What is best to quench tool steel in to harden it?
Different types of steel require different quench media.
Some require oil some require water some require air quench.
It is best to refer to the manufactures guidelines and quench as close as possible to their recommendations.
If you quench an oil hardening steel in water it will be too brittle. If you quench the same steel in air it will be too soft.
The thickness of the steel will play a roll as well. Thicker sections usually mean you can quench in the next faster medium.