Models and Characteristics of Discontinuous Transformation of Austenite in Iron–Carbon Alloys

Abstract

The analysis of significant amount of works dealing with intermittent transformation of austenite allows determining the following basic characteristics of such transformation. The lamellar structure of perlite formed at intermittent transformation of austenite is thermodynamically unstable, since it has the very large extent of interphase boundaries. Perlite is formed as the separate colonies, i.e. areas, where α-iron and cementite plates are approximately parallel and interplate (interlamellar) distance (S0) depends on cooling speed. Zener has suggested a relation for the interlamellar distance as a function of the steel overcooling value, S0∝1/ΔT. The experimental results demonstrate, however, that the measured values of interlamellar distance of perlite are much larger than the values, which are proposed within the Zener model. The authors found the temperature dependence of interlamellar distance of perlite as a function of the overcooling value of alloy, S0∝(D/ΔT)^1/2, that is in agreement with experimental data. The growth rate of perlite v is an important kinetic characteristic of austenite transformation process. The value of the growth rate has been obtained by Меhl as v=K/S0. In the subsequent works, the scientists found various solutions of equation for carbon diffusion in austenite, which allow determining the value of the K-coefficient and to take into account effects of additional factors — structural strains (B. Ya. Lyubov), non-metallic inclusions (V. E. Ol’shanetskiy), herewith improving the accuracy of calculations. The authors develop the diffusion model of intermittent transformation of austenite, which allows explaining the formation of perlite and bainitе in iron–carbon alloys in the same temperature interval. The temperature dependence of the growth rate of α-phase on the steel overcooling value is found theoretically as v∝ΔTexp(−Q/(2RT)).

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