FNC vs nitriding — stop confusing them

Typical FNC compound layer is 8-25 µm. That is thicker than a controlled straight nitride compound layer (5-15 µm typical) because FNC cycles aim for compound-layer formation as the primary product rather than the diffusion-zone development that drives long nitriding cycles. Surface microhardness on the compound layer is HV 0.05 in the range 800-1100, depending on substrate and phase purity. Case depth (diffusion zone, NHD per DIN 50190-3) is 0.05-0.30 mm. That is shallower than a long-cycle straight gas nitride (0.30-0.60 mm) because FNC cycles run shorter: 90 minutes to 6 hours for salt-bath, 2-8 hours for gas FNC. The short cycle is the reason FNC dominates the high-volume end of the work.

Corrosion resistance is the other lever. A bare FNC compound layer holds 24-72 hours of salt spray (ASTM B117). QPQ (FNC plus a post-oxidation pass that grows Fe₃O₄ on the compound layer and seals the porous zone) holds 200-600 hours and approaches hard chrome on neutral salt spray, at a fraction of the chrome environmental load. The post-oxide step is what differentiates QPQ from plain FNC. Lesson 4 covers the salt-bath chemistry in detail.