@_Darthsoldier_
It's called quenching! It's a common practice in metalworks to quickly cool down steel to make it harder. It's possible to quench in water, gas or in this case probably mineral oil.
Note: I've only worked with aluminum heat treatment and not steel.
Holy shit guys I've made it to 800 followers! Thanks for you guys for the continued support... Now I wonder what to share (416 is still on the way!)
多謝大家繼續支持,現在有800跟蹤!唔知800人應該放咩福利~ (請放心,416是未到)