Development of the compound layer during nitriding and nitrocarburising of iron and iron-carbon alloys

The development of the compound layer during gaseous nitriding, gaseous nitrocarburising and salt bath nitrocarburising of Fe-based material is described. For gaseous processes the first nucleation of the compound layer at the surface depends on the competition between the dissociation of ammonia and the removal of nitrogen from the surface by solid state diffusion and desorption or competition with a carburising reaction.

For salt bath nitrocarburising the first phase to develop is iron carbide. During compound layer growth the kinetics are determined by a combination of solid state diffusion of interstitial elements and the kinetics of the surface reactions in the gas or the salt bath. Moreover, phase transformations within the compound layer play a role in determining the overall layer growth kinetics during nitrocarburising..