Sigma phase sometimes occurs in 18-8 type chromium-nickel austenitic stainless steel. When the 18-8 steel contains titanium, niobium, molybdenum, silicon and other elements to form ferrite, because the ferrite is rich in chromium, it promotes the formation of σ phase from the chromium-rich ferrite. The formation of σ phase must have a certain temperature and time.
The σ phase is not found in the 18-8 steel with pure austenite structure, while the σ phase is often found in the as-cast 18-8 titanium-added steel, which may be related to the composition segregation of the casting. That is, the local σ phase, which may be related to the composition segregation of the casting. The ferrite is enriched with more chromium, which makes the σ phase easy to nucleate and grow. The formation of the σ phase leads to the brittleness of the steel, and at the same time reduces the σ phase that can be dissolved in the ferrite at high temperatures. In iron-chromium alloys, the upper limit temperature is about 820°C, so the brittleness caused by the σ phase can pass the temperature above 820°C. Heating or solution treatment to eliminate.
Due to the different composition of steel, the upper limit dissolution temperature of σ phase also changes, so the specific temperature can be determined by experiment. For example, in 1Crl8Ni11Nb steel, the σ phase has begun to dissolve at 800 °C, and the σ phase can disappear when heated at 850 °C.
