# Nitrocarburizing

Ferritic nitrocarburizing systems, compound and diffusion layers, post-treatment choices and grade-specific acceptance.

Reviewed: 2026-11-01

Tags: processes, heat-treatment, nitrocarburizing, surface-hardening

Uddeholm's **Formvar, Edition 4, September 2018**, lists **0.13 mm** for gas nitrocarburizing at **580°C for two hours** and **0.08 mm** for salt at **580°C for one hour**. Both use **50 HV0.2 above base hardness** as the depth criterion. Specify compound-layer thickness and hardness depth separately. [Uddeholm Formvar, printed page 6](https://www.voestalpine.com/highperformancemetals/mexico/app/uploads/sites/294/2025/05/Tech-Uddeholm-Formvar-EN.pdf)

Ferritic nitrocarburizing introduces nitrogen and carbon without austenitic hardening. It forms a surface compound layer and underlying diffusion zone for scuffing and adhesive-wear control. [Bodycote ferritic gas nitrocarburising](https://www.bodycote.com/what-we-do/precision-heat-treatment/case-hardening-atmosphere-vacuum/ferritic-nitrocarburising-gas/)

## Distinguish neighboring treatments

| Route | Main distinction | What must be specified separately |
| --- | --- | --- |
| Ferritic nitrocarburizing | Nitrogen/carbon uptake without the conventional austenitize-and-harden route | Compound layer, diffusion profile, core condition and any oxidation/finishing |
| Nitriding | Nitrogen is the principal introduced element | Required compound-layer condition and diffusion/hardness depth |
| Carbonitriding | Austenitic case treatment followed by hardening | Case/core requirements, quench and tempering |
| Austenitic nitrocarburizing | A separately identified route that can develop an austenitic region near the surface | Exact material, transformation and cooling/post-treatment requirements |
| Low-temperature stainless surface hardening | May develop expanded austenite or related supersaturated phases | Specific alloy and process qualification, including corrosion behavior |

Bodycote identifies carbonitriding as an **austenitic** case-hardening process. Similar spelling and use of both nitrogen and carbon do not make it interchangeable with ferritic nitrocarburizing. [Bodycote carbonitriding](https://www.bodycote.com/what-we-do/precision-heat-treatment/case-hardening-atmosphere-vacuum/carbonitriding/)

Austenitic nitrocarburizing is not merely an erroneous name for carbonitriding. Flodström's **Chalmers report 76/2012**, performed at SKF, compared ferritic and austenitic gas nitrocarburizing of selected bearing steels and observed an austenitic layer beneath the compound layer in the latter route. This bounded research does not supply a universal treatment for other steels, and “austenitic” here should not be confused with the starting designation of an austenitic stainless alloy. [Original thesis, 2012](https://publications.lib.chalmers.se/records/fulltext/155234.pdf)

## Compound layer and diffusion zone

The two regions serve different functions. Somers' **2013** technical contribution describes iron (carbo-)nitrides in the compound layer and nitrogen in solution or nitride precipitates in the diffusion zone. It relates the compound layer to tribological/corrosion performance and the diffusion zone to fatigue-related effects. The ferritic hardening mechanism is not a martensitic transformation. The same paper separately discusses expanded-phase treatments for stainless steels; those cannot be presumed equivalent to conventional ferritic treatment. [Somers, *Nitriding and Nitrocarburizing: Status and Future Challenges*, DTU author manuscript](https://backend.orbit.dtu.dk/ws/portalfiles/portal/57262608/Nitriding_and_Nitrocarburizing_Current_Status_and_Future_Challenges.pdf)

Avoid a single unqualified “case depth” value. Compound-layer thickness, a hardness-defined diffusion depth and a total affected depth are different characteristics. Surface hardness also requires a method and load: a thin layer measured under one load cannot be compared indiscriminately with a bulk Rockwell value.

## Gas, salt and plasma routes

Struers' metallography guidance distinguishes salt-bath, gas and plasma nitrocarburizing. Gas processing supplies carbon alongside the nitrogen-bearing atmosphere; plasma processing adds carbon-containing gas to a discharge-assisted treatment. Its salt-bath example includes a subsequent oxidizing stage. These identify process families, not interchangeable operating instructions. [Struers nitrided/nitrocarburized components guidance, undated](https://cdnstruersproduction.azureedge.net/en/Knowledge/Materials/Nitrided-coatings)

Choose the route with the supplier using actual alloy, part geometry, loading and required surface state. Identify no-treatment regions and verify masking or process exclusion for the chosen route. Include holes, blind features and assembled interfaces when evaluating access, cleaning and retained chemistry. Use current supplier technical instructions and SDS for gases, salts, cleaning chemicals and post-treatments; the family name does not define those handling requirements.

## Post-oxidation, impregnation and finishing

HEF's **undated CLIN Technologies sheet** describes liquid nitrocarburizing combined with post-oxidation and impregnation. It identifies an iron-oxide layer and retention of organic compounds in microporosity as elements of the finished system. HEF explicitly makes properties substrate-dependent. Consequently, a black surface, trade name or generic salt-spray maximum does not establish an equivalent corrosion result on another material or after removing the final surface. [HEF CLIN sheet](https://hef.group/wp-content/uploads/2024/06/data_sheet_CLIN%E2%84%A2-Technologies.pdf)

Specify whether polishing, oxidation and impregnation are required, their sequence and the final inspected state. Removing material can change roughness, porosity, compound-layer thickness or an oxide/impregnated surface. Validate corrosion and friction performance for that complete state, including any cleaning that removes temporary organic protection. Do not convert accelerated corrosion hours into service years without application evidence.

## Substrate and core-condition limits

Low distortion does not mean zero growth or unchanged core properties. Prior tempering, stress state and the time/temperature of the selected cycle require review. A deep load-bearing case cannot be inferred from high hardness at the outer surface.

A useful producer-specific example is **Uddeholm Formvar, Edition 4, September 2018**. It lists approximate **1100 HV0.2** surface hardness after the described nitrocarburizing treatments. Its table gives **0.13 mm** for gas treatment at **580°C for 2 hours** and **0.08 mm** for salt treatment at **580°C for 1 hour**, defining depth at **50 HV0.2 above base hardness**. These are Formvar guidance values with stated conditions, not universal recipes or compound-layer thicknesses. [Uddeholm Formvar, printed page 6](https://www.voestalpine.com/highperformancemetals/mexico/app/uploads/sites/294/2025/05/Tech-Uddeholm-Formvar-EN.pdf)

Crucially, the same sheet warns that a brittle white/compound layer can crack or spall under heavy mechanical or thermal loads and generally should be avoided for the discussed tooling. It also requires relating the treatment temperature to prior tempering to avoid loss of core properties or dimensional tolerance. A layer deliberately retained in an anti-scuffing system is therefore not automatically desirable on every die. [Uddeholm Formvar](https://www.voestalpine.com/highperformancemetals/mexico/app/uploads/sites/294/2025/05/Tech-Uddeholm-Formvar-EN.pdf)

## Inspection and dimensional acceptance

**ISO 18203:2026**, edition 2, **January 2026**, replaces the 2016 edition and covers hardness-defined depths and total surface-hardened thickness for the processes in its scope. Select the applicable depth definition and procedure for the treatment being specified. Do not treat a compound-layer micrograph measurement as automatically identical to nitriding-hardness depth. Agree hardness load, core reference, test location and final surface condition. [ISO 18203:2026](https://www.iso.org/standard/89021.html)

Metallographic preparation itself can distort the evidence. Struers warns that chipping, cracking and rounded edges introduced during cutting, mounting or polishing can compromise layer assessment. Evaluate suspected layer defects with suitable preparation before concluding they occurred in service or processing. [Struers preparation guidance](https://cdnstruersproduction.azureedge.net/en/Knowledge/Materials/Nitrided-coatings)

**Original allowance example:** assume a qualified trial measures **0.006 mm outward radial growth** on a shaft. Starting from **20.000 mm diameter**, uniform growth would produce `20.000 + 2 × 0.006 = 20.012 mm`. If the required final maximum is **20.010 mm**, this illustrative result exceeds it by **0.002 mm**. The assumed growth is not a published material constant and is not the compound-layer thickness. Grinding to recover size could change the qualified layer or post-treatment; account for it in the original process plan rather than treating it as a neutral correction.

## Manufacturing handoff

Provide alloy and prior condition, significant surfaces, no-treatment zones, required layer structure, hardness/depth definitions, final texture and dimensional limits, and any oxidation/impregnation requirements. Agree representative samples, section locations, corrosion/wear test conditions and lot acceptance. Record the complete route and final state, so a generic “nitrocarburized” certificate cannot substitute for the properties required by the component.

## Related reference

[Grades, tempers and conditions](../../materials/grades-tempers-and-conditions.md)