A computational model for the prediction of steel hardenability
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I.
INTRODUCTION
ONE of the primary goals of computational metallurgy and mechanics is to model and analyze the metallurgical and mechanical changes that have occurred during thermomechanical processes. Such computational models can provide powerful tools to understand fundamental factors that control the quality of the manufacturing process and performance of products. The objective of this study was to develop such a computational model for the prediction of phase transformations in steels under arbitrary nonisothermal cooling conditions, so that the microstructural evolution during heat treating processes and the resultant room-temperature hardness can be readily predicted. Although in this article the computational model was only applied to analyze end-quench bars, commonly known as Jominy bars, the principles should be applicable to practical heat treating and welding processes. In these processes, the workpieces are typically subjected to a thermal cycle that includes heating up and cooling down sections. The steel chemistry and the actual thermal cycle experienced by the workpiece are the necessary information to extend work presented in this article to the modeling and analysis of practical heat treating and welding processes. There is a considerable volume of literature on the prediction of the hardness distribution in end-quench bars (Jominy hardness) of heat treatable steels. One of the authors[1,2] previously examined existing models in the open literature for the prediction of Jominy hardness of steels. One subset of these hardness prediction models exhibits the capability of predicting transient phase transformations. In M. VICTOR LI, Research Scientist, is with Battelle, Columbus, OH 43201-2693. DAVID V. NIEBUHR, Tribologist, is with the Technology and Engineering Department, Quantum Corporation, Milpitas, CA 95035. LEMMY L. MEEKISHO, Associate Professor, and DAVID G. ATTERIDGE, Professor, are with the Department of Materials Science and Engineering, Oregon Graduate Institute of Science and Technology, Portland, OR 97006. Manuscript submitted May 1, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
this perspective, the computational phase transformation model originally developed by Kirkaldy and Venugopalan[3] exhibited the greatest predictive potential. It was found in the previous studies by the authors,[1,2] as well as by other researchers,[4] that the Kirkaldy model works reasonably well for less hardenable steels that are characterized with small amounts of alloying element additions. However, it underestimates the hardenability of steels with a moderate to high concentration of alloying elements. The inadequacy of the Kirkaldy model was primarily attributed to its reaction kinetics model for austenite decomposition. The authors thus reconstructed the reaction kinetics model with a better balance between the phase transformation theories and empiricism in order to improve the model predictions under continuous cooling conditions. II.
GLOBAL MODEL DESCRIPTION
The major steps of predicting Jom
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