Carburization of Steel: Why Use High Temp?

[Rafa]

The process of carburization works via the implantation of carbon atoms into the surface layers of a metal. As metals are made up of atoms bound tightly into a metallic crystalline lattice, the implanted carbon atoms force their way into the crystal structure.

The carburization is produce for difusion of C in the Steel. I read that temperature for carburization are around 900ºC.

The difusion of C depends of Fick'a law. For:
bcc Fe , the difusion of C is D=220x10^-6*exp(122000/RT)
fcc Fe , the difusion of C is D=20x10^-6*exp(142000/RT)

Then difusion in austenitic steel (fcc Fe) is lower than ferritic steel (bcc Fe)

¿Why do it use a higher temperature (900ºC) if the diffusivity in fcc-fe is lower than in bcc-fe?

 

[Matcon]

It boils down to obtaining a desirable microstructure than kinetics of diffusion.

Carbon is far more soluble in gamma iron than in alpha iron (where the maximum solubility is about 0,02%). If you make carbon react with alpha iron, you would expect to get a thin, brittle surface layer of iron carbide and, with longer times, large iron carbide precipitates just below the surface. This might be OK if you just want a hard surface but it would crack readily under a tensile load.

If your carbon is in solution, you can quench and temper your alloy to get a fine distribution of carbides and the properties obtained from this microstructure have a much wider engineering application.

 

[Matcon]

An additional thought. Whether or not you actually get iron carbide on the surface will, of course, depend on the temperature. Above the eutectoid temperature, the alpha iron transforms to gamma iron

 

[Rafa]

Thanks Macon, now I understand. The diffusion is lower but the solubility is higher and improves the posibility of treatment.