Gas nitriding
Nitrogen diffusion hardening with distinct compound-layer, diffusion-zone, core-property and dimensional acceptance requirements.
Tags: processes, heat-treatment, nitriding
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An example 19.994 mm journal with 0.012 mm diametral growth finishes at 20.006 mm, 0.006 mm oversize against a 20.000 mm maximum. Axisymmetric radial growth would be 0.006 mm. The separately measured 0.300 mm nitriding-hardness depth is a depth into the part. Verify geometry and hardness depth independently.
Process and intended result
Gas nitriding diffuses nitrogen into the surface. Nitride precipitation can improve wear, scuffing and fatigue resistance on suitable ferrous parts. Qualify those outcomes separately. Bodycote gas nitriding
Gas and plasma nitriding share nitrogen diffusion as an objective but supply nitrogen differently. Nitrex describes ammonia in the gas-nitriding atmosphere, while plasma processing uses an electrical-discharge route. They are distinct process selections; meeting a similar surface-hardness value does not by itself authorize substitution. Nitrex comparison of gas and plasma nitriding
Nitriding is also distinct from carbonitriding, nitrocarburizing and an applied coating. Bodycote identifies addition of a carbon-supplying gas as gas nitrocarburizing. Define the requested route and any post-oxidation, polishing or coating separately instead of using “nitrided” to describe every nitrogen-bearing surface treatment. Bodycote process distinctions
Compound layer and diffusion zone
Nitrex distinguishes an outer compound layer, often called the white layer, from the underlying diffusion zone. Both can contribute to the overall treated case, but they have different structures and properties. A quoted case depth must therefore identify which layer or measurement is meant. Nitrex nitriding effect
| Requirement | Information needed |
|---|---|
| Compound layer | Permitted thickness/range, continuity, phase or porosity requirements where applicable, and inspection method |
| Diffusion-hardened zone | Required depth definition, hardness threshold, test force and section location |
| Surface hardness | Test method, force, location, surface condition and applicable correction/evaluation |
| Core condition | Required mechanical or hardness condition after the full thermal route |
| Finished part | Dimensions, surface texture, local defects and functional acceptance |
There is no universal requirement to maximize or eliminate the compound layer. Nitrex describes potential-controlled processing that can vary compound-layer development relative to the diffusion zone. Its named Nitreg system illustrates a control approach, not permission to replace a drawing's specified layer with another configuration. Nitrex controlled nitriding
Material and prior heat treatment
Identify grade, product form, heat treatment, core hardness, prior machining and surface condition. Bodycote highlights steels containing nitride-forming chromium, molybdenum, vanadium or aluminum and recommends a suitable hardened-and-tempered starting state for its described applications. Do not infer an identical hardness response from equal starting hardness in two different alloys. Bodycote materials guidance
Uddeholm's Heat Treatment of Uddeholm Tool Steels, edition 10, April 2018, says the core must remain mechanically and microstructurally stable. For its hardened tool-steel context, it calls for prior tempering above the nitriding temperature to avoid core softening. This is producer guidance for a defined material family, not a complete cycle for every alloy. Review the actual steel and allowed property changes with the heat treater. Uddeholm guide, printed pages 12–13
Stainless steel requires a separate corrosion assessment. The same guide warns that nitriding can impair corrosion resistance of high-chromium corrosion-resistant steels. In contrast, Nitrex describes its Nano-S nitrogen/carbon S-phase treatment as avoiding chromium nitride/carbide formation to retain inherent corrosion resistance. That is a named controlled route; it does not demonstrate that conventional nitriding preserves corrosion resistance on an arbitrary stainless part. Specify and qualify both wear and corrosion requirements for the actual alloy and treatment. Uddeholm, printed page 12, Nitrex Nano-S
Process control and geometry
Nitrex connects nitriding potential, temperature and time to the resulting layer configuration. Preserve the approved atmosphere-control method and load conditions rather than reducing the instruction to a furnace setpoint or ammonia-flow percentage. A fixed time does not independently establish the required compound layer and hardness-depth profile on a different material. Nitrex control discussion
Specify cleaning, significant surfaces, selective protection, fixture contacts and required internal coverage. Discuss holes, grooves, abrupt transitions and sharp edges with the processor, and inspect the critical locations during qualification. Uddeholm identifies excess layer development at sharp corners and possible substrate over-tempering as concerns in conventional nitriding. A measurement on an easy exterior face does not prove acceptance at a sheltered internal feature. Uddeholm, printed page 13
Gas nitriding is commonly selected for low distortion, but retain final dimensional inspection. Agree any finishing, polishing or later heating before qualification. Removing part of the treated surface changes the remaining layer system; grinding, welding or coating afterward should not be assumed equivalent to the inspected as-nitrided state. The complete manufacturing route is the unit to approve.
Depth measurement and acceptance
ISO 18203:2026, edition 2, January 2026, covers thickness determination for surface-hardened steel layers, including nitriding-hardness depth. Its public record identifies the 2016 edition as withdrawn. The scope distinguishes several depth types; it does not specify one universal acceptance depth for every nitrided component. Use the contractually required edition and definition. ISO 18203:2026
Struers describes cross-sectional hardness traverses and distinguishes nitrided-part evaluation from carburized or induction-hardened evaluation. Its nitriding example uses a limit related to core hardness. Record the actual specified threshold, test force, preparation, indentation spacing and location. Do not silently reuse a carburized-case threshold or treat the compound-layer thickness as the hardness-depth result. Struers hardness-depth guidance
Surface hardness, layer thickness, core condition and dimensions need separate evidence. Where service depends on fatigue, wear or corrosion behavior, qualify the corresponding component/system under relevant conditions; a hardness certificate alone is not a design allowable or life prediction.
Original dimensional example
Apply the example’s diametral growth to the starting journal:
19.994 + 0.012 = 20.006 mm.
The example exceeds the 19.990–20.000 mm interval. The radial calculation assumes axisymmetry.
Adding twice the 0.300 mm hardness depth to diameter would confuse a subsurface measurement with outward growth. Qualify actual variation, shape change and measurement uncertainty.
Production handoff
Supply the drawing revision, alloy and starting condition, significant surfaces, protected regions, approved route, layer/depth/hardness requirements, final dimensions and functional tests. Agree sample representation, section locations, lot definition and sampling with the processor. Retain load, atmosphere and temperature records, material traceability and the measured results in the required finished condition.
Source dates are distinguished from upload paths: the Uddeholm PDF states edition 10, 04.2018, despite its 2024 URL. No verified technical issue date is assigned here to the supplier web pages. The numerical example is original and separate from manufacturer performance claims.