Annealing is defined as heating material that has previously been hardened to a temperature between room temperature and its lower transformation temperature, holding it at this temperature, then cooling it. If we selected a hardening temperature of 1080 °C and oil as the hardener, for instance, this would result in a hardness of approx. 64 HRC (Rockwell hardness) directly after quenching at room temperature. However, this would not result in the desired “tetragonal martensite”. Ultimately, however, we want to obtain cubic martensite, which is less subject to cracking and breaking.
To achieve this, we must “anneal” the steel. The transformation of tetragonal martensite to cubic martensite occurs at an approximate temperature of 200 °C. This means that we must heat our steel again and hold it at the selected temperature for a certain period of time. For instance, if we want to achieve a hardness of approx. 59 HRC after annealing, we must bring the steel back to a temperature of approx. 230 °C and hold it there for approx. 1 hour.
Heat treatment that is generally carried out after hardening or another heat treatment in order to achieve desired values for certain properties. This consists of warming to a specified temperature once or multiple times, holding at this temperature, and then cooling depending on the intended purpose. Annealing generally reduces hardness, however in some cases it can increase hardness.