Carburizing is a heat treatment process used to introduce carbon into the surface layer of a steel component, creating a high-carbon, hardened case while maintaining a relatively low-carbon, ductile core. The main reason carburizing is carried out in austenite and not in ferrite is due to the difference in carbon diffusion rates between the two phases.
Austenite is a high-temperature steel phase with a face-centered cubic (FCC) crystal structure. It has a higher carbon solubility than ferrite, a low-temperature phase with a body-centered cubic (BCC) crystal structure. The carbon diffusion rate in austenite is significantly higher than in ferrite.
In order to guarantee austenite formation, the steel component is heated during the carburizing process to a temperature higher than the transformation temperature, usually between 850°C and 950°C. The high carbon potential environment, usually achieved by introducing a carbon-rich gas atmosphere, allows carbon atoms to diffuse into the surface of the steel.
In austenite, the carbon atoms have a higher mobility due to the FCC crystal structure, allowing them to diffuse into the lattice rapidly. This results in a higher carbon concentration in the austenite phase than ferrite. As a result, the carbon atoms can more readily diffuse into the lattice and form iron carbides, such as cementite (Fe3C), which contribute to the formation of a hardened case during subsequent quenching.
Conversely, ferrite has lower carbon solubility and a slower carbon diffusion rate due to its BCC crystal structure. If carburizing were to be carried out in ferrite, the carbon diffusion would be relatively slow, leading to a lower carbon concentration and limited formation of iron carbides. This would result in a less effective case-hardening process.
Therefore, carburizing is primarily carried out in austenite to ensure efficient carbon diffusion and the formation of a hardened case with sufficient carbon content.