Max. operating temperatures are limited by the susceptibility of the ferritic phase to 475 °C (885 °F) embrittlement and other embrittlement mechanisms. Most Codes applicable to refinery equipment and piping limit the various DSS grades to between 260 °C to 340 °C , maximum, to avoid these problems . WELDS ARE MORE PRONE to sigma formation than base materials.
Prolonged exposures at above 260 °C to 340 °C depending on grade, may initiate embrittlement.
The mechanism causing the embrittlement is decomposition of the ferrite into a brittle, Cr-rich alpha prime phase. This is the primary reason that the Code temperature limits are in the 260 °C to 340 °C range and indicates that the DSSs should not be used for long durations above the Code limits.
The 25 % Cr and 27 % Cr alloys are more susceptible to this embrittlement than are UNS S32205 or S32304, and they can embrittle from a room temperature toughness level of 150 J to 27J from 10 hours of exposure to. Similar levels of embrittlement would take 10,000 hours at 300 °C.
The effect on toughness is not pronounced at the operating temperature, but is significant at lower temperatures experienced during plant shutdowns, startups or upsets.
API TR 938 C
API RP 571