On the Nature of the Increased Propensity of Chromium Steels to Carbide Formation During Cementation

The purpose. Investigation of the propensity of chromium steels to carbide formation during cementation and determination of the factors determining the increased propensity of chromium steels to carbide formation.

Methods. For the study, 12 melts of 0.5 kg of iron alloys with manganese, chromium and vanadium were melted in an induction furnace. The charge was technically pure iron and ferroalloys of these metals. Cementation was carried out in a wood-coal carburetor (grain size 1...3 mm) containing 30% of VaSО₃. Cementation mode: 920°C - 6 h, air cooling in an unpacked container.

Results. It is noted that the release of cementite in the granular form is facilitated by large positive deviations from the ideal in solid solutions of chromium in iron. The activity coefficient of chromium in Fe-Cr alloys is much higher than unity and is at 1000°C for alloys containing less than 10% Cr, 2.38. A large value of the chromium activity coefficient indicates a strong tendency for the separation of Fe-Cr solutions into two phases and the formation of chromium fluctuations, on which the nucleation of carbides is facilitated. Manganese stabilizes cementite so weakly that the latter, as in unalloyed austenite, cannot form in a granular form. Vanadium, in contrast to chromium and manganese, does not stabilize cementite, since it practically does not dissolve in it (the solubility of V in Fe₃C is only 0.4...0.5%, and forms a more stable carbide VC compared to chromium carbides. For this reason, Fe₃C is not formed in vanadium steel during carburization, and the total content of the carbide phase formed during carburization is determined by the content of carbide particles VC, which is determined by the content of vanadium in the steel.

Conclusion. The increased tendency of chromium steels to carbide formation during cementation is due to the stabilization of cementite with chromium. As a result of the dissolution of chromium, the value  ΔG₀ of cementite increases so much that it becomes energetically possible to form it in a granular form. The formation of cementite in granular form in chromium steels is also facilitated by a significant value of the coefficient of thermodynamic activity of chromium in austenite.

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