The purpose of heating the workpiece to appropriate temperature, adopting different holding time according to material and workpiece size, and then cooling slowly (the slowest cooling rate, most commonly cooling in air or cooling with furnace) is to make the internal structure of the metal reach or close to equilibrium state, to obtain good technological performance and service performance, or to prepare the structure for further quenching. In short, make the steel really soft. But soft is not the easiest status to cut.