Stainless Steels Above 1000°F
At elevated temperatures, all stainless steels with high chromium contents will develop some “sigma phase” which causes embrittlement at lower temperatures. Sigma phase is very hard, nonmagnetic, and brittle. The composition of sigma phase varies depending on the alloy from which it formed. Sigma phase normally does not affect the steel’s elevated temperature properties but may make it so brittle at lower temperatures that failures will occur during startup or shutdown.
The straight chromium ferritic and martensitic stainless steels containing 13% and more chromium are very susceptible to extensive sigma phase formation at temperatures above about 1000°F. The austenitic stainless steels are not as susceptible because of their high nickel content, but they can develop damaging amounts of sigma phase when held between about 1000°F to 1550°F for long periods of time. Certain highly susceptible austenitic alloys, such as castings and welds, may develop serious embrittlement in a few hours at temperatures of 1200°F to 1300°F. Duplex stainless steels are also very susceptible to sigma embrittlement.
Sigma embrittlement is controlled by minimizing ferrite content of stainless steel welds. Refer to specifications PVM-MS-1322 and PVM-MS-4748. Duplex stainless steel is limited to 650°F maximum service temperature to avoid embrittlement.
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