Flame hardening
Localized transformation hardening with material, geometry, quench, tempering and finished-depth requirements defined together.
Tags: processes, heat-treatment, flame-hardening, surface-hardening
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An example journal has 2.20 mm depth to an agreed hardness criterion before grinding. Removing 0.30 mm radially leaves 1.90 mm if the remaining profile is unchanged, 0.10 mm below an assumed 2.00 mm minimum. The diameter decreases by 0.60 mm. Inspect the actual profile from the finished surface and assess thermal effects of grinding.
Local flame heating and quenching can produce a martensitic surface in hardenable material. Specify material, dimensions and required hardness pattern before selecting equipment. Flame Treating Systems, machine guidance
Do not specify flame hardening as a generic way to make any steel wear-resistant. The starting material must support the intended transformation, and a final hardness value does not by itself establish depth, pattern, toughness or freedom from cracking.
Transformation, diffusion and coating routes
| Route | What distinguishes it | Information that cannot be inferred from the name |
|---|---|---|
| Flame hardening | Flame heats selected material; cooling produces transformation hardening | Surface hardness, depth and transition pattern |
| Induction hardening | Electromagnetic heating instead of a flame | Equivalent result without a separately qualified cycle |
| Carburizing/carbonitriding | Thermochemical treatment changes surface composition before hardening | Same case/core structure as a flame-hardened component |
| Coating or hardfacing | Adds material to the component | Same dimensions, adhesion behavior or substrate condition as transformation hardening |
Bodycote distinguishes steel hardenability—the response and depth achievable for a given cooling route—from maximum hardness, which is strongly related to carbon content. Its glossary also identifies carburizing/carbonitriding as composition-changing treatments. Use those distinctions when selecting a route rather than treating all “case hardening” terminology as equivalent. Bodycote technical glossary
Material and starting condition
Flame and Induction Hardening Services' material-selection page, marked 19 July 2020, gives representative flame-hardened results of 45–55 HRC for 1045 and 50–60 HRC for 4140. The supplier expressly makes these dependent on chemistry, quenchant and section thickness. They are useful selection examples, not guaranteed acceptance ranges for every heat or component. The same guidance relates transformation volume change to stress, distortion and possible cracking. Supplier material-selection guidance
Identify the grade, actual material certification, product form and prior condition. Review decarburization, cast surface condition, weld repairs and previous heat treatment before deciding whether the selected region can meet the requirement. The hardening operation does not repair an unsuitable core, a defective casting or an incorrect alloy substitution.
For a producer-specific example, Ovako's C45 sheet, revised 30 January 2025, identifies variant 5155 with 0.44–0.49% carbon. Its induction/flame-hardening recommendation includes immediate tempering after quenching. The document explicitly applies to Ovako material and labels its recommendations illustrative rather than guaranteed. It also identifies its heat-treatment graphs as theoretical-model outputs; those graphs are not measured hardness profiles from a flame-hardened production part. Ovako C45 data sheet, variant 5155
Heating pattern and quenching
Flame Treating Systems describes several configurations: spinning circular components before quenching; treating one gear tooth at a time; progressively moving a flame/quench head along ways; and stationary treatment of a selected region. These produce different thermal histories and patterns. Specify which surfaces, tooth regions or tracks must harden and where the hardened region is allowed to end. Do not infer root coverage, transition location or full-circumference continuity merely from “flame harden.” Flame Treating Systems
Flame Technologies' equipment catalog identifies material, fuel, head design/operation and quenching as interacting variables. Its discussion includes different cooling media and self-quenching where sufficient cooler material can extract heat. Such alternatives require qualification; neither water nor self-quenching is a universal choice. The linked catalog has no verified issue date used here. Flame Technologies catalog, About Flame Hardening
Control heating and cooling with the installed equipment's approved procedure. Record relevant part motion, heat input, temperature assessment, quench delivery and setup condition. A visually similar flame or the same elapsed time does not demonstrate an equivalent part-temperature history on a different geometry.
Geometry, overheating and transitions
Flame and Induction Hardening Services' design page, marked 10 September 2020, warns that holes and section changes can create uneven heating and cooling, increasing crack risk. It recommends attention to sharp corners and notes that thin-walled bores and long shafts can move. These are reasons to agree radii, transitions, support and finishing allowance before processing—not universal dimensional rules to copy onto every drawing. Supplier design considerations
More heat is not automatically better. Dargacz and Starosta's 2018 Journal of KONES study varied flame-heating conditions on 30 mm diameter × 10 mm C45 specimens, then water-quenched them. It reported mean hardnesses spanning 428–694 HV5 and differences in martensitic structure. This bounded experiment demonstrates sensitivity to heating conditions; its small-specimen settings and fitted model are not a production recipe for gears, shafts or other alloys. Original study, volume 25, number 1, pages 41–48
Inspect start/stop regions and the boundary between hardened and unhardened material deliberately. Include adjacent surfaces whose existing properties must be preserved. If grinding follows, control both stock removal and its effect on the hardened region; specify acceptance after the final relevant operation.
Tempering and finished-depth acceptance
Treat tempering as a defined part of the required material condition, with hardness accepted after the specified tempering step. Ovako's immediate-tempering instruction for its C45 example should not be replaced by an arbitrary low-temperature cycle borrowed from another grade. Resolve the hardness/toughness requirement and permissible thermal history with the heat treater. Ovako C45
ISO 18203:2026, edition 2, January 2026, replaces the withdrawn 2016 edition. Its scope includes measurement of case-hardening depth, surface-hardening depth, nitriding-hardness depth and total surface-hardened thickness from thermal or thermochemical processes, explicitly including flame hardening. Select the appropriate defined depth and measurement procedure; “case depth” without a criterion is ambiguous. Specify test location, hardness method/load and condition of the surface from which depth is measured. ISO 18203:2026
A surface hardness test cannot establish the subsurface profile by itself. Agree representative destructive sections or other qualified verification, along with pattern, dimensional and crack inspection requirements. Define how test pieces represent critical geometry and how a process change triggers reassessment.
Original finishing-allowance example
Use the example’s 2.20 mm depth from the pre-grinding surface and 0.30 mm radial removal, assuming an unchanged profile:
2.20 − 0.30 = 1.90 mm.
The result misses the 2.00 mm minimum by 0.10 mm. If 0.30 mm were a diametral allowance, radial removal would instead be 0.15 mm.
Specify the finished reference surface. Include local profile variation, grinding heat and uncertainty in the acceptance plan.
Manufacturing handoff
Provide material and condition, drawing-defined hardened areas and transitions, final hardness range, depth definition, test locations, core requirements, permitted distortion, tempering and finishing sequence. Agree representative trials and acceptance before production. Keep equipment settings as process controls and measured final properties as acceptance evidence; one does not substitute for the other.