Phase-Field Modeling of Austenite Formation from a Ferrite plus Pearlite Microstructure during Annealing of Cold-Rolled
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AUSTENITE formation during heating and annealing of dual-phase steels is a diffusion-controlled phase transformation that is a combination of nucleation and growth processes. The kinetics of austenitization in steel strongly depends on the starting microstructure.[1–4] In the case of a ferrite plus pearlite initial microstructure, the formation of austenite occurs in two steps: the first is the fast dissolution of pearlite; and the second is the ferrite-to-austenite transformation, which is a much slower process and defines the overall kinetics of austenite formation.[4–8] During dissolution of pearlite, austenite nucleation takes place on the ferrite/cementite interfaces and austenite rapidly overgrows the pearlite regions. In this process, the growth rate is controlled by carbon diffusion in austenite and thus also depends on the average interlamellar spacing of pearlite: the austenitization starts earlier and proceeds faster in a finer initial pearlite microstructure.[9] Dissolution of pearlite usually takes anywhere between a few seconds and a few minutes, depending on the pearlite morphology and the austenitization temperature, as well as the alloying elements in the steel. During the formation of austenite J. RUDNIZKI, PhD Researcher, U. PRAHL, Head of the Group, and W. BLECK, Head of the Department, are with the Department of Ferrous Metallurgy, IEHK, RWTH Aachen University, Aachen, Germany. Contact e-mail: [email protected] B. BO¨TTGER, Postdoctoral Researcher, ACCESS e.V., is with RWTH Aachen University. Manuscript submitted July 26, 2010. Article published online February 12, 2011 2516—VOLUME 42A, AUGUST 2011
from ferrite, the nucleation of new austenite grains on the ferrite grain boundaries can take place only under certain conditions, for example, during fast heating when a high thermodynamic driving force is available. The rate of austenite growth from ferrite, as mentioned previously, is much lower compared to the austenite formation from pearlite and is controlled by carbon diffusion in austenite as well as by diffusion of substitutional elements in ferrite.[8] Typically, substitutional elements affect austenitization by their influence on the transformation temperature. Thus, austenite stabilizing elements such as Mn decrease the AC3 temperature, which serves as a thermodynamic explanation for the faster completion of austenite formation. In contrast, ferrite stabilizing elements such as Si increase the AC3 temperature and, therefore, delay austenite formation. The influence of alloying elements on the transformation temperatures was studied with empirical or thermodynamic approaches.[10,11] However, more than the transformation temperature is influenced by the alloying elements: the diffusion of substitutional elements can also strongly affect the kinetics of phase transformations. Different types of growth kinetics can be distinguished, depending on the redistribution behavior of the substitutional elements: paraequilibrium (PE), local equilibrium (LE), and LE negligible partitioning (LENP
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