Oil for heat treatment of steels to adjust defined hardness (see tempering oil, burnishing oil). Depending on service temperature and requirements tempering oils (bath temperature up to approx. 320 °C) are used, martempering oils (bath temperature from 100 °C to 230 °C), high-performance quenching oils (addition of hydrocarbon resins to reduce the vapour phase), bright hardening oils (for bright metal surfaces), which can be water soluble or emulsifiable by adding emulsifiers (see bright hardening oils).