Expressions for Solubility Products of Fe₃Nb₃C carbide and Fe₂Nb Laves Phase in Niobium Alloyed Ferritic Stainless Steels

抄録

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; log₁₀{[Nb]³[C]} = −11613/T+5.2178
Fe₂Nb; log₁₀[Nb] = −3780.3/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^αβ, are given by:

Fe₃Nb₃C (β); ΔG^β = −222509−RT{6.423+ln(x_Nb^αβ)³(x_c^αβ)}
Fe₂Nb (ω); ΔG^ω = −72334−RT{0.5469+ln x_Nb^αω}