Carburizing
Carbon diffusion and subsequent hardening, with route, core response and finished-case acceptance boundaries.
Tags: processes, heat-treatment, case-hardening
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In the example, 620 HV at 0.600 mm and 500 HV at 0.900 mm give 0.775 mm depth at a specified 550 HV threshold and common test force, using linear interpolation. Removing 0.080 mm normal to the surface leaves 0.695 mm if the remaining profile is unchanged. That misses the assumed 0.700 mm finished minimum. Check finished case depth and grinding effects using the ordered inspection.
Process and intended result
Carburizing diffuses carbon into the surface. Quenching hardens the enriched region, and tempering and any subsequent operations complete the specified route. Bodycote atmospheric carburizing
Order the required case and core properties together. A carbon-rich surface before quenching is not proof of a martensitic case, and surface hardness does not establish core strength. The intended wear or fatigue benefit must be qualified for the actual material, geometry, loading and final condition rather than inferred from a heat-treatment certificate alone.
Atmosphere and low-pressure routes
| Route | Primary-source description | Boundary to preserve |
|---|---|---|
| Atmospheric carburizing | Carbon-rich furnace atmosphere followed by hardening and tempering | Specify atmosphere control, load, quench and finished properties; the name alone does not define a cycle. |
| Low-pressure carburizing, LPC | Hydrocarbon treatment in a vacuum furnace, with carburizing and diffusion stages | Qualify gas access, case distribution, thermal sequence and the actual quench configuration. |
| Carbonitriding | Carbon and nitrogen enrichment before hardening | Nitrogen addition makes this a different route, not a synonym for carburizing. |
Bodycote presents LPC as an alternative offering case-uniformity, cleanliness and dimensional-control benefits. It also attributes avoidance of intergranular oxidation to the lack of oxygen in that process. These are route-specific benefits, not a guarantee that every blind hole, tooth root or thin section meets an arbitrary case-depth tolerance without development work. Bodycote atmospheric route, Bodycote LPC, Linde furnace-atmosphere guide, printed page 8
LPC does not necessarily imply gas quenching. SECO/WARWICK's current CaseMaster Evolution OQ page describes a double- or triple-chamber LPC furnace with integral oil quenching, particularly for lower-hardenability steels or components requiring relatively high core hardness. Bodycote separately describes LPC with high-pressure gas quenching. Specify the installed equipment and approved cooling route instead of transferring one configuration's claims to another. CaseMaster Evolution OQ, Bodycote LPC process details
Alloy, core and section response
Linde distinguishes surface enrichment from core response, which depends on hardenability and cooling. “Unaffected core” does not mean its incoming properties survive unchanged. Specify its final requirements. Linde guide, printed pages 9–12
Provide alloy/heat identity, incoming condition, machining and cold-work history, section dimensions and required final core hardness or mechanical properties. Compare critical thick and thin regions, intersections and gear features when developing a sampling plan. A convenient coupon should represent the surface exposure and cooling response of the accepted feature; otherwise its result needs a demonstrated relationship to the actual part.
Linde explains that retained austenite can reduce hardness at high carbon enrichment, with alloy-dependent response, and identifies internal oxidation under conventional carburizing conditions. Specify microstructure and surface integrity rather than maximizing carbon potential. Linde guide, printed pages 9 and 33
Distortion, selective treatment and final finishing
Review protected surfaces, selective treatment boundaries, loading and quench orientation before release. Preserve final size, form, alignment and surface-integrity requirements through the entire route. Claims of reduced distortion do not supply a numerical machining allowance for another part or eliminate inspection after hardening. Bodycote presents improved dimensional control as an LPC benefit, not a statement that every component needs no finishing. Bodycote LPC
Linde connects depth planning to the case remaining after grinding. State the required finished condition and permitted removal. Separate normal removal from diameter change: removing 0.05 mm radially from a cylindrical outside surface reduces diameter by 0.10 mm. That geometric relation does not itself establish a safe removal allowance. Linde guide, printed page 12
Case-depth inspection
ISO 18203:2026, edition 2, January 2026, covers determination of surface-hardened-layer thickness quantities, including case-hardening depth after carburizing and hardening. The record identifies ISO 18203:2016 as withdrawn. State the contractual edition, applicable depth definition, hardness criterion, test force and finished condition; the public scope does not create a universal case-depth or surface-hardness limit. ISO 18203:2026
Struers describes case-depth assessment using a hardness traverse from the surface toward the core. Specimen preparation, indentation spacing and test conditions affect the measurement. Record section location, surface reference, force, individual results and the approved determination method. Distinguish a hardness-defined case depth from total metallographic case or depth to a carbon concentration. Struers Vickers and case-depth guidance
Original bounded finishing example
Use the example 550 HV threshold, common test force and stated descending traverse. Interpolate linearly:
depth = 0.600 + (620 − 550)/(620 − 500) × (0.900 − 0.600) = 0.775 mm.
Assume 0.080 mm grinding normal to the surface, without altering the profile or causing thermal damage:
remaining depth = 0.775 − 0.080 = 0.695 mm.
The example misses the finished minimum by 0.005 mm. The approved method must account for local variation, nonlinear profiles, grinding effects and uncertainty.
Manufacturing handoff and source versions
Provide the part revision, alloy/condition, significant and protected surfaces, case-depth definition/range, surface/core requirements, microstructure and crack-inspection requirements, final dimensions and permitted finishing. Retain atmosphere/load/quench records, subsequent treatments, representative sampling and results. Agree rework or additional heating with the responsible process authority.
The Linde document identifies itself as Expert edition, Furnace atmospheres no. 1: Gas carburising and carbonitriding; no verified technical issue date is assigned here. Historical references inside it do not date its current issue. The cited supplier web pages also have no verified technical revision date. ISO's January 2026 edition is distinguished from its withdrawn predecessor; the calculations above are original explanations.