Examinations to the sigma-phase

 

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 )          

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