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Abstract: The dissolution of Fe3C in ferrite has been shown to be controlled by a slow first order interfacial reaction. The rate constant,K, is Arrhenius in behavior and involves two activation enthalpies; one applies to a jump process with a short relaxation time, another to a long relaxation time process which is tentatively identified as a change in kink density on ledges in the interface. The heat of solution of Fe3C in ferrite also appears in the exponential. A single process, activated jump theory is not adequate to explain the behavior ofK with temperature; a dissolution model based on a ledge-double kink mechanism appears to be in agreement with experiment. These features point to significant differences between the mechanism of interphase boundary migration and grain boundary migration. In addition, the Fe3C-ferrite interface is felt to be at equilibrium with respect to carbon but not iron, leading to small departures from stoichiometry in the Fe3C at the interface.

The rate of dissolution of spheroidized Fe3C in ferrite in both high purity and commercial purity plain carbon eutectoid steels has been investigated. The experiments consisted of equilibrating wire specimens of spheroidized eutectoid steels at 650°C or 700°C and then jumping the temperature (in 2 to 7 m-sec) 4 to 32 C deg above the equilibration temperature. After heating, the specimens were maintained constant at the new temperature, and the dissolution of Fe3C was followed by observing the changes in electrical resistivity. The data are correlated to changes in the average carbon concentration in ferrite and are compared to the predictions of a generalized phenomenological model. Two important facts have been determined. First, the Fe3C ferrite interfaces are not in a state of local equilibrium during dissolution; the deviations from equilibrium in all cases were large. Second, the kinetics of dissolution are first order and the log of the reaction rate constants are a linear function of (1/T), whereTis the absolute temperature. These results lead to the conclusion that under the present experimental conditions, the dissolution of Fe3C in ferrite is controlled entirely by an interfacial reaction. A rather detailed description of the experimental procedure is included in this paper because the technique is new, and it is broadly applicable in the study of many metallurgical phenomena.