4. Conclusion

The formation of combination layers (ε and γ′) on the surface of steels can be described using a thermodynamic approach to binary (iron-nitrogen) and ternary (iron-nitrogen-carbon) systems and knowledge of material transfer mechanisms relating to nitrogen diffusion in polyphase systems.

Thanks to work carried out over the last twenty years, thermodynamic databases have been enriched to such an extent that it is now possible to implement calculation codes (such as Thermo-Calc) that can predict and explain, with very high accuracy, the domains of existence of ε and γ′ phases on the surface of iron and steels.

Recent knowledge of material transfer mechanisms and control of boundary conditions at interfaces have enabled the development of numerical, or analytical, models of the growth kinetics of ε and γ′ combination layers.

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