Sigma phase is a chromium-rich intermetallic phase found in stainless steels. It forms when an alloy is cooled slowly through the range from around 1000°C to 550°C.
As these chromium-rich particles form, they denude the surrounding metal of chromium, reducing its resistance to pitting corrosion. They also significantly reduce the impact toughness of the alloy at lower temperatures.
Avoiding sigma phase
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 is an important part of our purchasing specifications, and also forms the basis of industry approvals such as Norsok M-650.
Achieving a fast enough cooling rate becomes more challenging at larger diameters and in thicker section castings. We routinely stock super duplex stainless steels up to 16" (406.4mm) diameter, where production capabilities are greater.
Detecting sigma phase
Sigma phase can be detected with high-magnification microscopy, but it is often easier to test for its effect on the alloy's properties than to observe it directly. Charpy impact toughness is a quick, low-cost test undertaken during final production, alongside an ASTM G48 corrosion test.