Induction Hardening
Case & surface hardening · Surface hardening
Induction hardening uses an alternating magnetic field to austenitize selected steel surfaces, followed immediately by quenching.
Induction harden at a glance
Four scores, each 0–100 against every heat treatment in this catalog. Hardness is derived from the HRC target (20 HRC = 0, 70 HRC = 100) — treatments without a hardness target (anneals, stress relief, HIP, cryo) are not rated. Toughness, dimensional stability and cost are FabDigit's engineering assessment.
What is Induction harden?
Induction hardening uses an alternating magnetic field to austenitize selected steel surfaces, followed immediately by quenching. It provides repeatable wear and rolling-contact hardness while preserving a tough core.
How it works
Alternating current in a purpose-built copper coil induces eddy currents in the workpiece. Electrical resistance heats the surface rapidly above the austenitizing range while the interior remains cooler; higher frequency concentrates heating nearer the surface. An integral spray or immersion quench then transforms the heated zone to martensite. Coil shape, frequency, power density, coupling gap, dwell or scan speed, and quench delay define the pattern and depth. Because no carbon is added, the steel's carbon content and hardenability set the attainable hardness. A low temper is often used to relieve quench stress.
Why specify it
- Produces about 48–60 HRC on the common 0.35–0.60% carbon steel range
- Localizes hardness without changing the full section
- Supports automated, repeatable production cycles
- Allows effective case depths from about 0.75 to 6 mm by frequency and dwell selection
Where it falls short
- Requires a dedicated coil and validated coupling geometry
- Sharp corners and diameter changes can overheat
- Quench cracking remains possible at holes, keyways, and hardening boundaries
- Low-carbon steel cannot reach high surface hardness without prior carbon enrichment
The Induction harden cycle
Typical furnace parameters from the governing specifications and shop practice; the heat treater sets the exact recipe from the alloy, section size and required result.
| Temperature | 800–950 °C (1,472–1,742 °F) — Rapid surface austenitizing window; grade-specific control is required. |
|---|---|
| Soak | 0–0.03 h — About 1 second to 2 minutes, depending on single-shot or scan hardening. |
| Cooling | Polymer quench |
| Atmosphere | Induction coil |
| Door to door | 0.02–1 h — Part processing time excluding coil development and setup. |
Stages
- 1Locate the part in the qualified coil
- 2Apply power or scan to the validated thermal profile
- 3Quench immediately
- 4Temper when specified
What Induction harden does to the part
| Hardness | 48–60 HRC — ASM practice for steels containing about 0.35–0.60% carbon after a 180 °C temper. |
|---|---|
| Case depth | 0.75–6 mm — Effective case depth to the drawing-specified threshold, commonly 50 HRC, measured per SAE J423. |
| Toughness | The core retains its prior strength and toughness; temper the martensitic case when impact or crack resistance matters. |
| Distortion | Medium risk. Short localized heating reduces bulk movement, but nonuniform quench and pattern transitions can move thin sections. |
| Surface after treatment | Usually darkened or lightly oxidized at the hardened band; protected-atmosphere systems can reduce scale. |
| Machining sequence | Finish soft areas before treatment and grind precision hardened diameters afterward without removing the minimum effective case. |
Specifications for Induction harden
The documents a drawing cites to define this treatment. Each links to a page explaining the standard, its classes and the tests it invokes.
| Standard | Role | Classes used | Status |
|---|---|---|---|
Induction Hardening of Steel Parts | Governing | — | Active |
Test and related methods: ASTM E18 (Standard Test Methods for Rockwell Hardness of Metallic Materials), ASTM E384 (Standard Test Method for Microindentation Hardness of Materials), SAE J423 (Methods of Measuring Case Depth), AMS 2759 (Heat Treatment of Steel Parts, General Requirements), ASTM E140 (Standard Hardness Conversion Tables for Metals Relationship Among Brinell Hardness, Vickers Hardness, Rockwell Hardness, Superficial Hardness, Knoop Hardness, Scleroscope Hardness, and Leeb Hardness).
Drawing call-out
INDUCTION HARDEN PER AMS 2745 JOURNAL SHOWN TO 52–58 HRC; 1.5–2.5 MM EFFECTIVE CASE DEPTH TO 50 HRC PER SAE J423; VERIFY SURFACE PER ASTM E18Which alloys take Induction harden?
Verdict per material family, with the grades that behave differently. Each family links to its datasheets in the material library. Typical grades: 1035, 1045, 1050, 4140, 4150, 4340, 5140, 52100.
| Material family | Verdict | Notes |
|---|---|---|
| Yes | Medium-carbon and alloy steels are the primary candidates.
| |
| With care | Fully sintered steel can be considered if carbon and density are verified. | |
| With care | Printable martensitic steels may respond after density, chemistry, and cracking are qualified. | |
| Limited | Only martensitic stainless grades respond directly.
| |
| No | Aluminum cannot form martensite by induction quenching. | |
| No | Brass is not martensitically hardenable. | |
| No | Bronze is not martensitically hardenable. | |
| No | Cobalt-chrome is not induction quenched to a martensitic case. | |
| No | Copper alloys require different precipitation or cold-work strengthening. | |
| No | Magnesium uses alloy-specific solution and aging cycles. | |
| No | Nickel superalloys do not use this martensitic route. | |
| No | Precious metals do not form a martensitic induction-hardened case. | |
| No | Refractory metals require protected alloy-specific thermal processing. | |
| No | Conventional induction surface hardening is not a titanium strengthening treatment. | |
| No | Tungsten heavy alloys do not respond to steel induction hardening. | |
| No | Zinc alloys melt far below steel austenitizing temperatures. |
Design rules for Induction harden
What to decide on the drawing before the part goes to the heat treater.
- Show the hardened pattern and transition boundaries.The coil produces hardness only where electromagnetic coupling is qualified.
- Keep holes, keyways, and sharp notches out of the hardening boundary.Current concentration and quench stress raise crack risk.
- Specify effective depth and threshold together.For example, 2.0 mm ECD at 50 HRC is inspectable; 2.0 mm case alone is ambiguous.
- Use generous radii at diameter transitions.Smooth geometry reduces electromagnetic hot spots and stress concentration.
- Control runout before coil design.A changing coupling gap changes power density and case depth around the part.
Induction harden versus the alternatives
Treatments engineers weigh against Induction harden, and when each wins.
| Treatment | Hardness | Toughness | Stability | Cost | Choose it when |
|---|---|---|---|---|---|
Induction harden this page | 67 | 50 | 60 | $$ | — |
| 68 | 45 | 25 | $$ | when the part is too large or volume does not justify a dedicated coil | |
| 85 | 50 | 85 | $$$$ | when lower distortion and no quench are more important than deep case |
Cost, lead time and applications
Relative cost
Induction Hardening is a standard-cost treatment ($$) relative to the other heat treatments in this catalog.
Heat treatment is priced per lot with a furnace minimum. The exact figure appears on the instant quote once the part is uploaded.
Lead time
| Added lead time | 3–7 working days |
|---|---|
| Furnace time | 0.02–1 h |
What Induction harden is used for
- Gear teeth and sprockets
- Crankshaft and camshaft journals
- Axle and transmission shafts
- Bearing races
- Pins and track links
- Valve seats
Induction harden — frequently asked
What steels induction harden well?
ASM identifies roughly 0.35–0.60% carbon as the common range, including 1045, 4140, and 4150. Lower-carbon steels produce less martensitic hardness unless they were previously carburized.
How deep can induction hardening go?
Typical engineered depths span about 0.75–6 mm. Lower frequency and longer heating generally increase depth, but geometry and hardenability must be demonstrated on coupons or sacrificial parts.
Does induction hardening affect the core?
The intended cycle heats only the surface above the transformation range, so the core retains its previous condition. On thin sections, thermal penetration can transform the full thickness and must be checked.
Why specify 50 HRC with case depth?
SAE J423 defines effective depth by a stated hardness threshold. The conventional 50 HRC threshold is common, but lower-carbon steels may require the standard's composition-dependent criterion.
Is induction hardening distortion-free?
No. Local heating is brief, but phase transformation and spray quenching still create movement; precision journals are commonly finish ground after treatment.
Is a new coil needed for every part?
A coil is normally designed around the feature and coupling gap. Families of similar diameters may share tooling, but the qualified recipe must still demonstrate hardness and depth.