Effect of composition and process variables on Nb(C, N) precipitation in niobium microalloyed austenite

Abstract

Nucleation theory and the solubility product of niobium, carbon, and nitrogen in austenite have been used to derive equations for the start of Nb (C, N) precipitation as a function of temperature and composition. The predicted curves have been compared with the experimental observations of several authors to determine the effects of thermomechanical processing variables on the density of preferred nucleation sites and to incorporate these in the equations. Good agreement between the predicted and observed forms of precipitation curve is obtained with consistent constants in the equations when account is taken of the influence of different methods of detecting the onset of precipitation. Combining the calculated precipitation start curves with the dependence of recrystallization kinetics on composition and thermomechanical process variables when all niobium is in solution leads to prediction of the lower temperature limit for complete recrystallization and of the upper temperature limit for effective stoppage of recrystallization by precipitation. The predictions are in good agreement with observed results.

MST/495