of microstructures, austenite and ferrite. Hence the name duplex. In simple terms, the combination of these two phases is what causes duplex alloys to be highly sensitive with respect to the formation of the so-called “Sigma phase” (σ-phase), because the -phase primarily will form in the boundary between austenite and ferrite, growing into the ferrite.

Sigma phase leaves the material in a stage with unfortunate material properties and must be prevented. Sigma phase forms primarily at 800-850C, but is able to form at temperature between 600 and 1000C, and will do so at varying rates, depending on the specific alloy. Please refer to the below TTT (Time-Temperature-Transformation) diagram for SAF2205 and the speed of which foreign phases forms. For example, then chromium nitrides, chi-phase and carbides forms if a part made from SAF2205 is held at 800C for more than 1 min. The critical σ-phase precipitation will form after approximately 16 min.During our tests, Zeron100 SuperDuplex alloy proved worst results as it initiated formation of σ-phase after 3.5 min. at 850C.

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