Abstract:Phase diagrams of the Fe-Cr-Ni system have been calculated in order to examine the sigma(σ)phase composition range and its stability. Calculation indicates that σ phase starts to precipitate at 977℃ and then disappears after the invariant reaction γ+σ→α₁+α₂ occurs at 464℃. Here α₁and α₂ represent Fe-rich and Cr-rich ferrites respectively. Particularly, effect of Cr and Ni contents on the σ precipitation in the 18-8 type stainless steels was formulated by the present calculation. The a precipitation was also examined in a super duplex stainless steel Fe-25 Cr-7. 2 Ni-3. 6 Mo-0. 25 N (wt. -%), and calculated phase diagram shows that the σ precipitation takes place only when annealing this steel at temperature below 920℃. The addition of nitrogen content up to 0.6 wt. -% can not prevent the σ precipitation in such steel.

Key words: Fe-Cr-Ni phase diagram σ-phase precipitation