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Lesson 06·Process variants

FNC vs nitriding — stop confusing them

How carbon in the atmosphere changes the compound layer, why FNC owns stamping and powder-metal tooling, and how to tell the two processes apart on a quote and a microsection.

8 min readLesson 6 of 12

Tying it together

What this means on the shop floor

  • Stamping dies on D2, A2, or pre-hard 4140 running galvanized, aluminized, or stainless stock: FNC is the default. Salt-bath if the volume is high and the part is rough enough to tolerate a salt-handling shop; gas FNC if cleanliness or geometry rules out salt. Cap the compound layer at 15 µm for blanking, 20 µm for drawing.
  • Powdered-metal compaction tooling: FNC with a 10-15 µm ε compound layer is the standard, often followed by a light polish for surface finish without removing the compound layer.
  • Forge dies (H13, H11, Nitralloy): straight gas or plasma nitriding with a controlled compound layer, not FNC.
  • Plastic molds going to a polished finish or a PVD coater: straight plasma nitriding at low Kn, with the compound layer removed before coating.
  • Aluminum extrusion dies: straight gas or plasma nitride with a controlled compound layer, then a light bearing-surface polish. FNC is occasionally used for low-volume extrusion work; vendor practice varies.

Pushback questions for the vendor

  1. Is your atmosphere carbon-bearing? If yes (CO₂, propane, endogas, or salt-bath cyanate), this is FNC, not pure nitriding. Confirm the carbon source and concentration.
  2. Are you running NHT (nitriding hardness depth) to a hardness target, a phase target, or both? An FNC house should commit to ε-phase purity. A straight nitriding house should commit to a Kn-controlled phase.
  3. What is the target compound layer phase, and how do you verify it: optical metallography, XRD, EDX, or witness-coupon hardness only?
  4. If this is FNC, do you run a post-oxidation step (QPQ) and is it specified on the certificate?

A vendor who answers all four is running a process. A vendor who says "we just nitride it" is running a furnace.

Common confusions

"Nitriding" used generically for any sub-590°C diffusion treatment of nitrogen-bearing atmosphere. Vendors and buyers do this constantly. SAE J2745 and AMS 2759/10 keep nitriding separate; AMS 2759/12 covers controlled gaseous FNC specifically. If the PO references AMS 2759/10, the deliverable is straight nitriding. If it references AMS 2759/12, the deliverable is FNC with potential control (Kn and Kc). If it references neither, the deliverable is whatever the vendor's marketing department writes on the certificate.

Tufftride, Tenifer, Melonite, Nitrotec, Nitemper, Sursulf, and Arcor are all FNC variants under trade names. Tufftride and Tenifer are the original Degussa/Durferrit cyanate salt-bath processes. Melonite is the Houghton/HEF licensee version, often run as QPQ. Nitrotec is a gas FNC variant (Lucas/BOC). The chemistry differs at the margin, the operating temperature is 570-580°C, the result on the part is an ε-dominant compound layer with a porous outer zone over a shallow diffusion zone. If the quote uses any of these trade names, you are buying FNC regardless of what the line item says.

"Liquid nitriding" almost always means salt-bath FNC, not straight nitriding. The salt chemistry inevitably introduces carbon from the carbonate, so the compound layer is ε-Fe₂₋₃(N,C) even if the vendor calls the process "nitriding."

Up next: reading a microsection.

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