Formation and hardening of the diffusion layer by nitriding and nitrocarburizing - Iron-chromium-nitrogen and iron-chromium-carbon-nitrogen systems

The formation of ε and γ′ combination layers by thermochemical nitriding treatment of ferrous alloys is accompanied by nitrogen diffusion into the ferritic phase at temperatures below 570°C. A nitrogen concentration gradient is formed at the interface between the combination layer and the ferritic matrix. The formation of this nitrogen concentration gradient is accompanied by the precipitation of nitrides whose crystallographic structure, composition, morphology and distribution depend on the composition of the steel and the temperature at which nitrogen diffusion takes place. This precipitation, which establishes the nitrogen concentration gradient, results in the appearance of a hardness and compressive stress gradient, which in turn increases the mechanical strength of the surface and resistance to various fatigue phenomena. Knowledge of precipitation and hardening mechanisms is fundamental to optimize both the composition of steels (nature and concentration of alloying elements) and the parameters of nitriding treatment (composition of gaseous atmosphere, temperature, etc.), which condition nitrogen transfer to the steel surface.

The most recent knowledge of thermodynamic equilibria and matter transfer mechanisms, together with methods of microstructural study using high-definition electron microscopy, have been brought together to elucidate all these mechanisms and enable them to be presented in a way that is rational and useful for their understanding and practical use.