a Fe-Al ϭϪ1 112/TϪ0.585 g°A l ϭϪ185.0/Tϩ0.165 KEY WORDS: liquid iron; aluminum; nitrogen; AlN; equilibrium constant; interaction parameters. ISIJ International, Vol. 47 (2007), No. 7, pp. 945-954 945 © 2007 ISIJ In the present study, thermodynamics of aluminum, nitrogen and AlN formation in liquid iron was investigated using the metal-nitride-gas equilibration technique in the temperature range of 1 873-1 973 K. The sampling method utilizing a high frequency induction furnace was used to determine the equilibrium relations of aluminum and nitrogen in iron melts containing up to 4.53 mass% Al at different nitrogen partial pressures. The effect of aluminum content on nitrogen solubility in liquid iron was measured. The solubility product of aluminum and nitrogen in liquid iron saturated with pure solid AlN was also measured as a function of temperature. Using Wagner's formalism, 9) the interaction parameters of aluminum and nitrogen and the equilibrium constant of AlN formation in liquid iron were determined and compared with previous investigations. In addition, thermodynamics of aluminum in liquid iron determined in the present study were examined by Darken's quadratic formalism. 15,16) Experimental ProceduresFive hundred grams of high purity electrolytic iron (99.95 mass% purity, 200 mass ppm O, Ͻ5 mass ppm N, 18 mass ppm C, Ͻ10 mass ppm Si), contained in an Al 2 O 3 crucible (OD: 56 mm, ID: 50 mm, H: 96 mm) was melted in the temperature range of 1 873-1 973 K by a 15 kW/30 kHz high frequency induction furnace as shown in Fig. 2. The alumina crucible was placed in a graphite crucible and a porous alumina crucible as outer crucibles. The reaction chamber consisted of a 96 mm OD quartz tube open at both ends and was connected to a gas delivery system.After melting the iron, the temperature of the melt was monitored by a Pt/Pt-13 mass%Rh thermocouple sheathed with an 8 mm OD alumina tube immersed in the melt. Any possible influence of high frequency noise on the temperature reading was avoided by grounding the circuit of the thermocouple. Preliminary trials confirmed that no significant noise was detected. The temperature fluctuation of iron melt could be controlled within 2 K during experiment by the PID controller of the induction furnace. The temperature reading of the PID controller was calibrated by the sourcing DC voltage calibrator for the thermal EMF of Rtype thermocouple.After the temperature of melt was reached to a desired value, an Ar-10%H 2 gas was blown onto the melt surface at a high flow rate of ϳ5 000 mL/min for 2 h to deoxidize the iron melt. The oxygen content in the melt after this procedure was in the range of 15-20 mass ppm. Then the gas was switched to a N 2 -3%H 2 gas (P N 2 ϭ0.97 atm) or a mixture of Ar-10%H 2 and N 2 gases (P N 2 ϭ0.2 atm). The flow rate of gas mixture was controlled by a mass flow controller in the range of 1 000 to 2 000 mL/min depending on nitrogen partial pressures in the gas.Strong agitation of melt by an induction furnace resulted in a fast attainment of equil...

Using thermodynamic data determined in present study, a stability diagram for Al 2 O 3 , Ti 2 O 3 and Ti 3 O 5 phases was constructed as a function of dissolved aluminum, titanium and oxygen contents in liquid iron. The result indicated that at equilibrium condition titanium oxides would not form on titanium addition in Al-killed liquid steels with soluble aluminum content of 0.02-0.05 mass% at 1 873 K.