What process is used to decrease the hardness and increase the toughness of steel?

Tempering is the correct process for decreasing the hardness and increasing the toughness of steel after it has been hardened. During the hardening process, steel is heated to a high temperature and then rapidly cooled, usually through quenching. This can result in a very hard but brittle material. Tempering involves reheating the hardened steel to a lower temperature and then allowing it to cool down gradually. This heat treatment process relieves internal stresses and reduces brittleness, thereby improving the toughness of the steel while still maintaining a certain level of hardness.

In this way, tempering effectively balances the properties of steel to achieve the desired performance characteristics for various applications. The result is a more versatile material that can withstand impacts and stresses better than untreated hardened steel.