SOLUBILITY OF NBC IN A NI-16CR-8FE ALLOY

The solubility of niobium carbide NbC in the nickel-rich phase of the Ni-16Cr-8Fe system, which is a base system of commercial Inconel alloy 600, was experimentally determined at a temperature range between 1 273 and 1 473 K. A ternary Ni-16Cr-8Fe alloy was prepared from three elemental metals. Thirteen Ni-16Cr-8Fe-Nb quaternary alloys with niobium concentrations between 0 and 6.0 mass% were prepared from the Ni-16Cr-8Fe alloy and pure niobium. The Ni-16Cr-8Fe-Nb alloys were carburized by a sealed capsule method at equilibrating temperatures of 1 273, 1 373 and 1 473 K for various times between 3.1 x 10(5) and 7.8 x 10(5) sec. Carbon activity in each capsule was determined from the carbon concentration of the pure iron sealed in the capsule together with the alloys. Only niobium carbide NbC(x) was observed as a second phase in the equilibrated alloys under the present experimental conditions. The carbon concentration x of NbC(x) was lower than that of the stoichiometric carbide NbC and was estimated to be x=0.81 from the experimentally determined solubility of NbC(x) in the nickel-rich phase. The solubility was described as a solubility product and its temperature dependence was evaluated as (mass% Nb)(mass% C)0.81 = exp{7.5 - (14 000/T)} or Y(Nb)(Y(C))0.81 = exp{0.053 - (14 000/T)}, where the concentration parameters of niobium and carbon, Y(Nb) and Y(C), are related to the mole fractions, X(Nb) and X(C), by the equations Y(Nb) = X(Nb)/(1 - X(C)) and Y(C) = X(C)/(1 - X(C)). The present results were compared with those for commercial Inconel alloy 600.