Serving the high temperature heat processing industry since 1979 !
Sigma-phase Embrittlement: The short answer is catastrophic failure from embrittled alloy.
The technical answer: Embrittlement of iron-chromium alloys (most notably austenitic stainless steels Type 301,302, 303, 304, 308, 309, 316, 321) caused by precipitation at the grain boundaries of the hard, brittle intermetallic sigma phase during long periods of exposure to temperatures between approximately 560 and 980 ºC ( 1050 and 1800 ºF). Sigma-phase embrittlement results in severe loss in toughness and ductility, and can make the embrittled material susceptible to intergranular corrosion.
We can eliminate this in your process with the correct alloy.