セメンタイト生成の遊離エネルギー變化より見たる黒鉛化に就て

抄録

Free energy change of the reaction 3Fe+C=Fe3C may be calculated from the heat of formation of cementite at 25°, specific heats of cementite, graphite and iron (α, β, and γ) at 0-1, 100° together with the heats of transformation of A₀, A₂ and A₃ in iron carbon alloy or from the equilibrium constants of the reactions 3Fe+2CO=Fe₃C+CO₂ and 2CO=C+CO₂ at higher temperatures. As these thermo-chemical and dynamical values were measured by many investigators and are in good agreement with each other, in the present papers the free energy change above mentioned has been calculated by using these data. The calculation has been made in two ways according to the following thermodynamical relations, namely, (1) ΔF⁰=-RT ln K_p and (2) ΔC_p=(∂ΔH/ΔT)_p and-ΔH/TT²=[∂(ΔF/T)/∂T]_p, In carrying out the calculation (2) proper correction has been made as the γ-iron dissolves carbon and forms austenite. This has been done by applying the Nernst's formula of dilution i.e. ΔF⁰ (dilution)=-RT ln N₁/N₂. The results of these two ways of the calculation showed a very good agreement with each other, that is, ΔF⁰ of tbe above reaction becomes zero at (1) 960°C and (2) 964°C respectively, and is positive below these temperatures. This means that the cementite is unstable and should decompose into austenite and graphite below these temperatures while above these its decomposition can not take place as it is stabler than the equivalent mixture of austenite and graphite. Hence it may be said that the cementite is meta-stable at lower temperatures but at higher temperatures graphite is meta-stable. By precisely reviewing the experimental data here cited, further calculation of ΔF⁰ of the above reaction has been made by assuming some probable errors or corrections.