The structure of steel is much hard and brittle for a lot of applications after hardening. Thus tempering process is applied for relieving this brittleness and getting structure tough.
Tempering is a heating process that steel is heated to any temperature under 723°C (A1 temperature). This process can be applied in great period to A1 temperature. Consequently some changes will be occured in mechanical properties and microstructure of steel. In tempering period, while temperature is increasing, hardness decreases and toughness increases.