When the cycle has commenced, the development of the nitrided case begins with the nucleation of gamma-prime into small “islands” at the interface between the ammonia atmosphere (decomposing) and the component being nitrided. This is followed with a similar formation, only this time it is by the nucleation of “islands” of epsilon nitride until there is a buildup at the surface of the complete compound layer of the two phases.
Nitrogen (from the decomposition of the ammonia) diffuses through the forming compound layer to react with the alloying elements to form the stable nitrides. The morphology of the forming compound layer is being controlled by the amount of carbon present in the steel or the amount of carbon present in the ammonia atmosphere (FNC).
There is generally some porosity associated with the surface compound layer of a gas-nitrided component, and there is not yet a very clear understanding of the formation of this porosity. It is generally thought to be due to pressure from the atomic nitrogen in the epsilon phase recombining with other atomic nitrogen to form molecular nitrogen in the surface layer. This creates a high enough pressure within the compound layer to cause small microscopic high-pressure bursts in the compound layer.
