Sigma Formation

Do you know what elements cause sigma formation? Will 310S or 304H have more resistance to sigma formation?
All of our nickel-bearing stainless and nickel base alloys have an austenitic structure, and are ductile and non-magnetic when they are placed in service. Ideally, a heat resistant alloy should retain these qualities throughout its service life.

Some materials change after a few hundred or thousand hours in service, and become brittle instead of tough and ductile. This usually happens with high chromium, low nickel grades such as 309 and 310. The most common problem is that the alloy forms a hard, brittle nonmagnetic phase, called sigma. The overall chemical composition of the alloy remains the same. Sigma forms in the 1100-1600°F (600-870°C) temperature range. It happens more quickly, and embrittles more severely, when the alloy has been cold worked. Sigma may not seriously harm the alloy while it is operating at high temperature. But enough sigma can completely embrittle the alloy when it reaches room temperature.

Chromium, silicon, molybdenum, columbium, aluminum and titanium promote sigma. Nickel, carbon and nitrogen retard its formation.  Although RA330 might normally be regarded as overkill for a 1200°F (650°C) application, RA330 does not form sigma or embrittle at any temperature range.