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 1273 and 1473 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 1273, 1373 and 1473 K for various times between 3.1×10
5 and 7.8×10
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-(14000/
T)} or
YNb(
YC)
0.81=exp{0.053-(14000/
T)}, where the concentration parameters of niobium and carbon,
YNb and
YC, are related to the mole fractions,
XNb and
XC, by the equations
YNb/(1-
XC) and
YC=
XC/(1-
XC). The present results were compared with those for commercial Inconel alloy 600.
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