The sigma-phase is a brittle, intermetallic phase with high
hardness. They arises, when a cubic body-centered and a cubic body-centered metal
coincide, whose atomic radius correspond with a low deviation (8%). Sigma-phase
forms predominantly out of delta-ferrite, because in high alloyed cr-ni-steel the
composition of the delta-ferrite is similar. A possible composition consists of
(24 % Cr, 18 % Mo, 6 % Ni, 52 % Fe), another consists of (48 % Cr, 52 % Fe). At
temperatures between 600 and 900 °C the undesirable Phase precipitates in high
alloyed steels. Sigma-phase is not only undesirable because of her embrittled effect,
but also because of its characteristic, to withdraw chromium out of the matrix.
The consequence of this is a drastic deterioration of the stability against corrosion.
When sigma-phase precipitates out of the delta-ferrite, also austenite is formed
through cr- and mo-decrease (ferrite-forming elements). The arising mixture consists
of sigma-phase and austenite, with possible existing residues of delta-ferrite,
has evident the character of a eutectic microstructure. Because the sigma-phase
goes in solution at temperatures above 900 °C, it can get removed through a
corresponding annealing. Nitrogen in the steel reduces the danger of the formation
of sigma-phase or suppresses it completely. Characteristic of the sigma-phase :
not magnetizable ; very brittle
- bad impact strength
very low corrosion resistance - especially in the direct surroundings
through decrease of Cr and Mo
The following examinations and the micrographs are realized at the following materials.
1.4435 ( X2CrNiMo18-14-3 ) 1.4462 ( X2CrNiMoN22-5-3 ) 1.4872 ( X25CrMnNiN25-9-7 )
1.4427 ( X12CrNiMoS18-11 )