What is sigma phase in duplex stainless steel?

Sigma phase is a chromium-rich intermetallic phase found in stainless steel. It 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.

How Does the Sigma Phase Form?

The formation of the 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 Alloy’s purchasing specifications. 

Sigma phase 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 the sigma phase on an alloy’s 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.

Get in Touch for More Information

Langley Alloy’s experience of super duplex stainless steel is unrivalled. We hold the most complete product range as solid bars up to 16” (406.4mm) in diameter, super duplex plates up to 3” (76.2mm) in thickness, as well as pipes and fittings.

For further information about the sigma phase in duplex stainless steel, please contact a member of our team today.