What is sigma phase in duplex stainless steel? Stainless Steels

Sigma phase is a chromium-rich intermetallic phase found in stainless steels, that forms when an alloy is cooled slowly through the range from around 1000degC to 550degC. The formation of these chromium-rich particles denudes the surrounding metal of chromium, thereby reducing its resistance to pitting corrosion. In addition, these particles significantly reduce the impact toughness of the alloy at lower temperatures.

The formation of sigma phase can be avoided during initial production by solution annealing at a temperature above the formation range, followed by a rapid quench with minimal delay. Careful control of heat treatment and quenching conditions is an important part of Langley Alloys purchasing specifications, and also forms the basis for industry approvals such as Norsok M-650. Achieving a fast enough cooling rate can become more challenging at larger diameters, and for thicker section castings. Therefore, Langley Alloys routinely stocks super duplex stainless steels up to 16” (406.4mm) diameter, where production capabilities are greater.

Sigma phase can be detected with high magnification microscopy. However, it is often easier to test for the effect of sigma phase on an alloys property rather than try to observe it. Therefore, Charpy impact toughness is a quick and low-cost test undertaken during final production, along with an ASTM G48 corrosion test.

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Langley Alloys experience of super duplex stainless steels is unrivalled, holding the most complete product range as solid bars up to 16” (406.4mm) diameter, plate up to 3” (76.2mm) in thickness, plus pipes and fittings.