Sigma phase embrittlement

Austenitic stainless steels are often susceptible to sigma-phase embrittlement after extended exposure at 1050°F to 1700°F (565°C to 925°C). Occurrence depends primarily on the temperature and the presence of ferrite. While Type 304 SS can develop sigma-phase embrittlement, it is more common in austenitic products that contain small amounts of ferrite. Examples include austenitic weld metal and castings. 

Severe loss of ductility of a metal (or alloy) loss of load carrying capacity of a metal or alloy the severe loss of ductility or toughness or both, of a material, usually a metal or alloy. Many forms of embrittlement can lead to brittle fracture and many can occur during thermal treatment or elevated-temperature service (thermally induced embrittlement). Some of these forms of embrittlement, which affect steels, include blue brittleness, 885 °F (475 °C) embrittlement, quench-age embrittlement, sigma-phase embrittlement, strain-age embrittlement, temper embrittlement, tempered martensite embrittlement, and thermal embrittlement. In addition, steels and other metals and alloys can be embrittled by environmental conditions (environmentally assisted embrittlement). Forms of environmental embrittlement include acid embrittlement, caustic embrittlement, corrosion embrittlement, creep-rupture embrittlement, hydrogen embrittlement, bquid metal embrittlement, neutron embrittlement, solder embrittlement, sobd metal embrittlement, and stress-corrosion cracking. 

Embrittlement of iron-chromium alloys (most notably austenitic stainless steels) caused by precipitation at grain boundaries of the hard, brittle intermetallic sigma phase during long periods of exposure to temperatures between approximately 560 and 980 C (1050 and 1800 °F). Sigma-phase embrittlement results in severe loss of toughness and ductility, and can make the embrittled material susceptible to intergranular corrosion. See also sensitization. 

Type 315-This has a composition that provides a similar oxidation resistance to type 309 but has less liability to embrittlement due to sigma formation if used for long periods in the range of 425 to 815°C. (Sigma phase is the hard and brittle intermetallic compound FeCr formed in chromium rich alloys when used for long periods in the temperature range of 650 to 850°.)... 

ISCnSNi SS is required to resist oxidation however, embrittlement due to formation of sigma phase possible in some grades. Avoid cold work. 

Duplex stainless steels are susceptible to 885°F (475°C) embrittlement and to sigma-phase formation, and they are usually not selected for temperatures above 650°F (345°C). Because of their ferrite content, they are susceptible to low-temperature embrittlement. However, the duplex stainless steels tend to have relatively low brittle-ductile transition temperatures. The engineering codes typically require the duplex stainless steels to be qualified for low-temperature service by impact testing. They can be susceptible to hydrogen embrittlement, but are less susceptible than are the ferritic and martensitic stainless steels. 

Do not use ferritic stainless steels containing more than 16% chromium in the 750 F to 1,000 F temperature zone. They invariably embrittle, because of precipitation of a chromium-rich constituent. Even the 11 to 13% chromium alloy sometimes seems to embrittle in this way, for reasons not clear. Unfortunately, this is a common temperature zone for many refinery and chemical plant processes. These ferritic stainless steels should not be used above 1,000°F, either. They will embrittle for a somewhat different reason the formation of an iron-chromium intermetallic compound called sigma phase. 

The nickel-rich type 310 alloy is less susceptible to sigma-phase formation. Above 1598 (870°C), there is little risk of embrittlement. Alloy compositions... 

The low chromium favors less sensitivity to 855°F (475°C) embrittlement and sigma phase formation. 

---