AMS 2759/6
Gas Nitriding and Heat Treatment of Low-Alloy Steel Parts
Gas nitriding of low-alloy steel parts (Nitralloy 135M, 4140, 4340, H11 and similar), including the prior hardening and tempering and the verification of case depth, surface hardness and white layer.
What AMS 2759/6 specifies
AMS 2759/6 covers conventional gas nitriding in dissociated ammonia at about 925–1050 °F (495–565 °C), single stage or two-stage (Floe) process, of low-alloy steels that form hard alloy nitrides: Nitralloy 135M (AMS 6470/6472), 4140, 4340, H11 and similar grades. Parts are hardened and tempered first, and the tempering temperature must sit above the nitriding temperature so the core does not soften during the 20–80 h cycle; surfaces must be clean, free of decarburization and, where required, activated. The sheet controls ammonia dissociation, case depth, surface hardness (over 900 HV on Nitralloy 135M, about 550–650 HV on 4140) and the brittle white (compound) layer, which the drawing may limit or require to be removed by grinding or lapping. Case depth is verified by a microhardness traverse (ASTM E384) on a coupon of the same alloy and prior condition processed with the lot; core hardness is re-checked afterwards.
Tests and the drawing call-out
Tests the document imposes
- Effective and total case depth by microhardness traverse per ASTM E384 on a lot coupon
- Surface hardness by ASTM E18 superficial scale (HR15N) or ASTM E384 with light load
- White (compound) layer thickness by metallographic cross-section at 400–500× (ASTM E3)
- Core hardness per ASTM E18 to confirm the prior temper survived the cycle
- Ammonia dissociation and furnace records; pyrometry per AMS 2750
How to cite it
Drawing call-out
GAS NITRIDE PER AMS 2759/6, NITRALLOY 135M PREHARDENED 28–34 HRC, CASE DEPTH 0.012–0.018 IN, SURFACE 90 HR15N MIN, WHITE LAYER 0.0005 IN MAX; MASK AREAS SHOWNAngle brackets mark the choices the drawing must make; every finish page shows a completed example.
Heat treatments specified to AMS 2759/6
| Treatment | Role | Cost |
|---|---|---|
Gas nitriding diffuses nitrogen into finished ferrous parts below the transformation temperature, producing a hard compound layer and diffusion zone without quenching. | Governing | $$$$ |
AMS 2759/6 — frequently asked
Which steels nitride well?
Steels containing aluminum, chromium, molybdenum or vanadium: Nitralloy 135M (about 1 % Al) reaches roughly 1000–1100 HV, H11/H13 about 1000 HV, 4140 and 4340 about 550–650 HV. Plain carbon steel forms only a thin, soft iron-nitride layer and is not what the sheet is written for.
How deep can the case be?
Typical cases are 0.008–0.025 in (0.2–0.6 mm) effective and take 20–80 hours because nitrogen diffuses slowly at 500–565 °C; deeper cases add hours quickly and thicken the white layer. Specify the depth you need at a stated hardness threshold, not the maximum the process can give.
What is the white layer and should it be removed?
The compound layer of epsilon and gamma-prime iron nitrides, about 0.0002–0.001 in (5–25 µm) thick, very hard but brittle; it helps wear and corrosion but can spall under point loads or bending. Fatigue-critical and gear-tooth surfaces usually limit it or grind it off; the two-stage Floe process (second stage at 65–85 % dissociation) keeps it thin.
Why must the prior temper be hotter than the nitriding temperature?
If the temper was lower than the nitriding temperature, the core keeps tempering during the long cycle, hardness drops and dimensions move. For 4140 that means tempering near 1050–1100 °F (565–595 °C) and accepting a core of about 28–34 HRC; a 45 HRC core cannot be kept under a 975 °F nitride.
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