When steel is hardened, it is heated and then cooled rapidly. The quenching oil decides how fast heat leaves the part, which affects hardness, distortion and cracking.
The three cooling stages
- Vapour stage – a vapour blanket forms around the hot part and cooling is slow.
- Boiling stage – the blanket collapses and cooling is fastest.
- Convection stage – cooling slows as the part approaches the oil temperature.
Fast quenching oils break the vapour stage quickly and suit low-hardenability steels and thick sections. Normal oils are used for general work, while hot oils are run at higher bath temperatures to reduce distortion.
How to select
- Match oil speed to steel grade and section thickness.
- Choose a bright-quenching oil if a clean, light surface is needed.
- Check the flash point and keep the bath well below it.
Bath care
Control bath temperature and agitation, keep water out, and test the oil periodically, because oxidation slowly changes its cooling behaviour.
Need help choosing?
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