Nucleation kinetics of Ti carbonitride in microalloyed austenite
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I. I N T R O D U C T I O N
IT is widely known that carbonitride precipitation in microalloyed austenite takes place by diffusion controlled nucleation and growth. Understanding the nature of these processes is therefore important in manipulating the microstrncture of H S L A steels. For this reason, the growth and coarsening mechanisms of carbonitride precipitation have been extensively investigated, u-gJ By contrast, nucleation, the initial stage of the process, has received much less attention to date, both theoretically and experimentally. It is only recently that Dutta and Sellars u~ have proposed a kinetic model for Nb(CN) precipitation based on classical nucleation theory. Their model is consistent with the experimental precipitation-timetemperature (PTT) diagrams obtained by several authors using different methods and alloys, but has not been applied to Ti-modified steels. The aim of tile present study was to throw some light on the nucleation kinetics of Ti carbonitride in four grades of microalloyed steel. In earlier work by the present authors, Ti(CN) precipitation in deformed austenite was followed by both electron microscopy m and stress relaxation, m] The results showed that the relevant PTT diagrams are C-shaped, and that, in the temperature range above the nose of the C-curve, the precipitation rate is sensitive to Ti concentration. By contrast, the precipitation start time, Ps, at lower temperatures is relatively independent of the compositions of the steels tested. However, attempts to predict Ps times for Ti(CN) precipitation using conventional nucleation theory did not lead to satisfactory results. This is partly because, in the treatment of Dutta and Sellars, u~ for example, the chemical driving force for nucleation is simply related to the solubility product for the precipitating species. The latter is not known for Ti(CN) (although the expressions for TiC and TiN are both available) and, in any event, the driving force is not related in a simple way to the Ti(CN) solubility product because of the concentration W.J. LIU, Research Associate, and J.J. JONAS, Professor, are with the Department of Metallurgical Engineering, McGill University, 3450 University Street, Montreal H3A 2A7, Canada. Manuscript submitted M a y 23, 1988. METALLURGICAL TRANSACTIONS A
of N that occurs in the critical nucleus. For these reasons, several modifications, which are described below, were made to the classical theory. With the aid of these changes, reasonable agreement was obtained between the predictions and experimental observations. A number of the parameters governing the nucleation of Ti(CN) were also evaluated, and the observations are given a detailed physical interpretation below in terms of these quantities. II. THEORETICAL
TREATMENT
In terms of classical nucleation theory, the C-shaped PTT curves obtained experimentally suggest that the steady state nucleation rate J of Ti(CN) per unit volume can be described by the following equation: E12,13,143 J = Z/3*| e x p ( - A G * / k T )
[1]
Here Z is the Zel
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