Influence of pearlite morphology and heating rate on the kinetics of continuously heated austenite formation in a eutect
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I. INTRODUCTION
MOST commercial processes rely to some extent on heat treatments, which cause the steel to revert to the austenitic condition. This includes the processes involved in the manufacture of wrought steels and in the fabrication of steel components by welding. It is useful, therefore, to be able to model quantitatively the transformation of an ambienttemperature steel microstructure into austenite.[1] The microstructure from which austenite may form can be infinitely varied (ferrite, martensite, carbide, and various morphologies and aggregates of each, e.g., pearlite and bainite). Many variables are, therefore, needed to describe the kinetics of austenite formation. Factors such as particle size, the distribution and chemistry of individual phases, homogeneity, and the presence of nonmetallic inclusions should all be important.[2–5] Thus, in the case of the formation of austenite from pearlite, one of the most relevant structural factors to be considered is the interlamellar spacing of pearlite.[6] In fully pearlitic steel, austenite nucleates heterogeneously at the junctions between pearlite colonies. This is in spite of the relatively large amount of interlamellar surfaces available within the pearlite colonies, which seem to be much less effective as sites for the nucleation of austenite.[7] The rate of growth of the austenite is controlled primarily by the rate of carbon diffusion in the austenite between adjacent pearlitic cementite lamellae, but may also be influenced by boundary diffusion of substitutional alloying elements at low temperatures.[3] Models of specific metallurgical approaches exist for isothermal austenite formation from different initial microstructures.[3,7–13] However, none of these is likely to be of general applicability, except perhaps at slow heating rates consistent with the achievement of equilibrium. In this work, a model is presented for the austenite formation during continuous F.G. CABALLERO and C. CAPDEVILA, Research Associates, and C. GARCI´A DE ANDRE´S, Research Scientist, are with the Department of Physical Metallurgy, Centro Nacional de Investigaciones Metalurgicas (CENIM-CSIC), Avda. Gregorio del Amo, 8, 28040 Madrid, Spain. Manuscript submitted June 9, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
heating in a eutectoid steel with a fully pearlitic initial microstructure. The influence of parameters such as interlamellar spacing of pearlite, edge length of pearlite colonies, and heating rate on the transformation kinetics has been considered in the model. Results of modeling have been experimentally validated at three different heating rates and for three different morphologies of pearlite. II. EXPERIMENTAL PROCEDURE A. Morphological Characterization of Lamellar Pearlite A eutectoid carbon steel with an actual composition (wt pct) of 0.76C, 0.24Si, 0.91Mn, and 0.013P was used. The alloy was prepared as a 2500 kg vacuum-induction melt from high-purity base material. After casting and cropping, the ingot was hot rolled down to a 50-mm-square bar. The following h
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