Duplex stainless steels (DSSs) are the dual phase stainless steels with approximately equal volume fractions of ferrite and austenite, and characterized by an unusual combination of mechanical strength, toughness and corrosion resistance. The phase fractions of δ ferrite and γ austenite in DSS have the considerable effects on the most properties of DSS, and are mainly determined by compositions and heat treatments. It has been reported that Cr, Mo, W, N and some other alloying elements not only have positive effect on corrosion resistance of DSS but accelerate the precipitation of secondary phases that have detrimental effects to the mechanical and corrosion properties of DSS.
In this study, the effect of N addition in Fe-25Cr-4.5Ni-0.5Mo-4.5W-4.5Mn-0.8Si-0.5Cu-0.03C-xN duplex alloy particularly on the stability and precipitation behavior of Cr_2N phase has been calculated using Thermo-Calc and the results were compared with the experiments. The database used in the Thermo-Calc calculation was SGTE solution database. Electron and optical microscopy and X-ray diffraction were used to measure th volume fraction of austenite, ferrite and $Cr_2N$ phase and precipitation kinetics of $Cr_2N$ phase.
The results of calculation of stability of ferrite and austenite was generally in good agreement with the experimental measurements. The result of calculation of composition of ε phase ($Cr_2N$) was also in good agreement with the experiment, showing, in particular, that \epsilon phase can contain a large amount of W up to about 18wt.%.
However, the calculation tended to largely overestimate the solvus temperature of \epsilon phase ($Cr_2N$) and to significantly underestimate the volume fraction of $Cr_2N$ phase at temperature below about 1000℃. As a result, no precipitation of $Cr_2N$ phase observed at 1050℃ up to 600min in an alloy containing up to 0.3N despite the fact that the equilibrium calculation predicts the precipitation of 2 vol.% at this temperature.
