quenching

A blacksmith heats a blade to glowing orange, then plunges it hissing into a bucket of oil or water. That sudden dunk is quenching: cooling hot steel so fast that it has no time to settle into its soft, comfortable structure and is instead frozen into a hard, strained one. Quenching is how steel is made hard.

Here is the mechanism in plain steps. Heated above its critical temperature, steel becomes austenite, which holds carbon dissolved in a face-centered-cubic lattice. Cool it slowly and the carbon has time to move out and form soft pearlite. Cool it fast enough, though, and the carbon is trapped: the iron snaps into a distorted body-centered structure with carbon jammed inside, called martensite, very hard and very brittle. The quench medium sets the speed: brine and water pull heat out fastest (hardest, but risk cracking), oil is gentler, and still air is slowest. Faster is harder but also means more thermal shock, distortion, and risk of quench cracks.

Quenching almost never stands alone, because as-quenched martensite is so brittle it can crack on its own. It is nearly always followed by tempering, a gentle reheat that trades a little hardness for much-needed toughness. The picture to hold: quenching then tempering is shocking the steel glass-hard, then relaxing it just enough not to be glassy. And crucially, whether a quench hardens the whole cross-section or only the skin depends on the steel's hardenability, not just on how cold the bucket is.