Diffusion

� At 927 C (1200 K) iron is fcc Austenite (γ) for which: D(1200) = 1.5 x 10^-11 m² sec^-1. This gives a penetration depth = 2.94 x 10^-3 m, for t = 10 hrs.
� At 1200 K the maximum solubility of carbon in Austenite is 1.3 weight %. This limits the surface carbon concentration in Austenite to this value and makes the process relatively insensitive to carbon carrier gas pressure.
� The surface region will have second phase Fe₃C, iron carbide, present in the Austenite matrix if this carbon concentration limit is passed. The iron carbide is a brittle ceramic phase.

From: Callister,
"Materials Science & Engineering," Wiley (1994)