By using the Fick's second law, the carbon concentration of phase γ at non-equilibrium interface and its distribution in austenite during the decomposition of austenite to ferrite in Fe-C alloys were calculated. For example, it takes 160 sat 1 025 K, 210 s at 975 K, 270 s at 925 K and 580 s at 875 K for reaching the equilibrium carbon concentrations of phase γ at the α/γ interface in Fe-0.2C alloy, and the relationship between the ratio Cγ int/Cγ/α and the time meets the parabola law with parabola exponents of 0.167 39, 0.235 97, 0.319 87 and 0.389 44 at 1*!025 K, 975 K, 925 K and 875 K respectively. The decrease of the diffusion velocity of carbon in austenite with the increase of under-cooling would be counteracted by the increase of driving force for pro-eutectoid ferrite formation before soft impingement of ferrite grains with each other; but the effect of diffusion is more intensive than that of driving force at the later stage.
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