Sigma Phase(σ)

in 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 σ-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.