Carburization kinetics of heat-resistant steels

A series of 21 commercial austenitic heat-resistant alloys was carburized in H₂-C₃H₆ atmospheres which were reducing to Cr₂O₃ and SiO₂ and which provided a carbon activity of one. Reactions were carried out at 900T1100°C and the resulting depth of carburization was measured metallographically. All alloys showed parabolic carburization kinetics after an initial brief period, at low temperatures, of more rapid reaction. The apparent activation energies for carburization are discussed in terms of carbon solubilities and diffusion coefficients. It is concluded that Wagner's theory of internal oxidation is quantitatively applicable to simple alloys but not to alloys which contain additions of reactive elements. The effectiveness of additions of niobium, aluminum, titanium, and silicon is demonstrated and is discussed in the context of Wagner's theory.