Calculation of incubation time for compound layer formation during gaseous nitriding of tempered martensitic steels

A compound layer comprised of ε and γ’ iron nitrides can form at the surface of tempered martensitic steels during the gaseous nitriding process. The compound layer is formed when the nitrogen surface concentration in the alloy reaches the ‘critical value’. This paper presents a method to calculate the incubation time for compound layer formation for several steels being nitrided with various nitriding process parameters of temperature and K_N. The time dependent nitrogen surface concentration is calculated by applying the nitrogen flux balance boundary condition and solving Fick’s second law at the alloy surface. The key input parameters to the model are the nitrogen flux from the atmosphere and the nitrogen diffusivity in the steel. The ‘critical value’ at the alloy surface is determined from the thermodynamically calculated alloy specific Lehrer diagram and alloy isopleth. The incubation time is the time at which the nitrogen surface concentration reaches the ‘critical value’. The method of determining the temperature and K_N dependent nitrogen flux from the atmosphere into the steel is discussed. The calculated incubation time predictions are compared with experimental results.