Relation between phase composition of compound zone and growth kinetics of diffusion zone during nitriding of steel

The present work is devoted to an investigation of the influence of the micro-structural evolution of the compound zone (iron (carbo)nitrides zone) upon the development of hardness profiles in the diffusion zone. A different phase structure of iron (carbo)nitrides zone on steel as compared to iron, further changing with the process, may result in upsetting of the quasi-equilibrium of nitrogen concentration in the iron (carbo)nitrides zone/diffusion zone interface. This, as a result, may have an impact on the kinetics of this layerʹs growth. The research carried out was aimed at solving this problem. It was to evaluate the influence of (carbo)nitrides zone, with an intentionally created diametrically different phase composition, on hardness profiles in the diffusion zone. The nitrided layers were characterised by a light optical microscopy, X-ray diffraction and measurements of hardness. It was observed that the evolution of the phase structure and phase composition of the compound zone contributes significantly, regardless of nitrogen potential and temperature, to the formation of the diffusion zone and in particular to its effective thickness. It makes this complex picture of nitrided case development on steel even more intricate.