Heat treatment and surface
Heat treatment (quench and temper, normalise, anneal): what goes wrong and where to inspect
Heat treatment gives steel the properties the designer specified. It is a special process: the result cannot be fully verified by inspecting the part afterwards.
01How the process works
Parts are heated in furnaces to a defined temperature, soaked, and cooled at a controlled rate: in oil, water or polymer for hardening, in air for normalising, in the furnace for annealing. Tempering after hardening adjusts strength and toughness.
Furnace temperature uniformity, thermocouple calibration, load arrangement and quench medium condition determine the result. Hardness on the surface does not prove core properties; test coupons or sacrificial parts are needed for critical components.
- Loading. Parts arranged on fixtures with thermocouples.
- Heating and soaking. Controlled ramp and soak time.
- Quenching. Rapid transfer to quench medium.
- Tempering. Reheating to final hardness.
- Testing. Hardness, mechanical tests, distortion check.
02What typically goes wrong
| Defect | Typical cause | How it is found |
|---|---|---|
| Low or uneven hardness | Temperature, quench, section size | Hardness survey |
| Quench cracks | Severe quench, sharp corners | MT |
| Distortion | Load support, quench | Dimensional check |
| Decarburisation | Furnace atmosphere | Microhardness, metallography |
| Low toughness | Wrong tempering | Impact test |
| Untraceable loads | Missing records | Furnace chart review |
03Where an inspector should look
Before production
- Heat treatment specification
- Furnace survey and calibration (TUS, SAT)
- Test coupon arrangement
During production
- Charts with load thermocouples
- Quench delay and medium temperature
After production
- Hardness per load
- Mechanical tests on coupons
- MT for cracks on critical parts
04Standards and references
CQI-9 (heat treat system assessment), AMS 2750 (pyrometry), EN 10083 (quench and temper steels), ISO 6508 and ISO 6506 (hardness).
Standards are named for orientation. The applicable edition and acceptance criteria are those in your contract and specification.