Carbon and nitrogen are the most important interstitial alloying elements in steel and other iron-base alloys. Superficial comparison of the metastable phase diagrams Fe-C and Fe-N implies strong similarities both with respect to the solid solutions and with respect to occurrence of intermediate carbides and nitrides.
The presentation will focus on the crystallographic, thermodynamic, kinetic (e.g. diffusivities) and other physical properties of the iron nitrides and carbides in view of recent experimental insight. Examples are the low-temperature mobility of N in e-Fe3N, the homogeneity range of Fe3C, and the elastic anisotropy of various nitrides and carbides. Of particular interest are the similarities and differences of the behaviour of N and C. Finally, it is shown how the detailed knowledge becomes relevant upon investigating the phase transformations occurring during gaseous nitriding of ledeburitic iron (e.g. surface-remelted cast iron). This nitriding leads, in particular, to transformation of ledeburitic and pearlitic cementite into iron (carbo)nitrides.