Tempering of Steel
After a steel is hardened, it is too brittle for ordinary purposes. Some of the badness should be removed and toughness induced. This process of reheating quench hardened steel to a temperature below the transformation range and then, cooling it at any rate desired is called tempering. The metal must be heated uniformly to a predetermined temperature, depending on the toughness desired. As the tempering temperature increases, toughness increases and hardness decreases. The tempering range is usually between 370 and 750°F (188 and 399°C), but sometimes is as high as 1100°F (593°C).