Induction Hardening
Also: induction heat treating, induction case hardening, induction surface hardening, scan hardening, single-shot induction hardening, selective hardening
Induction hardening heats only a band of a steel part with a copper coil and quenches it: a 1-6 mm hard case on shafts, pins and gear teeth in seconds.
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Induction hardening passes high-frequency alternating current through a copper coil shaped around the part. The field induces eddy currents in the steel surface and heats it above the transformation temperature in a few seconds, and a water-polymer spray quench follows immediately to form martensite. Only the heated zone hardens, so a shaft can have hard bearing journals and a soft, machinable end, and a gear can have hard teeth on a tough hub.
Case depth is set mainly by frequency, power and heating time. Current concentrates near the surface, and higher frequencies concentrate it more: roughly 100-450 kHz for cases around 0.5-1.5 mm (0.020-0.060"), about 10 kHz for 1.5-3 mm (0.060-0.120"), and 1-3 kHz for deeper cases on large shafts. Parts are either scan hardened, with the coil or part moving progressively along the length, or single-shot hardened, with the whole zone heated at once. Case depth is defined at a stated hardness; ISO 18203 sets it at 80 percent of the minimum specified surface hardness.
Because heating is fast and local, distortion is lower than hardening the whole part in a furnace, and the process runs in line with machining at production volumes. The limits are coils and setup: each geometry needs its own coil or at least its own program, so it suits repeat parts, and sharp corners, cross holes and keyways inside the hardened zone overheat and crack. After quenching the part is tempered, usually at 150-200 °C (300-400 °F) in a furnace or by a second, low-power induction pass, to relieve quench stress.
At a glance
| Typical tolerances | Surface hardness is usually specified as a 5-point HRC range after temper, such as 50-55 HRC on 1045 or 4140; a 4-point band is achievable on a proven setup. Case depth is specified as a range about 1 mm (0.040") wide on cases of 2-5 mm, for example 2.0-3.0 mm (0.080-0.120"), and about 0.5 mm (0.020") wide on shallow cases. The start and stop of the hardened pattern along a shaft are typically held to about ±1.5-3 mm (±0.06-0.12"). Distortion is lower than through hardening, but long shafts still bow and are straightened, and bearing journals are ground afterwards with about 0.1-0.25 mm (0.004-0.010") of stock per side. |
|---|---|
| Size limits | Vertical scanners at job shops commonly take shafts up to about 1.5-3 m (5-10 ft) long and 150-300 mm (6-12") in diameter; larger shops scan 6 m (20 ft) and longer, and horizontal machines handle long bars. Single-shot hardening of large areas is limited by power supply, which can run to hundreds of kilowatts. Small gears are hardened all at once while spinning in the coil; large gears are hardened tooth by tooth. Very small parts are limited by coil design and the minimum practical case. |
| Surface finish | The hardened band comes out with a light oxide in straw to blue temper colours and a trace of quench residue; the rest of the part is unchanged. There is no heavy scale because heating takes seconds, and roughness is not changed. Parts are washed, and bearing journals are usually ground or polished afterwards. |
| Lead time | Parts with existing coils and programs in 3-7 business days. New parts that need a coil designed and built, plus development sections to prove the pattern, take 2-4 weeks for the first lot, then days for repeats. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Medium-carbon steels with about 0.35-0.55 percent carbon. 1045 and 1050 are the economical choices, 1541 is common for shafts, and 4140, 4150 and 4340 are used when the core must also be quenched and tempered for strength; a prehardened 4140 core at 28-32 HRC under an induction case is a common combination for heavily loaded shafts. High-carbon 52100 and 1095 induction harden but need care to avoid cracking. Ductile and pearlitic gray iron take induction hardening on wear surfaces such as bores, ways and cams. Martensitic stainless (410, 420) is induction hardened in specialist shops. Low-carbon steels (1018, 8620, 12L14) do not reach useful hardness without carburizing first, and austenitic stainless, aluminum and copper alloys cannot be hardened this way.
What drives the cost
- Coil design and fabrication for a new geometry, plus development parts sectioned to prove the pattern
- Quantity: setup and development amortize quickly, and at volume cycle times are seconds per part
- Number of zones: each separately hardened journal, band or tooth adds a cycle
- Case depth and diameter: deep cases on large parts need big power supplies and slower scans
- Tempering, straightening and final grinding
- Verification: sectioning for case depth and pattern, hardness mapping and crack inspection
When to use it
- Shafts, axles, pins, spindles and rolls that need hard bearing journals or wear bands and a tough, machinable remainder
- Gear and sprocket teeth, splines and cam lobes in medium-carbon steel
- Deep cases of 1.5-6 mm (0.060-0.240") for high contact, bending or torsional loads
- Repeat production, where one coil and program serve thousands of parts
- Parts too large to furnace harden and quench economically
When not to
- Low-carbon steels such as 1018 and 8620: carburize them instead
- One-off parts with complex geometry that would each need a custom coil: furnace hardening or nitriding avoids the tooling
- Thin sections, sharp corners, cross holes and keyways inside the hardened zone, which overheat and crack
- Parts that need an even, thin case over every surface, such as a complex die: nitriding covers everything evenly
Design tips
- Show the hardened zone on the drawing with its start and stop locations and their tolerances, the case depth range and the hardness it is measured to, and the surface hardness range after temper.
- Keep holes, keyways and sharp shoulders out of the hardened zone, or radius and chamfer them.
- Do not end a hardened pattern in a loaded fillet: either harden through the fillet deliberately or stop the pattern clear of it.
- Specify 4140 or 4340 prehardened to 28-32 HRC when the core carries high loads; specify 1045 when a softer core is acceptable and cost matters.
- Leave grind stock on bearing journals and allow for straightening on long shafts.
- Ask for magnetic particle inspection after hardening on safety-critical shafts; quench cracks can be invisible to the eye.
- Order a few development parts to section before production, and approve the pattern from the macro-etch photos.
Induction Hardening by material
Frequently asked questions
- How deep can induction hardening go?
- Typical cases run 1-6 mm (0.040-0.240"), and large shafts on low-frequency power supplies reach 10 mm (0.400") or more. Shallow cases, down to about 0.5 mm (0.020"), take high frequency. Depth is set by frequency, power and heating time, and limited by the hardenability of the steel.
- What steels can be induction hardened?
- Steels with about 0.35-0.55 percent carbon: 1045, 1050, 1541, 4140, 4150 and 4340 are the common ones. Ductile and pearlitic gray iron also respond. Low-carbon steels such as 1018 and 8620 do not harden usefully without carburizing, and austenitic stainless and aluminum cannot be hardened this way.
- Do induction hardened parts need tempering?
- Almost always. As-quenched martensite is highly stressed and prone to cracking; a temper at about 150-200 °C (300-400 °F), done soon after hardening, relieves it for the cost of a few points of hardness. The temper can be done in a furnace or by a second induction pass.
- Induction hardening or carburizing?
- Induction hardening is faster, cheaper at volume and selective by nature, and it suits medium-carbon steel shafts and gears that need a deep case in specific places. Carburizing hardens every exposed surface of low-carbon steel, follows complex gear geometry evenly, and leaves a tougher core. Heavy-duty gears are usually carburized; shafts with hardened journals are usually induction hardened.
