Kinetic Transition during Ferrite Growth in Fe-C-Mn Medium Carbon Steel
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INTRODUCTION
MEDIUM-CARBON manganese steels containing 0.2 to 0.5 mass pct of C and 1.0 to 2.0 Mn are used in large quantity as materials for hot forging parts. A prediction of austenite (c) decomposition into a microstructure consisting of proeutectoid ferrite (a), pearlite (P), and bainite (B) has long been a desired technology, which would be the basis for balancing mechanical properties and good machinability of final products. In the quest of an ultimate model that allows predicting accurately the phase fraction produced, an important milestone consists in determining the interface conditions of parent and product phases. On approaching the subject, one finds that the thermodynamics in the Fe-Mn-C ternary system have been well discussed in terms of partitioning of Mn, and the parameters describing diffusion-controlled transformation can now be calculated by the CALPHAD method for the ternary and even higher order system. However, there is still discussion on the type of equilibrium operating at the real interface between a and c,[1] although studies devoted to the influence of Mn (among others elements) on the diffusional austenite decomposition reactions in steels were the subject of C. CAPDEVILA, Senior Scientist, and J. CORNIDE, Postdoctoral Student, are with the Materalia Research Group, Centro Nacional de Investigaciones Metalu´rgicas (CENIM-CSIC), E-28040 Madrid, Spain. Contact e-mail: [email protected] K. TANAKA, Senior Researcher, and K. NAKANISHI, Senior Researcher and Research Manager, are with Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan, and CREST, Japan Science and Technology Agency, Tokyo 100-0004, Japan. E. URONES-GARROTE, Titulado Superior, is with the Centro de Microscopı´ a y Citometrı´ a, Universidad Complutense de Madrid, E-28040 Madrid, Spain. Manuscript submitted October 28, 2010. Article published online March 22, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
academic interest as well as industrial significance for more than a century.[2–9] In a ternary system in which growth of the ferrite phase is controlled by diffusion, it is possible to assume that the phases in contact at the interface are locally in equilibrium with each other; this requires that their compositions are given by the ends of a tie-line of the two-phase field in the equilibrium diagram. This can lead to two extremes. At low supersaturation, above the dotted line AB in Figure 1(a), the interface velocity depends on the diffusion coefficient of X. Below the dotted line AB, the growth velocity depends only on the diffusion coefficient of carbon. In the first case, the carbon concentration gradient in the matrix is negligible, whereas in the second, it extends far into the matrix. The X concentration gradient extends into the matrix in the first case but has only a very sharp and narrow ‘‘spike’’ in the second case. The first case is usually referred to as local equilibrium partition (LEP) and the second case as local equilibrium no partition (LENP). There is a third condition that represents a constrained equilibr
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