2011 Volume 97 Issue 3 Pages 130-135
In vacuum carburizing of steels, a short-time carburizing is usually followed by a diffusion period to eliminate the film-like cementite (θ) grown on austenite (γ) grain boundary surface. Due to a radical reaction with decomposed hydro-carbon gas, the surface C content in γ was found higher than the solubility of θ. It was recognized as the metastable equilibrium of supersaturated γ with graphite until the θ covered all the surfaces. Based on this finding, a calculation model predicting not only C concentration profiles but also quantitatively the grain-boundary θ has been developed in this work.
The existing model estimates the amount of θ with the equilibrium fractions for local C contents in a framework of finite difference method (FDM). Even with the equilibrium surface condition of γ plus θ with graphite, it overrates the amount of θ observed after several minutes of carburizing. In the developed model, a parabolic law was assumed for the thickening of θ, and the rate controlling process was considered Si diffusion rejected from θ under the isoactivity condition of γ/θ interface and supersaturated γ. The rate coefficient (αSi) has been validated using multicomponent diffusion simulation for moving velocity of the γ/θ interface. A 1-D FDM program calculates an increment of θ for all gridpoints using the updated diffusivities and local equilibrium by a coupled CALPHAD software. Predictions of carbon profile and volume fraction of cementite represent the experimental analysis much better than the existing models especially for short-time carburizing.