Influence of Morphology of Cementite on Kinetics of Austenitization in the Binary Fe–C System

抄録

When a binary Fe–C alloy with the ferrite (α) and cementite (θ) two-phase microstructure is isothermally annealed at a certain high temperature for the single-phase region of the austenite (γ) phase, the γ phase is produced at the α/θ interface by the reactive diffusion between the α and θ phases. Usually, this phenomenon is called austenitization. Owing to austenitization, the θ phase will completely dissolve into the γ phase at sufficiently long annealing times. For the flat plate of the γ phase produced between the α and θ lamellae, the one-dimensional diffusion of C occurs along the direction normal to the α/γ and γ/θ interfaces. In contrast, for the spherical particle of the θ phase distributed in the matrix of the α phase, the θ phase particle is covered with a spherical shell of the γ phase. In such a case, the three-dimensional diffusion of C in the spherical coordinate system occurs along the radial direction. The kinetics of the C diffusion is different from each other between the one-dimensional and three-dimensional coordinate systems. Consequently, the morphology of the θ phase will influence the growth behavior of the γ phase. To examine such influence, the dissolution of the θ phase was theoretically analyzed using kinetic models under various assumptions. On the basis of the analysis, the time-temperature-dissolution (TTD) diagram was constructed for each shape of the θ phase. This diagram provides quantitative information on the relationship between the dissolution time and the annealing temperature. According to the TTD diagram, the dissolution of the θ phase into the γ phase takes place much faster for the spherical morphology than for the flat one.

Fig. 14 The ratio t_d-S/t_d-P (Φ) versus the temperature T shown as dotted, dashed and solid curves for r₀ = 0.15, 0.25 and 0.50 µm, respectively.