Heat treating refers to several different processes that involve manipulating the temperature of steel to change its material properties.
Annealing, used to reduce hardness and increase ductility, making steel easier to work with. The annealing process slowly heats the steel to the desired temperature and holds it there for a period of time. The time and temperature required depend on the specific alloy and decrease with increasing carbon content. Finally, the steel is cooled slowly in a furnace or surrounded by insulation.
Normalizing, which relieves internal stress in steel while maintaining higher strength and hardness than annealed steel. During normalizing, steel is heated to a high temperature and then cooled to a moderate temperature to improve the hardness of the steel.
Quenching not only makes the steel harder and increases its strength, but also makes it more brittle. The hardening process involves slowly heating the steel, soaking it at high temperatures, and then rapidly cooling it by immersing the steel in a liquid such as water, oil, or a brine solution.
Tempering, which can be used to alleviate some of the brittleness that comes with steel hardening. Tempering of steel is almost identical to normalizing: first slowly heating it to a selected temperature, then air cooling the steel. The difference is that tempering is done at a lower temperature than other processes, which reduces the brittleness and hardness of the tempered steel.