The precipitation of sigma (σ) phase in austenitic stainless steels is a significant subject of many investigations. Exposure of these steels to elevated temperatures (from 600 °C to 900 °C) results in precipitation of σ phase (transformation product from delta (δ) ferrite) which is one of the main reasons for the deterioration austenitic stainless steels properties. It is therefore of considerable interest to study the conditions for σ phase formation, as well as the mechanism of the transformation. This review paper presents overview of precipitation characteristics (including morphologies and precipitation sites) as well as effect of σ phase on properties of austenitic stainless steels. It is particularly highlighted that the precipitation of hard, brittle and nonmagnetic σ phase occurring preferentially at δ/γ phase boundary and within δ ferrite islands (high Cr concentrated region) cause worse mechanical properties (impact toughness and elongation) and corrosion resistance.
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