Sigma Phase 

Due to the extremely rapid cooling, and the addition of nitrogen, σ-phase precipitation will not be an issue 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.