Phase Tranformation Modeling of Medium-carbon

The kinetics of phase transformations in medium-carbon forging steels (MCFS) have been modeled based on CALPHAD multicomponent thermodynamics and the classical nucleation-growth theory. New treatments include the time dependency of parabolic growth rate of proeutectoid ferrite (α) , which account for the soft impingement effect by carbon enrichment in austenite (γ). And a potential transition of γ/α interface equilibrium has also been considered depending on temperatures and velocity of the moving interface. To make a realistic prediction of the onset of pearlite (P) transformation, a normal distribution of γ grain size has been assumed and successive α→P transformation kinetics in each grain size have been summated. The developed program coupled with thermodynamic solver, 'ThermoCalc', calculated the isothermal kinetics of MCFS and has been found to predict well the effect of minor difference of chemical composition / holding temperatures. INTRODUCTION Medium carbon steels containing 0.2~0.5 mass percent carbon are used in a large quantity as materials for hot forging parts. A prediction of the microstructure consisting of proeutectoid ferrite (α), 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. The α/P/B fraction after forging is mainly affected by both carbon and manganese content, austenite (γ) grain size (AGS), and cooling schedules. Thermodynamics in 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 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 α and γ. On the other hand, AGS and cooling rate are strongly related to hot working conditions and have been evaluated in the course of thermal-mechanically coupled FEM analysis. Senuma et al.[1] reported experimental equations for AGS for low carbon steel based on the relationship between rolling conditions and fractions of dynamic/static recrystallized γ grain, and Yogo[2] et al. recently modified the equation to be applicable to forging process. A lot of work has been devoted to understand the kinetics of α, P and B transformations in the framework of the classical nucleation-growth theory, and there are some integrated models[3-6] reported to be able to predict the evolution of complex α+P+B structure. These models so far have been developed for low-carbon steel strips, and therefore, focused on the prediction of α grain size as Materials Science Forum Vols. 539-543 (2007) pp. 2443-2448 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 161.111.90.248-27/02/07,11:19:16) well as the fraction of widmanstätten α or B. In view of the circumstances, we have firstly constructed an integrated physical model of the successive α→P transformation for medium-carbon forging steel (MCFS). And a trial program empowered by CALPHAD method has been developed to evaluate the usefulness of such computerized predictions. This article presents the newly introduced features of treating the above diffusion-controlled phenomena in the model, together with the current status in predicting α→P transformation behaviors of MCFS by the program. The possible B transformation from the remaining γ are also to be considered and will be included in a separate paper.