Carbonitriding
Austenitic carbon-and-nitrogen case hardening, with material selection, atmosphere control, quenching and case-depth acceptance boundaries.
Tags: processes, heat-treatment, case-hardening
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In the example, 660 HV at 0.200 mm and 580 HV at 0.300 mm give 0.275 mm depth at a purchaser-defined 600 HV threshold and common test force, using linear interpolation. Removing 0.030 mm normal to the surface leaves 0.245 mm, below an assumed 0.250 mm minimum, if the remaining profile is unchanged. Verify finished depth using the ordered method, force and reference surface.
Process and intended result
Carbonitriding diffuses carbon and nitrogen into austenitic steel before hardening. Specify the case and core together for the selected plain-carbon or low-alloy steel. Bodycote carbonitriding
A process name does not establish final surface hardness, case depth, core strength, distortion or component life. Define the actual alloy, incoming condition, geometry and performance requirements before selecting the route. A gear tooth, thin wall and solid shaft can have different heat-treatment responses even when processed in the same load.
Related processes have different boundaries
| Route | Essential distinction | Procurement implication |
|---|---|---|
| Carburizing followed by hardening | Carbon enrichment before the hardening transformation | Do not infer nitrogen enrichment from the process name |
| Austenitic carbonitriding | Carbon and nitrogen enrichment in austenite, followed by hardening | Identify the full diffusion, quench and subsequent treatment route |
| Ferritic nitrocarburizing | Nitrogen/carbon treatment within the ferritic condition | A distinct layer system and thermal route; not an interchangeable spelling |
| Electroplating or sprayed coating | Material is deposited onto a substrate | Coating thickness is not the same measurement as diffusion-case depth |
Linde's Furnace atmospheres no. 1 describes carbonitriding as transferring both carbon and nitrogen from gas to steel, in contrast to its carburizing discussion. Bodycote's ferritic nitrocarburizing page describes a diffusion zone and surface compound layer formed within the ferrite phase. Similar element names therefore do not make the routes equivalent. Linde guide, printed page 8, Bodycote ferritic nitrocarburizing
Material, hardenability and transformation
Provide the grade, composition limits where relevant, prior heat treatment, cold work, machining history and existing surface condition. Bodycote explains that carbonitriding's attained hardened depth depends on hardening temperature, quench rate, steel hardenability and component dimensions, rather than diffusion time alone. Its published application ranges are service examples, not guaranteed capability for every geometry or alloy. Bodycote process details
Surface enrichment and the subsequent transformation are different controls. Linde explains that carbon and nitrogen lower the martensite-start temperature and that retained austenite can alter the near-surface hardness profile. A higher enrichment setting cannot therefore be assumed to produce a monotonic increase in final hardness. Require appropriate microstructure and hardness evidence for the actual material and route. Linde guide, printed page 9
Agree the required core condition independently. A shallow hardened case does not establish through-section strength, and a hard core does not prove that the required surface case was achieved. Review thin sections and intersections where cases from adjacent faces may interact; use representative sections when developing acceptance.
Atmosphere and process sequence
The atmosphere is a controlled process variable, not a generic gas recipe. Linde's carbonitriding discussion explains that ammonia dissociation dilutes the atmosphere and affects interpretation of carbon-potential control; residual ammonia and its relationship to nitrogen uptake also matter. An inlet percentage alone is insufficient evidence of a reproducible surface condition. Specify the qualified equipment, control and verification route without copying settings from an unrelated furnace example. Linde guide, section 5.1.5, printed page 32
Establish cleaning, surface preparation, selective protection where required, loading, enrichment, hardening/quenching and subsequent tempering or other approved treatment as one route. Define the surfaces intended to remain untreated and the acceptance at transitions. A surface hardness result on an exposed face does not show the same result in a protected region or narrow internal feature.
Review final geometry after the full route. Bodycote describes reduced distortion relative to some carburizing routes through lower processing temperatures and less severe quenching. Reduced distortion does not mean zero distortion or a numerical allowance for another part. Qualify fixtures, loading and any permitted straightening or final machining against the actual dimensional requirements. Bodycote carbonitriding benefits and process details
What case-depth inspection establishes
ISO 18203:2026, edition 2, January 2026, specifies measurement of case-hardening depth, surface-hardening depth, nitriding-hardness depth and total thickness of surface-hardened layers. Its public scope explicitly includes carbonitriding. The ISO record identifies ISO 18203:2016 as withdrawn. Use the ordered edition and applicable definition; the public scope does not supply a universal hardness or depth requirement for a carbonitrided component. ISO 18203:2026
Struers describes a hardness traverse on a cross-section from the surface toward the core, plotting hardness against distance to determine a limiting-depth result. It also distinguishes procedures for different hardening processes and warns that indentation spacing affects measurements. Record the threshold definition, test force, section location, preparation, spacing and calculation method; do not report an unspecified “case depth” as though it uniquely identified the measurement. Struers Vickers and case-depth guidance
Surface hardness, depth to a specified hardness, total metallographic case, core hardness and microstructure answer different questions. State which are required and in what finished condition. A coupon should represent the material, surface exposure and thermal response relevant to the production feature; its convenient flat face does not automatically represent every part section.
Original bounded traverse and finishing example
Use the example purchaser’s 600 HV threshold, common test force and stated readings from the same reference surface. Interpolate linearly:
depth = 0.200 + (660 − 600) / (660 − 580) × (0.300 − 0.200) = 0.275 mm.
Assume 0.030 mm grinding normal to the surface with an unchanged hardness profile, threshold and no thermal damage:
0.275 − 0.030 = 0.245 mm.
The example fails the 0.250 mm finished minimum. Account for local case variation, nonlinear profiles and uncertainty in finished-part inspection.
Manufacturing handoff
Provide the drawing revision, alloy and incoming condition, significant surfaces, selective protection, required surface and core properties, depth definition/range, microstructure requirements, final dimensions and permitted finishing. Agree representative specimens, test locations, lot definition, sampling, traceability and any crack or other surface-integrity inspection.
Record atmosphere/process control, load and quench history, subsequent treatments and inspection results against the actual parts. Review rework, straightening and additional heating or material removal with the responsible process authority. A process certificate should identify the ordered route and requirements; it does not independently establish a fatigue or wear-life allowable.
The supplier web pages and linked Linde guide have no verified technical issue date assigned here; historical references inside a guide do not date its current issue. The ISO publication date is taken from its public record. The numerical traverse and finishing example is original and deliberately separated from product ratings and standard limits.