Nitriding of White‐Solidified Fe–C–Si Alloys: Diffusion Path Concept Applied to Inhomogeneous Microstructures

The microstructure formation during nitriding of white‐solidified Fe–C–Si alloys, relevant to advanced surface treatments for cast irons, is studied. The as‐cast microstructures mainly contain Si‐free, C‐rich eutectic cementite and Si‐enriched, C‐lean ferrite/pearlite. The spatial extent of these microstructure regions is significantly larger than the diffusion distance of Si during nitriding. The phases and microstructure forming during nitriding vary with the local Si content in the substrate. Nitriding conditions only allowing to form γ′–Fe₄N from pure α‐Fe generate a compound layer composed of γ′–Fe₄(C, N), ε–Fe₃(C, N)_1+x , and Si‐rich nitride, X. ε is the main constituent, whereas γ′ only forms from Si‐free eutectic cementite. Different nitriding experiments elucidate the interrelated thermodynamic and kinetic effects of C and Si on the phase evolution and the compound layer growth. A schematic model of the microstructure evolution is developed. Information on the topology of the largely unknown Fe–C–Si–N phase diagram is derived from the experimental observations. For this, the diffusion path concept is adapted to describe the microstructure formation during nitriding of compositionally inhomogeneous alloys, in which substitutional alloying elements are heterogeneously distributed and largely immobile. α + ε and α + ε + X phase fields are identified as particular features of the Fe–C–Si–N phase diagram at 540 °C.