粒界セメンタイトの成長速度論にもとづく真空浸炭組織の計算予測

抄録

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 (α_Si) 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.