Gas Nitride
Case & surface hardening · Thermochemical diffusion
Gas nitriding diffuses nitrogen into finished ferrous parts below the transformation temperature, producing a hard compound layer and diffusion zone without quenching.
Gas nitride 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 Gas nitride?
Gas nitriding diffuses nitrogen into finished ferrous parts below the transformation temperature, producing a hard compound layer and diffusion zone without quenching. It is chosen for wear, fatigue strength, and dimensional stability on alloy, tool, and nitriding steels.
How it works
Clean, previously hardened and tempered steel is exposed to dissociated ammonia, usually between 495 and 565 °C. Nitrogen enters the ferritic matrix and reacts with aluminum, chromium, molybdenum, and vanadium to form fine alloy nitrides. The outer compound or white layer contains iron nitrides; beneath it, the diffusion zone carries the useful hardness gradient and compressive stress. Nitriding potential, temperature, and time control phase balance and depth. Because there is no austenitizing transformation or quench, the core microstructure remains unchanged and movement is low, provided the prior tempering temperature exceeds the nitriding temperature.
Why specify it
- Produces about 600–1200 HV depending on alloy
- Runs below the steel transformation range with no quench
- Provides about 0.10–0.50 mm nitriding hardness depth on 4140 and nitriding steels
- Improves wear and bending-fatigue performance through a hard compressive case
Where it falls short
- Cycles commonly run 10–80 hours and deep cases can require longer
- Best hardness requires nitride-forming alloy elements
- Compound-layer thickness and porosity must be controlled for contact fatigue
- Final grinding can remove a meaningful fraction of the shallow case
The Gas nitride 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 | 495–565 °C (923–1,049 °F) — Common controlled gas-nitriding window; stay below the prior tempering temperature. |
|---|---|
| Soak | 10–80 h — About 48 h is typical for a 0.5 mm case on suitable steel; some cycles extend to 130 h. |
| Cooling | Still air |
| Atmosphere | Ammonia |
| Door to door | 14–100 h — Includes heat-up, multi-stage nitriding, and cooling. |
Stages
- 1Harden and temper the core above nitriding temperature
- 2Finish machine and stress relieve
- 3Clean and mask soft areas
- 4Nitride under controlled nitriding potential
- 5Controlled cool and inspect
What Gas nitride does to the part
| Hardness | 58–67 HRC — Approximate HRC equivalent; 4140 is commonly 600–800 HV and nitriding steels can exceed 1100 HV. |
|---|---|
| Case depth | 0.1–0.5 mm — Nitriding hardness depth to core hardness plus 50 HV, consistent with ISO practice; this is not the carburizing 50 HRC criterion. |
| Toughness | Core toughness remains that of the prior heat treatment; an excessively thick or porous compound layer can be brittle. |
| Distortion | Low risk. No quench is used, but predictable growth and stress relaxation can affect very tight tolerances. |
| Surface after treatment | Matte gray as nitrided; a thin compound layer is present unless the cycle specifies a diffusion-zone-only surface. |
| Machining sequence | Finish machine before nitriding; polish only lightly afterward and specify any permitted compound-layer removal. |
Specifications for Gas nitride
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 |
|---|---|---|---|
Gas Nitriding and Heat Treatment of Low-Alloy Steel Parts | Governing | — | Active |
Automated Gaseous Nitriding Controlled by Nitriding Potential | Alternative | — | Active |
Test and related methods: ASTM E384 (Standard Test Method for Microindentation Hardness of Materials), SAE J423 (Methods of Measuring Case Depth), AMS 2750 (Pyrometry), ASTM E140 (Standard Hardness Conversion Tables for Metals Relationship Among Brinell Hardness, Vickers Hardness, Rockwell Hardness, Superficial Hardness, Knoop Hardness, Scleroscope Hardness, and Leeb Hardness), ISO 15787 (Technical product documentation — Heat-treated ferrous parts — Presentation and indications).
Drawing call-out
GAS NITRIDE PER AMS 2759/6; 900 HV0.5 MIN SURFACE; 0.30–0.45 MM NHD TO CORE + 50 HV ON SURFACES SHOWN; COMPOUND LAYER 0.013 MM MAXWhich alloys take Gas nitride?
Verdict per material family, with the grades that behave differently. Each family links to its datasheets in the material library. Typical grades: Nitralloy 135M, 4140, 4340, H11, H13, M2, 17-4 PH, 410.
| Material family | Verdict | Notes |
|---|---|---|
| Yes | Alloy, tool, and dedicated nitriding steels respond best.
| |
| With care | Sintered ferrous parts can respond, but open porosity changes gas uptake. | |
| With care | AM tool and alloy steels can be nitrided after density and prior heat treatment are validated.
| |
| With care | Martensitic and PH grades can be nitrided with corrosion and prior-aging effects reviewed.
| |
| No | Conventional ammonia gas nitriding is a ferrous process. | |
| No | Brass is not suitable for ammonia gas nitriding. | |
| No | Bronze is not suitable for steel gas nitriding. | |
| No | Cobalt-chrome requires a different surface-engineering route. | |
| No | Copper alloys do not form the required iron/alloy nitrides. | |
| No | Magnesium is outside the process metallurgy and atmosphere scope. | |
| No | Nickel alloys do not respond to conventional steel gas nitriding. | |
| No | Precious metals do not form a steel-type nitrided case. | |
| No | Refractory-metal nitriding uses different high-temperature processes. | |
| No | Titanium nitriding uses different plasma or high-temperature methods. | |
| No | Tungsten heavy alloys are not conventionally gas nitrided. | |
| No | The process temperature exceeds zinc alloy capability. |
Design rules for Gas nitride
What to decide on the drawing before the part goes to the heat treater.
- Temper the core above the nitriding temperature before final machining.Otherwise the long nitriding hold can overtemper or destabilize the core.
- Specify NHD to core hardness plus 50 HV.A nitrided case should not be accepted using the carburizing 50 HRC convention.
- State the maximum compound-layer thickness.A thick porous white layer can spall under rolling or impact contact.
- Finish machine before nitriding.A 0.10–0.50 mm diffusion zone leaves little stock for later removal.
- Mask threads and ductile assembly features where hardness is unwanted.The hard case can reduce local compliance and complicate rework.
- Use coupons with the same alloy and prior heat treatment.Alloy nitride formers strongly control hardness and depth.
Gas nitride versus the alternatives
Treatments engineers weigh against Gas nitride, and when each wins.
| Treatment | Hardness | Toughness | Stability | Cost | Choose it when |
|---|---|---|---|---|---|
Gas nitride this page | 85 | 50 | 85 | $$$$ | — |
| 85 | 52 | 88 | $$$$ | when selective treatment and tighter compound-layer control are needed | |
| 68 | 55 | 88 | $$$ | when a short cycle and thin compound layer suit low-carbon steel |
Cost, lead time and applications
Relative cost
Gas Nitride is a high-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 | 7–14 working days |
|---|---|
| Furnace time | 14–100 h |
What Gas nitride is used for
- Extrusion screws and barrels
- Die-casting dies
- Crankshafts
- Aircraft gears and shafts
- Valve components
- Mold tooling
Gas nitride — frequently asked
How hard is gas nitriding?
4140 commonly reaches about 600–800 HV, while aluminum-bearing Nitralloy can exceed 1100 HV. Vickers microhardness is preferred because the case may be too thin for a valid Rockwell C impression.
How deep is a nitrided case?
For 4140 and Nitralloy, about 0.10–0.50 mm is a useful commercial range, with 0.305–0.380 mm common. A roughly 0.5 mm case can require around 48 hours.
Does nitriding require a quench?
No. Gas nitriding operates around 495–565 °C in the ferritic range and normally air cools, which is why distortion is much lower than carburizing.
What is the white layer?
It is the outer iron-nitride compound layer, typically only micrometers thick. Its phase balance, thickness, and porosity are controlled because an excessive layer can crack or spall.
Can 17-4 PH be nitrided?
Yes, it can be processed with a qualified stainless-steel route. The nitriding temperature must be reconciled with the precipitation-hardening condition so the core properties are not unintentionally changed.
Can nitrided parts be ground?
Only lightly unless the drawing allows case removal. Removing 0.05 mm from a 0.20 mm case consumes one quarter of the specified depth.