Induction Hardening & Tempering
Selective surface or through hardening of steel and cast iron by rapid austenitising and quenching, with optional induction tempering for toughness.
How It Works
The workpiece surface is heated above the austenitising temperature — typically 850–1050°C for carbon and alloy steels — using high power density at a frequency chosen to match the desired case depth.
Skin depth determines the heated layer. High frequency (100–400 kHz) produces shallow cases of 0.5–2 mm. Medium frequency (3–30 kHz) reaches 3–10 mm. Dual-frequency systems combine both for complex profiles like gear teeth, where the root and tip require different heat penetration.
After heating, the part is quenched — typically with water, polymer solution, or forced air — to transform austenite into martensite. The unhardened core retains its original ductility and toughness.
Induction tempering follows hardening: a second, lower-power pass at 150–350°C for 1–10 seconds reduces brittleness while retaining most of the hardness. This replaces long furnace tempering cycles (1–2 hours) with seconds of inline processing.
Typical Parameters
| Process Variant | Frequency | Power Density | Temp Range | Cycle Time |
|---|---|---|---|---|
| Surface hardening, thin case (< 1 mm) | 100 – 400 kHz | 5 – 15 kW/cm² | 850 – 1000°C | 0.5 – 3 s |
| Surface hardening, medium case (1–3 mm) | 10 – 50 kHz | 2 – 8 kW/cm² | 850 – 1000°C | 2 – 10 s |
| Through hardening (small parts) | 3 – 30 kHz | 1 – 3 kW/cm² | 850 – 1050°C | 5 – 30 s |
| Gear tooth hardening (single-shot) | 100 – 400 kHz | 5 – 20 kW/cm² | 860 – 950°C | 0.5 – 5 s |
| Gear tooth hardening (dual-freq) | 10 kHz + 200 kHz | 5 – 15 kW/cm² | 860 – 950°C | 1 – 5 s |
| Crankshaft journal hardening | 3 – 10 kHz | 2 – 5 kW/cm² | 850 – 950°C | 3 – 15 s |
| Induction tempering | 3 – 50 kHz | 0.5 – 2 kW/cm² | 150 – 350°C | 1 – 10 s |
Key Considerations
- Carbon content must be sufficient — steel needs > 0.3% C for meaningful hardness increase. Low-carbon steels will not form martensite regardless of quench rate.
- The Curie transition at ~770°C causes relative permeability to drop from ~200 to 1, requiring 5–10× more power above this temperature. Power supplies must handle this step change.
- Quench delay must be minimized (< 1 second) to prevent pearlite or bainite formation in the cooling curve. Integrated spray quench rings are standard.
- Geometry affects uniformity — sharp corners, keyways, oil holes, and splines create flux concentration hot spots. Controlled rotation and coil profiling compensate.
- Induction leaves compressive residual stress on the hardened surface, which is beneficial for fatigue life — a key advantage over furnace hardening which produces tensile surface stress.
- Scan hardening (moving part through coil) gives more uniform case depth than single-shot for long shafts and axles. Speed control is critical — typically 5–25 mm/s.
Common Coil Geometries
Solenoid (Single/Multi-Turn)
Standard for shafts, pins, and cylindrical parts. Used in both single-shot and scanning modes. Turn count and spacing control the heat pattern.
Channel (U-Shaped)
For flat surfaces like machine tool ways or plate edges. The workpiece passes through or under the U-channel for linear hardening.
Profiled / Contour Coil
Custom-machined to match complex geometry — crankshaft journals, cam lobes, bearing races. CNC-profiled from copper for precise flux distribution.
Hairpin Coil
For selective hardening of one zone, such as a gear root or bearing inner race. Concentrates energy in a narrow band without heating adjacent areas.