Calculation of microstructure in vacuum carburizing incorporating kinetics modeling of grain-boundary cementite

Abstract

In the vacuum carburization of steels, short-time carburization is usually followed by a diffusion period to eliminate the filmy cementite (θ) grown on austenite (γ ) grain boundaries. The surface C content in γ was found to increase past the solubility of θdue to radical reaction with decomposed hydro-carbon gas. The thermodynamic condition was recognized as the metastable equilibrium of supersaturated γ with graphite. Based on these results, a calculation model for not only C concentration profiles but also quantitative evaluation of grain-boundary θwas developed. The existing model estimates the amount of θwith the equilibrium fractions for local C contents in a framework of the finite difference method (FDM). However, with the corresponding surface condition of equilibrium γ plus θwith graphite, it overrates the amount of θobserved after several minutes of carburization. In the developed model, a parabolic law was assumed for the thickening of θand a rate controlling process was considered for partitioning of Si. The local Si concentrations at the γ /θinterface were determined from the C isoactivity condition with supersaturated γ . The change in the rate coefficient (α Si) was also validated using multicomponent diffusion simulations. Coupled with the CALPHAD software, a onedimensional FDM program calculates the increment of θusing the updated local equilibrium. Predictions of the C profile and θvolume fraction corresponded much better with the experimental results than existing models. © 2012 ISIJ.