Expressions for solubility products of Fe3Nb3C carbide and Fe2Nb laves phase in niobium alloyed ferritic stainless steels

Precipitation behaviors in niobium alloyed ferritic stainless steels, which are widely used as heat resistant materials in automotive exhausts, were investigated. The solubility products of Fe3Nb3C and Fe2Nb, both of which are prominent precipitates in such steels, were experimentally obtained. The expressions of the solubility products with mass% are given by: Fe3Nb3C; log(10){[Nb](3)[C]}= -11 613/T+5.2178 Fe2Nb; log(10)[Nb] = -3 780.3/T+2.4646 These were verified with some published experimental data. An attempt was also made to get thermodynamic parameters using the solubility products. The free energy changes for the precipitation reactions from niobium-supersaturated ferrite have been obtained. The expressions with mole fractions in ferrite matrix, e.g. x(Nb)(alphabeta), are given by: Fe3Nb3C (beta); DeltaG(beta) = -222 509-RT {6,423+In(x(Nb)(alphabeta))(3)(x(c)(alphabeta))} Fe2Nb (omega); DeltaG(omega)= -72 334-RT {0.5469+In x(Nb)(alphaomega)}.