Heat treatment and surface
Induction hardening: what goes wrong and where to inspect
Shafts, axles, bearing seats and gear teeth are locally hardened by heating only the surface with an induction coil and quenching immediately.
01How the process works
High frequency current in a copper coil induces eddy currents that heat the part surface in seconds; a spray quench follows. Scanning or single shot techniques harden defined zones, leaving the rest of the part soft and tough.
Hardened pattern location and depth, surface hardness and absence of cracks are verified by sectioning and etching sample parts, hardness tests and MT. Power, frequency, scan speed and quench flow are monitored for every part.
- Set-up. Coil, frequency and power selected.
- Heating. Surface heated by induction.
- Quenching. Spray quench immediately.
- Tempering. Low temperature temper.
- Verification. Pattern, hardness, cracks.
02What typically goes wrong
| Defect | Typical cause | How it is found |
|---|---|---|
| Pattern in wrong position | Coil set-up | Etched section |
| Shallow hardened depth | Power, time, frequency | Hardness traverse |
| Quench cracks | Sharp edges, quench severity | MT |
| Soft zones | Quench nozzles blocked | Hardness survey |
| Overheating | Power too high | Metallography |
| Unmonitored parts | No energy monitoring | Process data review |
03Where an inspector should look
Before production
- Hardening pattern drawing
- Sample sectioning approval
During production
- Energy and quench monitoring per part
- Periodic section checks
After production
- Hardness and MT
- Straightness
04Standards and references
ISO 3754 (effective depth of hardening after flame or induction hardening), CQI-9.
Standards are named for orientation. The applicable edition and acceptance criteria are those in your contract and specification.