EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Nitriding of cast iron can improve mechanical surface properties. The non-nitridable graphite, however, causes heterogeneities in the nitride layer which limit improvements in corrosion resistance usually achievable upon nitriding of other Fe-based alloys. A strategy to overcome this problem for cast iron is surface remelting to form a white-solidified surface layer and subsequent nitriding.
This contribution reports the fundamental insights gained by microstructure investigations on nitrided Fe-Si-C alloys but also C-free variants and variants containing Cu and/or Mn. In case of C-containing alloys, ledeburitic cementite is Si-free, whereas the remaining pearlite is enriched in Si. Occasionally also Fe-Si carbide Fe23Si5C4 is formed. Since Si diffusion is only very short-range at the nitriding temperatures of 540°C, the heterogeneity in the Si distribution present after remelting is retained during nitriding. Whereas Si-free regions basically behave like Fe-C alloys upon nitriding, the behavior of the Si-enriched regions is much more complicated. Si in Fe is regarded as a “weak nitride former”, leading to nanoscopic precipitates of an amorphous nitride of composition classically believed to be Si3N4. In the course of our studies, it was made likely that the amorphous nitride referred to as X has a composition (Fe3N2)x(Si3N4)1-x with the Fe content increasing with the chemical potential of N, implying metastable local equilibrium between matrix and the amorphous nitride.
The slow formation of the amorphous nitrides also affects type and morphology of the Fe nitride, into which the Si-depleted Fe matrix transforms. Remaining Si appears to promote formation of the epsilon iron nitride at the cost of the gamma' iron nitride, an effect which is enhanced by remaining carbon. Presence of Cu, Mn or Cu+Mn change the situation characteristically which can be rationalized in terms of stabilization of epsilon or gamma' or due to acceleration of the precipitation of the amorphous nitride X.