AMS 2759/12
Automated Gaseous Nitrocarburizing Controlled by Potentials
Ferritic nitrocarburizing of steel parts in ammonia plus a carbon-bearing gas, with the nitriding potential (KN) and carburizing potential (KC) controlled automatically, including optional post-oxidation.
What AMS 2759/12 specifies
AMS 2759/12 covers ferritic nitrocarburizing (FNC) run under closed-loop atmosphere control: dissociated ammonia with a carbon source (carbon dioxide, endothermic gas or natural gas) at about 1000–1075 °F (540–580 °C), below the ferrite-to-austenite transformation, with the nitriding potential KN and the carburizing potential KC measured and held to set-points. The result is an epsilon carbonitride compound layer, typically 10–25 µm (0.0004–0.001 in) thick, over a nitrogen diffusion zone; the compound layer gives scuffing and corrosion resistance on plain carbon and low-alloy steels, powder-metal parts and cast iron, while alloy steels also gain a hardened diffusion zone. The sheet specifies pre-cleaning and prior heat-treatment requirements, coupon testing, and acceptance of compound-layer thickness and porosity, surface hardness and diffusion depth by metallography and microhardness traverse. An optional post-oxidation step produces the black magnetite finish of the oxidised-FNC family (QPQ, black nitriding) with salt-spray resistance far above plain nitriding.
Tests and the drawing call-out
Tests the document imposes
- Compound layer thickness and porous-zone fraction by metallographic cross-section at 500–1000× (ASTM E3 / E407)
- Diffusion-zone depth by microhardness traverse per ASTM E384
- Surface hardness per ASTM E18 superficial scale (HR15N) or ASTM E384 with light load
- Core hardness per ASTM E18
- KN / KC controller records, sensor calibration, pyrometry per AMS 2750
- Neutral salt spray per ASTM B117 when post-oxidation corrosion resistance is specified
How to cite it
Drawing call-out
NITROCARBURIZE PER AMS 2759/12, 1018, COMPOUND LAYER 0.0004–0.0008 IN, SURFACE 80 HR15N MIN, POST-OXIDIZE BLACK; MASK THREADSAngle brackets mark the choices the drawing must make; every finish page shows a completed example.
Heat treatments specified to AMS 2759/12
| Treatment | Role | Cost |
|---|---|---|
Ferritic nitrocarburizing diffuses nitrogen and carbon into ferrous surfaces below the transformation temperature. | Governing | $$$ |
AMS 2759/12 — frequently asked
How does nitrocarburizing differ from nitriding?
Both diffuse nitrogen below the transformation temperature, but FNC adds carbon so the compound layer forms as tough epsilon carbonitride even on plain carbon steel, which nitriding alone hardens only weakly. FNC runs short (about 1–6 h) and aims at the compound layer for wear and corrosion; nitriding runs 20–80 h and aims at a deep hardened diffusion zone on alloy steels.
What does potential control add?
KN and KC set the phase of the compound layer: too much carbon gives cementite and porosity, too little nitrogen gives a thin layer. Controlling both from gas analysis instead of fixed flow ratios makes the layer thickness repeatable within a few micrometres and lets the certification carry measured process variables — the reason the automated sheet exists alongside older manual practice.
Does the part change size?
Growth is small — about 0.0001–0.0003 in (2–8 µm) per surface from the compound layer and nitrogen uptake — and distortion is low because the part never transforms and is cooled slowly, in oil or in nitrogen. Thin sections can still move from relief of machining stress, so stress relieve before finish machining.
Why post-oxidize?
A thin magnetite (Fe3O4) film on the porous outer epsilon layer, often after a light polish and re-oxidation, raises neutral salt-spray resistance from tens of hours to several hundred hours and gives a uniform black finish. This is the QPQ / black-nitride route used on shafts, hydraulic pistons and firearm parts.
AMS 2759/12 is copyrighted by SAE International. This page explains the document so a drawing can be read; it does not reproduce the standard. Purchase the current edition from the publisher. Official source ↗