Computer simulation of VC
- PDF / 1,266,222 Bytes
- 13 Pages / 597 x 774 pts Page_size
- 42 Downloads / 282 Views
A computer simulation technique was developed on the basis of the interaction between carbide precipitation and moving y / a interfaces for predicting both the interphase precipitation kinetics and the microstructural evolution during austenite-to-ferrite transformation. Theoretical models for the calculation of the driving and pinning forces exerted on a moving interface boundary are described. The variations of the two forces lead to a phenomenon of periodic pinning and unpinning of the interface and, in turn, the characteristic microstructure of parallel sheet particles which is often associated with the interphase precipitation. The experimental data reported for a series of V-bearing steels were analyzed using the computer simulation technique. Three unknown physical parameters, i.e., the thickness of an incoherent interface or the height of the ledge of a coherent interface, the diffusion coefficient of V at the 7/a interface, and the coherence loss parameter of a VC nucleus, were determined. The calculated intersheet spacing, precipitate size, and precipitation start time all show a good correlation with the experimental observations.
I.
INTRODUCTION
DURING austenite-to-ferrite transformation in microalloyed steels, MI (M = Nb, Ti or V and I --- C or N) type precipitates often form at moving y / a interfaces, resulting in a characteristic microstructure of parallel particle sheets. It has long been recognized that such a microstructure can significantly affect both the strength and toughness of the material. For this reason, interphase precipitation has received extensive attention from metallurgists during the last 2 decades. Different mechanisms and models, including the ledge mechanism, m the quasiledge mechanism, m the bowing mechanism, t31 the eutectoid decomposition model, 141the solute depletion model, lSj the solute-drag nucleation model, 161 etc., have been proposed to interpret the unique microstructure. However, none of them provided a quantitative analysis. Only recently, attempts were made by Todd and co-workers to develop a quantitative model for precipitation at moving interphase boundaries, tT,s,91 This model is simply a diffusion analysis which treats the growth of the precipitates and ferrite grain simultaneously. Since other important factors, such as the nucleation of precipitates, were not considered, this model can only predict the ratio between the precipitate size and the intersheet spacing. Moreover, a number of assumptions and theoretical approaches in this model are questionable. For example, Todd and Su 191assumed that vanadium atoms redistribute between ferrite and austenite phases as the interface is moving through the material. This may only be true when the austenite-to-ferrite transformation rate is very slow at relatively high temperatures and is unlikely the case for the steels they examined, in which transformations were completed in less than a few tens of seconds, tl~ It has been widely agreed
W.J. LIU, Research Scientist, is with the Metals Technology Laboratories,
Data Loading...
