Sigma phase

From GAWDAwiki

Sigma phase is an iron-chromium (roughly 50-50) phase that can form from ferrite in austenitic stainless steel weld metal at elevated temperatures (approximately between 450°C and 850°C). It therefore forms during high temperature service and its presence in the stainless steel is only apparent at room temperature, during plant shutdown, when the stigmatized material becomes extremely brittle and will crack readily with low toughness.^[1]

References

1. http://www.bocindustrial.co.uk/bocindustrial/technical/glossary/a.asp