Induction hardening
Selective transformation hardening, with coil and quench qualification, pattern requirements and depth inspection.
Tags: processes, heat-treatment, surface-hardening
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An ideal 8.0 mm moving heating footprint gives 2.0 seconds of geometric exposure at 4.0 mm/s and 1.33 seconds at 6.0 mm/s, about 33.3% less. The model assumes steady translation and excludes dwell and end effects. Verify actual thermal and quench history, hardness pattern and hardened depth separately.
Process and intended result
A copper inductor heats the selected surface before quenching. Induction hardening transforms the existing steel. Coil geometry, frequency, power and quench influence the pattern. Bodycote induction hardening
Induction heating is not synonymous with hardening: heating can also support other operations. For hardening, identify the material, incoming structure, required transformation, quench and final condition. A power-supply rating or measured surface temperature alone does not establish a hardness pattern, residual-stress distribution or service-life improvement.
SAE ARP4715A provides guidance for establishing an induction-hardening process for selected areas of plain-carbon or alloy-steel components. Its scope recognizes that prior heat treatment may be needed for response to induction hardening or properties in the remainder of the component. The public record identifies it as stabilized April 3, 2018. It is a process-development reference, not a universal hardness or depth requirement. SAE ARP4715A
Material and heating response
Supply grade/heat identity, product form, incoming hardness, heat treatment, machining history and surface condition. Bodycote lists carbon/alloy steels, martensitic stainless steels and several cast-iron categories among applications, but that list does not make their thermal response interchangeable. The required surface and core properties must be attainable in the actual material and section. Bodycote material scope
Inductoheat's technical article explains that rapid induction heating and the initial microstructure influence austenite formation and the required heating conditions. An equilibrium phase diagram alone is therefore insufficient to choose the process temperature. Different starting structures within the same nominal steel grade can require different development work; do not import a furnace soak cycle into a rapid induction process. Rudnev, “Induction Hardening Temperatures”, May/June 2007, printed pages 15–17
The same article relates the hardness pattern to frequency, time, power and workpiece/coil geometry. Frequency affects electromagnetic heating, but the final hardened depth also requires a suitable temperature-time history and cooling response. Do not equate electrical penetration depth with a measured hardness-defined depth or assign one universal frequency-to-case-depth conversion. Inductoheat metallurgical guidance
Scanning versus single-shot processing
| Arrangement | Basic movement and exposure | Qualification focus |
|---|---|---|
| Scan hardening | The coil and workpiece translate relative to one another, progressively treating a region | Entry/exit conditions, scan motion, local dwell and moving quench relationship |
| Single-shot hardening | The intended region is heated together without relative linear scanning; rotation can still occur | Full-region heating pattern, coil coupling and circumferential/axial uniformity |
Inductotherm explicitly distinguishes single-shot processing from scanning and notes that the part may rotate in single-shot hardening. The term does not mean no motion, nor does it establish that any geometry can be hardened in one unrestricted exposure. Inductotherm equipment-selection discussion
A manufacturer's September 2017 stepped-shaft case study shows the importance of local geometry: its fixed-frequency scan produced satisfactory depths on straight regions but only 0.9 mm at an internal transition corner against a 1.5 mm minimum. The nearby external corner also overheated. These are results for that example, not general capability limits. They demonstrate why changing a shoulder, radius or section can invalidate a recipe established on a straight cylinder. Inductoheat study, printed page 4
Specify the hardening pattern
Define significant surfaces, hardened length, minimum/maximum depth, permitted transition zones and regions that must remain in their specified core condition. Identify fillets, shoulders, holes, keyways and ends in the development drawing. A surface-hardness check on a convenient straight region cannot qualify every transition or verify that a protected region avoided unwanted heating.
Include coil identity and condition, coupling/position, part rotation or scan program, power/frequency control and quench arrangement in the approved route. These are proposed reproducibility fields, not generic machine settings. Keep the quench and any tempering requirement with the heating process; a satisfactory heating trace alone does not establish the final microstructure.
Inductoheat's contract-processing guidance notes that tempering usually follows and emphasizes control of heating and quenching to limit cracking and distortion. Its treatment description is supplier guidance, not a universal tempering temperature/time or numerical straightening allowance. Agree the actual final hardness, dimensions, crack inspection and permitted correction with the responsible authority. Inductoheat Tewkesbury process FAQ
Original bounded scan-exposure example
Use the ideal 8.0 mm footprint with constant width and steady translation:
exposure time = footprint length / scan speed.
At 4.0 mm/s, this is 8.0 / 4.0 = 2.0 seconds. At 6.0 mm/s, it is 8.0 / 6.0 = 1.33 seconds, about 33.3% less time.
Use the actual field and thermal history to assess peak temperature, austenitization and cooling. The field is nonuniform, and conduction continues outside the nominal footprint.
Inspection and acceptance
ISO 18203:2026, edition 2, January 2026, covers determination of surface-hardened-layer thickness quantities and explicitly includes induction hardening. Its record identifies the 2016 edition as withdrawn. Specify the required depth definition, hardness criterion, test force, locations and finished condition using the applicable ordered edition; the public scope does not prescribe a universal acceptance depth. ISO 18203:2026
Struers distinguishes hardness-depth evaluation for induction/flame hardening from evaluation of carburized and nitrided parts. Document the cross-section, surface reference, traverse results, indentation spacing and approved evaluation method. Surface hardness, hardness-depth, total transformed region and core properties answer different questions. Struers Vickers and depth guidance
Relate qualification sections to the actual critical pattern, including start/stop and transition locations. Retain final size/form, roughness and surface-integrity requirements after any grinding or polishing. Removing material or adding subsequent heat can change the delivered condition; final fit does not prove that the required hardened layer remains.
For release, retain drawing/material traceability, incoming condition, approved setup and motion, heating/quench/tempering records, inspection locations, results and permitted rework. Qualify fatigue, wear or impact performance separately when the design depends on it.
Source versions: the Inductoheat PDFs print May/June 2007 and September 2017; these are technical examples, not current universal equipment ratings. SAE identifies ARP4715A's April 2018 stabilization; ISO identifies its January 2026 edition. No technical issue date is assigned to the undated supplier web descriptions. The scan-exposure calculation is original.