Kinetics of austenite-ferrite and austenite-pearlite transformations in a 1025 carbon steel
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INTRODUCTION
THE steel industry is rapidly adopting continuous processes including continuous casting, continuous annealing, and continuous heat treatment to minimize production costs and to improve control of product quality. The achievement of optimum processing conditions for producing steel with the desired properties can be assisted by the development of mathematical models that link the composition and thermomechanical treatment of the steel to its final mechanical properties. Thus, a program has been established at The University of British Columbia to develop a model of industrial heat-treatment processes such as Stelmor. The mathematical model requires accurate data on the phase-transformation kinetics of different steels. In a previous paper ~ the isothermal and continuous-cooling decomposition kinetics of austenite to pearlite in a eutectoid, plain-carbon steel were reported. These data then were employed to show the validity of applying the Additivity Principle to predict continuous-cooling transformation kinetics from isothermal-transformation measurements, as described by the Avrami equation 2. * Symbols are described at the end of the paper.
X = 1 - e x p ( - bt")
[1]
and a measured initiation time for the continuous-cooling conditions. Thus, at least for the eutectoid steel, isothermal data in combination with the Additivity Principle can be employed with confidence in the mathematical model. The model also consists of heat-flow equations, as described by Agarwal and Brimacombe, 3 so that the transient distributions of temperature and fraction transformed can be E.B. HAWBOLT, Professor, B. CHAU, Research Engineer, and J. K. BRIMACOMBE, Stelco Professor of Process Metallurgy, are with The Centre for Metallurgical Process Engineering, Department of Metallurgical Engineering, The University of British Columbia, Vancouver, BC V6T 1W5, Canada. Manuscript submitted July 11, 1984. METALLURGICAL TRANSACTIONS A
predicted simultaneously in shapes such as rods routinely cooled on a Stelmor line. The validity of the model has been confirmed by Iyer 4 by comparing model predictions to measurements of transient temperature at the centerline of 9 to 10 mm diameter rods subjected to forced-air cooling. Tamura et al. 5'6 also have developed models to predict transformations in steel, in which the Avrami equation is employed to characterize the isothermal decomposition of austenite to pearlite or bainite and the Additivity Principle is applied. In their characterization of the isothermal decomposition kinetics, the incubation (pre-transformation) period is combined with the transformation time to obtain the necessary kinetic parameters, n and b (Eq. [1]), from data in the literature. However, our study has shown that only the transformation event fulfills the criteria of the Additivity Principle, viz., dependence solely on the fraction transformed and transformation temperature. Thus inclusion of the incubation period in the calculation of continuouscooling transformation kinetics from isothermal data inevit
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