Sigma phase

The prototype FeCr sigma phase is of particular interest because the free atoms have very nearly the same size (ratio = 1.01), but they condense into a rather intricate structure. In the pure metals, the diameter of Cr is 2.50 A, while that of Fe is 2.48 A. (a difference of less than one percent), and both are bcc. Therefore, the existence of the sigma phase is determined by spd-hybridization of the electron orbitals. It is sometimes called a size-effect phase, but this is not really descriptive. 

The sigma phases are hard and brittle at below their Debye temperatures, but have some plasticity at higher temperatures. Thus there is some covalent bonding in them, and their glide planes are puckered, making it difficult for dislocations to move in them until they become partially disordered. Their structures are too complex to allow realistic hardness values to be calculated for them. Their shear moduli indicate their relative hardnesses. 

The hardness of precipitated sigma phase in stainless steels seems to vary with the composition. In Type 446 plain chromium steel it is about 9GPa (Guimaraes and Mei, 2004) whereas in Type 316 high Cr, Ni steel it is about 17GPa (Ohmura et al., 2006). Since they are ordered phases that do 

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Sigma Phase 800/1600"F Brittleness at Tubes, pipe, piping Austenitic steels. Precipitation of iron- Employ alloy... 

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°.)... 

The basic guidelines for preventing cracking would seem to be to operate at minimum stress levels, at as low an HjS concentration as possible and to make sure that welding procedures are adequately specified and followed. Furthermore, extensive periods of operation at temperatures that might cause sigma phase formation should be avoided. 

Hills, R. F. and Harries, D. P. (1960) Chem. and Proc. Eng. 41, 391. Sigma phase in austenitic stainless steel. 

A. A. Guimaraes and P. R. Mei, Precipitation of Carbides and Sigma Phase in AISI Type Stainless Steel under Working Conditions, Jour. Mater. Process. Tech., 155-156,1681 (2004). 

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

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