Composition of vanadium carbonitride precipitates in microalloyed austenite

Abstract

It is the purpose of this paper to present a model which describes equilibrium precipitation in austenite in the Fe-V-C-N system, and to discuss the implications of this model with regard to previous studies of precipitation behavior in this system. The equilibrium precipitates are taken to be compositionally homogeneous, and no attempt is made to calculate the initial precipitate composition or to describe the approach to equilibrium. The model which we propose assumes that the excess free energy of mixing of VC and VN can be described using the regular solution model developed by Roberts et al. At a given temperature, T, the steel, matrix, and precipitate compositions must lie on a common tie line (which will occur if certain mass-balance relationships are satisfied). Assuming that the vanadium carbonitrides are perfectly stoichiometric (i.e., VC /SUB x/ N /SUB 1-x/ )*, there are five unknowns for which to solve: the equilibrium precipitate composition (x), the precipitate mole fraction (f), and the equilibrium levels of vanadium, carbon, and nitrogen ((V /SUB m/ ), (C /SUB m/ ), and (N /SUB m/ )) in the austenite matrix. The equilibrium condition is determined by solving simultaneously the following nonlinear equations: In summary, a thermodynamicmore » model has been developed which describes equilibrium carbonitride precipitatin behavior in vanadium steels. Results obtained using the new model deviate substantially, in some cases, from those which have been predicted previously.« less