The solidification characteristics of the Super Duplex Stainless Steel (SDSS), 25Cr-7Ni-3Mo-2W-0.25N-Fe and the exchange rate constant of nitogen gas on 25Cr-7Ni-3Mo-2W-Fe alloy melt surface have been investigated, which have an improved pitting corrosion resistance due to the tungsten alloying. The liquidus and solidus temperature of the 25Cr-7Ni-3Mo-2W-0.25N alloy was determined by DTA and were 1480 and 1450℃ respectively. The interrupted quenching test showed that the alloy solidify in δ-ferrite single phase. During solidification Ni and N segregate in the rest melt, although N diffuses back into the δ-ferrite appreciably due to its high diffusivity. The specimen quenched only after solidification and above γ-transformation showed $Cr_2N$ precipitates, which was explained by the high N- and low Ni-concentration in the center of dendrites and the related change in the γ-and $Cr_2N$- transformation curves in TTT-diagram. The exchange rate constant of $N_2$ gas on 25Cr-7Ni-3Mo-2W-Fe alloy melt surface was about $2.4×10^{-6}$ mol $cm^{-2} s^{-1} atm^{-1}$ at 1620℃, and the amount of δ→γ transformation increased with nitrogen content.
