Nitrocarburize (Ferritic)
Case & surface hardening · Thermochemical diffusion
Ferritic nitrocarburizing diffuses nitrogen and carbon into ferrous surfaces below the transformation temperature.
FNC at a glance
Four scores, each 0–100 against every heat treatment in this catalog. Hardness is derived from the HRC target (20 HRC = 0, 70 HRC = 100) — treatments without a hardness target (anneals, stress relief, HIP, cryo) are not rated. Toughness, dimensional stability and cost are FabDigit's engineering assessment.
What is FNC?
Ferritic nitrocarburizing diffuses nitrogen and carbon into ferrous surfaces below the transformation temperature. It creates a thin hard compound layer over a diffusion zone for wear, scuffing, fatigue, and—when oxidized—corrosion resistance.
How it works
Steel is held in gas or salt near 560–590 °C, below the austenite transformation range. Nitrogen is the principal diffusing species while carbon stabilizes an epsilon iron-carbonitride compound layer. Beneath the 5–25 µm compound layer, dissolved nitrogen and alloy nitrides form a diffusion zone. A QPQ route adds oxidative quenching, polishing, and re-oxidation to create a dark porous oxide over the compound layer. Because no martensitic quench is required, core properties and dimensions change little. Potential control, time, alloy, and post-oxidation determine layer phase, porosity, depth, and corrosion behavior.
Why specify it
- Uses a short 2–6 hour hold compared with deep gas nitriding
- Produces about 450–1200 HV depending on steel
- Creates a 5–25 µm wear-resistant compound layer
- Provides roughly 0.05–0.80 mm diffusion depth without a transformation quench
Where it falls short
- Case is much shallower than carburizing
- Compound-layer porosity and thickness require explicit control
- A 560–590 °C cycle can soften previously low-tempered steel
- QPQ corrosion performance depends on polishing and oxidation details, not FNC alone
The FNC cycle
Typical furnace parameters from the governing specifications and shop practice; the heat treater sets the exact recipe from the alloy, section size and required result.
| Temperature | 560–590 °C (1,040–1,094 °F) — Ferritic gas FNC window; salt-bath QPQ commonly uses 580 °C. |
|---|---|
| Soak | 1–6 h — Salt-bath QPQ commonly holds 1–2 h; gas FNC commonly 2–6 h. |
| Cooling | Salt bath |
| Atmosphere | Ammonia / carbon-bearing gas or salt bath |
| Door to door | 4–12 h — Full QPQ sequence is longer than the nitrocarburizing hold. |
Stages
- 1Preclean and preheat
- 2Nitrocarburize below Ac1
- 3Oxidative quench when specified
- 4Polish for QPQ
- 5Re-oxidize and seal when specified
What FNC does to the part
| Hardness | 45–65 HRC — Approximate equivalent only; published FNC capability is about 450–1200 HV by alloy. |
|---|---|
| Case depth | 0.05–0.8 mm — Nitriding hardness depth to core hardness plus 50 HV; compound layer itself is typically 0.005–0.025 mm. |
| Toughness | The core remains in its prior condition; a thick porous compound layer is brittle and unsuitable for severe impact. |
| Distortion | Low risk. No austenitizing quench occurs, though thermal stress relief and predictable growth remain. |
| Surface after treatment | Matte gray after FNC or dark gray to black after oxidized QPQ; polishing changes gloss and bearing ratio. |
| Machining sequence | Finish machine first; QPQ includes controlled polishing, but later grinding can remove the 5–25 µm compound layer. |
Specifications for FNC
The documents a drawing cites to define this treatment. Each links to a page explaining the standard, its classes and the tests it invokes.
| Standard | Role | Classes used | Status |
|---|---|---|---|
Automated Gaseous Nitrocarburizing Controlled by Potentials | Governing | — | Active |
Test and related methods: ASTM E384 (Standard Test Method for Microindentation Hardness of Materials), AMS 2750 (Pyrometry), ISO 15787 (Technical product documentation — Heat-treated ferrous parts — Presentation and indications).
Drawing call-out
FERRITIC NITROCARBURIZE PER AMS 2759/12; 700 HV0.5 MIN SURFACE; 0.15–0.30 MM NHD TO CORE + 50 HV ON SURFACES SHOWN; COMPOUND LAYER 0.010–0.020 MMWhich alloys take FNC?
Verdict per material family, with the grades that behave differently. Each family links to its datasheets in the material library. Typical grades: 1015, 1018, 1045, 4140, 4340, H11, H13, 42CrMo4, Ductile iron ASTM A536.
| Material family | Verdict | Notes |
|---|---|---|
| Yes | Plain-carbon, alloy, tool, and nitriding steels are widely processed.
| |
| With care | Sintered ferrous parts can respond, but interconnected porosity changes uptake. | |
| With care | Dense AM steels can be treated after core heat treatment and porosity are validated.
| |
| With care | Some grades can be treated, but chromium-nitride formation can reduce corrosion resistance.
| |
| No | FNC is a ferrous thermochemical treatment. | |
| No | Brass is unsuitable at the FNC process temperature and chemistry. | |
| No | Bronze is outside ferritic FNC practice. | |
| No | Cobalt-chrome does not form a ferritic iron-carbonitride layer. | |
| No | Copper alloys do not form the intended iron-carbonitride layer. | |
| No | Magnesium is outside the metallurgy and bath-safety scope. | |
| No | Nickel alloys are outside conventional ferritic FNC metallurgy. | |
| No | Precious metals do not form the intended FNC case. | |
| No | Refractory metals require unrelated nitriding routes. | |
| No | Titanium requires a different nitriding process. | |
| No | Tungsten heavy alloys are not FNC substrates. | |
| No | The process temperature exceeds zinc alloy capability. |
Design rules for FNC
What to decide on the drawing before the part goes to the heat treater.
- Specify compound-layer thickness separately from NHD.A 0.015 mm compound layer and a 0.25 mm diffusion zone serve different functions.
- State whether oxidation or the full QPQ sequence is required.FNC alone does not guarantee the dark oxide or its corrosion benefit.
- Temper the core above the FNC temperature.A 580 °C hold can soften steel previously tempered below that temperature.
- Avoid heavy finishing after treatment.Grinding 0.02 mm can remove the entire compound layer.
- Define acceptable compound-layer porosity for loaded contacts.Porous outer nitride can initiate spalling under high Hertzian stress.
FNC versus the alternatives
Treatments engineers weigh against FNC, and when each wins.
| Treatment | Hardness | Toughness | Stability | Cost | Choose it when |
|---|---|---|---|---|---|
FNC this page | 68 | 55 | 88 | $$$ | — |
| 85 | 50 | 85 | $$$$ | when a deeper diffusion case and higher alloy-steel hardness are required | |
| 82 | 55 | 35 | $$$ | when deep load-bearing case depth is required |
Cost, lead time and applications
Relative cost
Nitrocarburize (Ferritic) is a premium-cost treatment ($$$) relative to the other heat treatments in this catalog.
Heat treatment is priced per lot with a furnace minimum. The exact figure appears on the instant quote once the part is uploaded.
Lead time
| Added lead time | 4–8 working days |
|---|---|
| Furnace time | 4–12 h |
What FNC is used for
- Hydraulic piston rods
- Automotive crankshafts
- Gears and sprockets
- Firearm components
- Stamping and forming tools
- Valve and pump parts
FNC — frequently asked
How thick is the FNC layer?
The compound layer is commonly about 5–25 µm. The underlying NHD is larger—often 0.05–0.80 mm depending on alloy and cycle—and must be stated separately.
What temperature is ferritic nitrocarburizing?
Gas FNC commonly runs at 560–590 °C, while salt-bath QPQ commonly uses 580 °C. These temperatures remain below Ac1, so there is no austenite-to-martensite quench.
How long does FNC take?
Gas holds are commonly 2–6 hours. A salt-bath QPQ nitrocarburizing step is often 60–120 minutes, with extra time for preheat, oxidative quench, polish, and re-oxidation.
How hard is FNC?
Published commercial ranges are roughly 450–1200 HV, controlled strongly by alloy nitride formers. A plain 1018 surface is softer than H13 under a comparable cycle.
Is FNC the same as carbonitriding?
No. FNC runs in the ferritic range near 580 °C and does not require a martensitic quench; carbonitriding runs around 800–900 °C in austenite and is quenched.
What does QPQ mean?
Quench-Polish-Quench adds an oxidative quench, mechanical polishing, and a second oxidation step around the FNC treatment. It is used to combine wear behavior with a dark corrosion-resistant surface.