FNC vs nitriding — stop confusing them

Ferritic nitrocarburizing is nitriding with a carbon source in the atmosphere. In gas FNC the carbon comes from CO₂ (1-3%), propane, or endogas blended with NH₃. In salt-bath FNC the carbon comes from cyanate and carbonate in the molten salt (typically a KCNO / K₂CO₃ system at 570-580°C). The result is the same: carbon enters the compound layer along with nitrogen and stabilizes the ε phase. The compound layer becomes ε-Fe₂₋₃(N,C) instead of Fe₂₋₃N, and the Lehrer diagram boundary that normally separates ε from γ' shifts so that ε forms at lower nitriding potentials than it would in a carbon-free atmosphere. A well-run FNC cycle produces a single-phase ε compound layer with little or no γ' sublayer, which removes the brittle ε/γ' interface that causes spalling in poorly controlled straight nitriding (Lesson 2).

The diffusion zone underneath is similar to a straight nitrided zone: interstitial nitrogen plus, on alloy steels, fine alloy-nitride precipitates. Carbon largely stays in the compound layer and does not contribute meaningfully to diffusion-zone hardness on most substrates.