Quenching and Tempering

Tempering means: first harden, then temper at high temperature. So you make the part hard and then give it back its toughness through tempering. The structure becomes finer and more uniform. If you don't hit the temperature or time during tempering, the part will be either too brittle or too soft. After tempering, you let the workpiece cool down slowly so it doesn't crack and the stresses are relieved.