Simulation of V(CN) Precipitation in Steels Allowing for Local Concentration Fluctuations

Abstract

From a “macroscopic” point of view, steel composition is assumed to vary smoothly along its microstructure. A closer look reveals that, at the atomic level, the material composition does not change so smoothly. Single atoms jump randomly along the crystal lattice due to their thermal energy, therefore creating sporadic zones of the crystal with higher concentration of certain elements. This composition fluctuations are responsible of many phenomena, such as precipitation, Ostwald ripening, some phase transformations. This work proposes a model to simulate the precipitation of V(CN) precipitates in microalloyed steels in the range of warm temperatures (800–900°C); when the matrix is fully austenitic (fcc), and taking into account for local composition fluctuations. The model works by dividing the space into very small cells, each containing a single fcc atomic cell. If during the random movement of atoms a small group of adjoining cells reach some critical composition, a nucleus of V(CN) appears. At the same time, if a cell touching an already existing precipitate reaches some critical vanadium composition, it is very easy to stick it to the precipitate by changing its “phase”. But it is also possible that some atoms escape from the precipitate by jumping to the austenitic matrix. The model considers both processes happening simultaneously, and which one is leading depends on the atoms energy, i.e. system temperature, leading to different possible situations, nucleation phenomena, Ostwald ripening or dissolution of precipitates.