A process model for the microstructure evolution in ductile cast iron: Part II. Applications of the model
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INTRODUCTION
CASTINGS are prime examples of components where the properties achieved depend on the characteristics of the microstructure.[1,2] When viewed from the sideline, the field seems to have reached a level where most of the underlying physical mechanisms are well established. At the same time, the recent advances in computer technology and numerical methods have made it possible to rationalize microstructural evolution and transport phenomena in terms of models based on the fundamental equations for energy, mass, and momentum conservation.[3,4] A synthesis of that knowledge has, in turn, been consolidated into various kinds of deterministic models to predict the as-cast microstructure.[5–14] The main idea here is to divide the computational space into a series of interconnected volume elements, each of them acting as an open system with respect to heat transfer but being autonomous when it comes to microstructural evolution. The process model developed in Part I of this investigation[14] is based on the same philosophy and combines information about the phase relations within the Fe-C system with kinetic data to describe the microstructural evolution in ductile cast iron. The reactions considered are the graphite/austenite eutectic transformation and the ledeburite eutectic transformation during solidification, the subsequent growth of the graphite phase in the austenite regime, and finally the decomposition of austenite into ferrite and pearlite during the eutectoid transformation. In this part of the investigation, the microstructural model will be linked to a well-proven experimental technique for assessment of inoculant performance in ductile cast iron (here, directional solidification). As a starting point, the model will be applied to the reference iron described in Part I[14] and validated by comparison with experimental microstructural data. Sub-
M.I. ONSØIEN and Ø. GUNDERSEN, Research Metallurgists, are with SINTEF Materials Technology, N-7034 Trondheim, Norway. Ø. GRONG, Professor, is with the Department of Metallurgy, Norwegian University of Science and Technology, N-7034 Trondheim, Norway. T. SKALAND, Research Metallurgist, is with Elkem a/s Research, N-4602 Kristiansand, Norway. Manuscript submitted October 20, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
sequently, its potential for optimization of melt treatment practice and casting conditions for ductile iron will be illustrated in different numerical examples and case studies. II. TOOLBOX FOR SIMULATION OF MICROSTRUCTURAL EVOLUTION The symbols and units used throughout the article are defined in the Nomeclature. In the present investigation, a special MATLAB* tool*MATLAB is a trademark of The MathWorks, Inc., Natick, MA
box has been developed to simulate the microstructural evolution during directional solidification of ductile iron. The toolbox consists of a numerical heat flow model and a series of microstructural models that are coupled. A. Heat Flow Model Calculation of the temperature-time pattern in different positions along
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