Induction hardening explainer

On CIMT2011 Eldec Has Unveiled: What Induction Hardening Machines Actually Change

On CIMT2011 Eldec has unveiled the MIND 750 modular machine, the VL-H automated line and SDF profile hardening. This page explains what each one changes at the quench zone, which parts benefit, and where the limits sit. Written for design and process engineers who need to pick a hardening route, not a trade-show slogan.

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Case depth 0.5–5 mmDistortion controlSelective hardeningPost-hardening machining

Induction hardening machine tools discussed as on CIMT2011 Eldec has unveiled for hardened steel parts

On this page

1
How induction hardening works, in one pass through the coil
2
What on CIMT2011 Eldec has unveiled, and what it changes
3
Which parts suit induction hardening, and which do not
4
What hardening does to your finishing and tolerance plan
5
Induction hardening against the furnace routes

Mechanism

How induction hardening works, in one pass through the coil

A copper coil carries alternating current at 1–400 kHz. The alternating magnetic field induces eddy currents in the steel surface. Resistance heating raises that skin layer above the austenitizing temperature, usually 850–950 °C for medium-carbon steel. A quench follows within a second. The surface becomes martensite. The core stays soft and tough.

Depth is not a fixed number. It follows frequency, power density, dwell time and the coil-to-part gap. High frequency concentrates heat in a shallow layer. Low frequency pushes heat deeper. So the same machine can produce a 0.5 mm case on a 10 mm shaft or a 5 mm case on a 60 mm bearing seat by changing the recipe.

The heat is generated inside the part, not delivered from outside. That is the difference from carburizing or nitriding, which are diffusion processes measured in hours. An induction cycle is measured in seconds. The metallurgy still has to be right: too slow a quench and you get ferrite or bainite instead of martensite.

One consequence matters for designers. Because heating is fast and local, the rest of the part never sees the austenitizing temperature. There is no through-hardening, no furnace load, no long ramp. You can harden a finished shaft without softening the splines at the other end.

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Frequency sets depthHigher kHz means a shallower case; lower kHz reaches deeper.
2
Power sets rateMore kW heats faster but narrows the process window.
3
Quench sets hardnessPolymer or water spray must remove heat fast enough for martensite.

CIMT2011

What on CIMT2011 Eldec has unveiled, and what it changes

On CIMT2011 Eldec has unveiled three things worth separating: a modular machine, an automated line and a profile-hardening method. They solve different problems. The MIND 750 is a modular vertical induction machine. It can run as a standalone hardening unit or be linked into a cell, so a shop can start with one part family and add modules later without replacing the base.

The VL-H is a fully automated hardening machine developed with EMAG for high-volume lines. Its value is repeatability at rate: load, heat, quench, unload, inspect, all in a fixed cycle. That matters when a supplier has to hold case depth across thousands of parts rather than prove it once on a sample.

SDF is Eldec's patented profile-hardening approach. It targets a known weakness of high-frequency, high-power hardening: uneven quench around a contour, which shows up as distortion or soft spots. Instead of one straight coil pass, the process shapes the heated profile along the part geometry. Contoured surfaces such as gear teeth, cam lobes and bearing races are where it earns its place.

None of these change the physics. They change how tightly you can control the physics on a production line. A modular frame widens the part envelope. Automation tightens cycle repeatability. Profile control tightens the hardened pattern.

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MIND 750Modular, standalone or inline; suits mixed part families.
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VL-HAutomated cell for serial production with fixed cycles.
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SDFProfile hardening for contoured surfaces, less distortion.

Selection

Which parts suit induction hardening, and which do not

Induction hardening fits parts where hardness is needed in one band or one contour, not through the whole section. Shafts, spindles, gear teeth, cam lobes, bearing journals, splines, valve stems and steering racks are typical. Medium-carbon and low-alloy steels respond well: 1045, 4140, 4340, 4130. Case-hardening steels work too if the surface carbon is already high enough.

It does not fit everything. Thin walls warp when one side heats before the other. Complex internal bores are hard to reach with a coil. Parts that need a deep case over 6 mm usually go to carburizing, because reaching that depth by induction needs low frequency and long dwell, which starts to compete with furnace routes on cost.

Geometry drives the decision more than material. A coil needs line of sight to the surface it heats. If a feature is hidden behind a shoulder or inside a closed cavity, the coil cannot follow it. In that case you either redesign for access, harden before the feature is formed, or choose a different process.

There is also a distortion budget. Induction heats a local band, so the surrounding cold metal constrains it. That usually means less distortion than through-hardening, but not zero. Long slender shafts still need straightening or a post-grind allowance of 0.05–0.15 mm on hardened diameters.

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Good fitLocal bands on shafts, teeth, journals and races.
2
Poor fitDeep cases over 6 mm, thin walls, hidden internal features.
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Budget allowanceLeave 0.05–0.15 mm on hardened diameters for grinding.

After hardening

What hardening does to your finishing and tolerance plan

A hardened surface is not a finished surface. Martensite at 58–62 HRC will not take a normal carbide cut cleanly, and the heat cycle moves the part a little. Plan the sequence so grinding, honing or hard turning comes after hardening. On a Ø50 mm journal with a 0.08 mm growth allowance, that is a routine grind stock, not a rescue operation.

Distortion is the number that surprises people. A 300 mm shaft with a hardened band can bend 0.05–0.20 mm depending on symmetry and quench. Symmetric heating around the axis reduces it. So does a rotating fixture, a controlled polymer quench instead of a water spray, and a slow cool after the martensite finish.

For our own machining work, the practical link is tolerance stack. We hold ±0.005 mm on machined features, but a hardened and ground diameter should be specified separately. Give the pre-hardening drawing with a grind allowance and the post-hardening drawing with final size and surface finish, for example Ra 0.8–1.6 μm on the bearing seat.

Inspection should be agreed before the first part. Case depth by microhardness traverse or a cut-and-etch sample, surface hardness in HRC, and a distortion check on the critical diameters. Those three cover most disputes. Without them, a supplier and a buyer can both be right and still disagree about the part.

1
SequenceHarden, then grind or hone. Never the reverse.
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Grind stock0.05–0.15 mm on diameters that will be ground.
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Two drawingsOne pre-hardening with allowance, one final.
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InspectionCase depth, surface HRC, distortion on critical sizes.

Process comparison

Induction hardening against the furnace routes

Case depth and cycle time decide most of these calls.

Process	Typical case depth	Cycle time	Best fit
Induction hardening	0.5–5 mm	Seconds	Local bands, high volume, contoured surfaces
Carburizing	0.5–3 mm	Hours	Complex geometry, deep uniform case
Nitriding	0.1–0.5 mm	Tens of hours	Finished parts, low distortion, wear surfaces
Through hardening	Full section	Hours	Small parts, uniform hardness needed
Flame hardening	1–6 mm	Seconds	Large simple surfaces, low volume

The call: local hardness or uniform case

If hardness is needed in one band or one contour on a medium-carbon steel part, choose induction hardening. If the whole section needs a deep uniform case, or the geometry hides the surface from a coil, choose carburizing or nitriding instead.

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FAQs

Induction hardening questions engineers ask

Can induction hardening replace carburizing?

Only where the case is local and under about 5 mm. Carburizing still wins for deep cases, complex internal geometry and parts where the whole surface must be hard.

If your part needs a 1 mm case on a shaft journal, induction is faster and cheaper at volume. If it needs a 2 mm case inside a bore, it is not.

How much distortion should I expect?

On a symmetric shaft, typically 0.05–0.20 mm of bend over 300 mm. Asymmetric heating, thin walls and a harsh quench push it higher.

A rotating fixture, a polymer quench and symmetric coil placement reduce it. Leave grind stock so the geometry can be corrected after hardening.

Which steels respond best?

Medium-carbon and low-alloy grades: 1045, 4130, 4140, 4340. They reach 58–62 HRC at the surface with a normal quench.

Low-carbon steels need a carburized surface first, or they will not harden enough. Stainless grades vary widely and should be tested on a sample.

Does hardening come before or after machining?

Rough machine, harden, then finish. Hardened surfaces at 58–62 HRC are ground, honed or hard turned, not cut with standard carbide.

Keep a 0.05–0.15 mm allowance on diameters that will be ground, and specify the final size on a separate post-hardening drawing.

What does SDF profile hardening actually do?

It shapes the heated zone along a contour instead of running one straight coil pass. The result is a more even austenitized layer around gear teeth, cam lobes and bearing races.

The payoff is fewer soft spots and less distortion on contoured parts. On a plain cylindrical shaft it adds little over a conventional single-shot coil.

How is case depth verified?

Microhardness traverse across a cut sample, or a cut-and-etch test on a sacrificial part. Both give a depth to a defined hardness limit, usually 50 HRC or 550 HV.

Agree the method and the limit before production. Surface HRC alone does not tell you how deep the case runs.