Calculation of nitrogen depth profiles in nitrided multi-component ferritic steel

Abstract

Nitrogen depth profiles in steels that were subjected to a nitriding treatment can be calculated both on the basis of precipitation and on the basis of trapping. Nitrogen depth profiles calculated on the basis of precipitation agree reasonably well with experimentally observed depth profiles, except for the occurrence of a tail advancing the main nitriding front, which is obtained in the calculations, but is not observed in the experiment. In calculations using a trapping model presented in the present paper the tail is absent, which suggests that for an accurate description of nitrogen diffusion in steel during nitriding trapping should be incorporated. Comparison of experimental nitrogen depth profiles in En40B, X38CrMoV5-1 and En19 steel with trapping model calculations shows that in general the trapping model is too simple to accurately describe the diffusion of nitrogen during nitriding, although for some steels already a very good agreement is obtained. The validity of the trapping model is discussed in view of the complicated microstructural changes occurring during the nitriding of steel. A realistic model to calculate nitrogen depth profiles in nitrided steel should incorporate the complicated precipitation sequence during nitriding, including both trapping and precipitation.