Published by Maney Publishing (c) IOM Communications Ltd Modelling of isothermal formation of pearlite and subsequent reaustenitisation in eutectoid steel during continuous heating F. G. Caballero, C. Capdevila, and C. Garcı´a de Andre ´s Three different morphologies of pearlite have been formed isothermally at three different temperatures in a eutectoid steel. Moreover, the interlamellar spacing of the pearlite was calculated using the Zener and Hillert theoretical method. Experimental results suggest that the growth of pearlite is mainly controlled by volume diffusion of carbon in austenite, in the temperature range studied in this steel. In addition, a model that describes pearlite to austenite transformation during continuous heating in a eutectoid steel has been developed. The influence of structural parameters, such as interlamellar spacing and edge length of pearlite colonies, on the transformation kinetics has been experimentally studied and considered in the modeling. It has been found that the kinetics of pearlite to austenite transformation are slower the coarser the initial pearlite microstructure. Experimental validation of this model has been carried out and a good agreement (an accuracy level of higher than 90% in square correlation factor) between the experimental and calculated values has been found. MST/4727 Dr F. G. Caballero and Dr C. Garcı ´a de Andre ´s are in the Department of Physical Metallurgy, Centro Nacional de Investigaciones Metalu ´rgicas (CENIM), Consejo Superior de Investigaciones Cientificas (CSIC), Avda. Gregorio del Amo 8, 28040 Madrid, Spain. Dr C. Capdevila is in the Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK. Manuscript received 7 June 2000; accepted 12 September 2000. # 2001 IoM Communications Ltd. Introduction Pearlite is a lamellar product of eutectoid decomposition, which can be formed in steel and non-ferrous alloys during transformations under isothermal or continuous cooling. 1,2 A pearlite nodule is composed of multiple colonies; each colony has parallel lamellae, which have different orientations with respect to lamellae in adjacent colonies. A wide range of interlamellar spacing in different colonies is also exhibited because of the intersection of pearlite colonies at different angles with the polishing plane. The interlamellar spacing is reflected by the diffusion kinetics at the transformation front and is a sensitive parameter which, in a particular steel, is larger as the transformation temperature increases. 3 Mehl and co-work- ers 3 demonstrated that the spacing decreases as the degree of undercooling DT below the eutectoid temperature, increases. Zener 4 provided the first theoretical analysis of these observations, which allows a calculation of the interlamellar spacing of pearlite as a function of under- cooling. Pearlite transformation in steel is reconstructive and known to have a constant growth rate. 5 The growth rate of pearlite is generally believed to be controlled by either the volume diffusion of carbon 4,6 or by the boundary diffusion of substitutional alloying elements. 7 There are some studies debating the importance of the boundary diffusion of carbon, especially at intermediate transformation tempera- tures, 8 however this possibility is not considered in the present work. In this paper, three different morphologies of pearlite were formed isothermally at three different temperatures in a eutectoid steel. As Mehl et al. 3 and Zener 4 reported, the pearlite is finer the lower the formation tempera- ture. Moreover, the interlamellar spacing was calculated using the theoretical method proposed by Zener and Hillert. 4,6,7 Experimental results suggest that the growth of pearlite is mainly controlled by the volume diffusion of carbon in austenite in the temperature range studied in this steel. On the other hand, the formation of austenite during heating differs in many ways from those transformations that occur during cooling. In this sense, the kinetics of austenite transformation during cooling can be described completely in terms of the chemical composition and grain size of this phase only. However, the microstructure from which austenite may grow during heating can be much more complex. Therefore, in order to describe the kinetics of austenite formation additional variables are needed. Factors such as particle size, the distribution and chemistry of individual phases, homogeneity, and the presence of non- metallic inclusions, are all important. 9 – 12 In the case of reaustenitisation from pearlite, the most relevant structural factor to be considered is the interlamellar spacing of pearlite. 13 In fully pearlitic steel, austenite nucleates heteroge- neously at the junctions between pearlite colonies. This is in spite of the relatively large amount of interlamellar surfaces available within the pearlite colonies that seem to be much less effective as sites for austenite nucleation. 14 The rate of austenite growth is primarily controlled by the rate of carbon diffusion in the austenite between adjacent pearlitic cementite lamellae, but it may also be influenced by the boundary diffusion of substitutional alloying elements at low temperature. 10 Models of specific metallurgical approaches exist for isothermal reaustenitisation from different initial micro- structures. 10,14 – 20 However, none of these is are likely to be applicable to non-isothermal conditions. In this work, a model is also presented for the reaustenitisation from pearlite during continuous heating in a eutectoid steel. The influence of parameters such as the interlamellar spacing of pearlite and the length of the edges of the pearlite colonies on the transformation kinetics has been considered in the model. The results of modeling have been experimentally validated for three different morphologies of pearlite previously formed isothermally. 686 Materials Science and Technology June 2001 Vol. 17 ISSN 0267 – 0836