system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How does quenching harden steel and how much harder does it get?
gpt
The chemistry of solid steel is kind of nuts.

In short, as it cools to room temperature, steel separates to form two kinds of crystal. As this happens, the carbon atoms move from some areas into others, forming the carbon-rich crystal and the carbon-poor crystal. Depending on the carbon content, the balance of these crystals varies.

There is, however, a third option. Martensite. This is what happens when the carbon does not have time to move, and so the two separate crystals can't form. By cooling the steel faster, you force martensite to form, which is much harder than the other two.