Changes in Mechanical Properties along with Tempering:
Tempering enhances the toughness and ductility of quenched steel though decreases hardness. Following figure demonstrates how these properties are influenced with tempering. The tempering temperature is consequently chosen such it results in the required combination of the properties. Several steel shows drop in impact values in the specific tempering temperatures range. Third drop is a suggestion of brittleness and that range should be ignored. For mild steel this brittleness as termed blue brittleness happens at a tempering temperature of about 300oC.
Figure: Tempering Diagram of Property Chart for Water Quenched 26 mm Diameter Bars of Eu-12 Steel as Scale for Stress Only