An analysis of static recrystallization during continuous, rapid heat treatment
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during continuous, rapid heat treatment is the classical Johnson-Mehl-Avrami-Kolmogorov expressiontT,8,9] for isothermal phase transformation kinetics controlled by nucleation and growth: X = 1 - exp {(-In 2) (t/to.s)"}
S.L. SEMIATIN, I.M. SUKONNIK, and V. SEETHARAMAN Because of its widespread use in a number of industrial processes, the static recrystallization of cold-worked metals is a phenomenon which has received extensive experimental and theoretical attention.V-sI Much of this work has dealt with isothermal heat treatment behavior in which recrystallization occurs at a nominally constant temperature for a fixed length of time. Laboratory work of this type has resuited in a number of so-called engineering laws of recrystallization which describe the effects of starting grain size of the deformed metal, level of prior cold work, and rate of heating to the annealing temperature on the recrystallization temperature and time and the recrystallized grain size. These observations have been complemented by theoretical analyses which have sought to explain recrystallization kinetics in terms of classical nucleation and growth phenomena as well as, in more recent times, the MonteCarlo and cellular automaton approaches which treat the statistical aspects of metallurgical transformations. Over the last 25 years, a number of primary metals producers have introduced (or evaluated on a laboratory scale) various types of continuous annealing processes in which the metal is heated to some peak temperature and then cooled without soaking at this temperature. This method has found wide application in particular for the recrystallization of cold-rolled sheet metals such as low carbon steel, stainless steels, copper alloys, and titanium alloys.[6I Heating techniques for sheet materials range from those based on gas-fired convection and indirect-resistance furnaces, with annealing times of the order of one-half to several minutes, to radiant and induction heating techniques which reduce heat treatment duration by one or several orders of magnitude, i.e., to times of the order of seconds or fractions of a second. The selection of the processing parameters for continuous lines is often done through an extensive series of laboratory trials in which the effects of variables such as heating rate and peak temperature on recrystallization are documented. The objective of the present work was to develop a theoretical basis for the description of recrystallization kinetics during continuous heat treatment operations. To this end, the recrystallization kinetics during very rapid heat treatments involving induction or direct-resistance heating were analyzed. To a great extent, these processes allow the study of the recrystallization process per se without the mitigating effects of grain growth following the completion of recrystallization. The starting point for the analysis of recrystallization
S.L. SEMIAT1N, Senior Scientist, is with the Metals and Ceramics Division, Wright Laboratory Materials Directorate, WL/MLLN, WrightPatterson Air
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