Changes in Mechanical Properties with Tempering

Tempering improves the ductility & toughness of quenched steel whereas decreases hardness. Figure illustrates how these properties are influenced by tempering. The tempering temperature is so selected that it results in the desired combination of the properties. Some steel illustrated drop in impact values in the certain tempering temperatures range. Third drop is an indication of brittleness and such range must be avoided. For mild steel this brittleness (termed blue brittleness) occurs at a tempering temperature of about 300^oC.

Figure: Tempering Diagram of Property Chart for Water Quenched 26 mm Diameter Bars of Eu-12 Steel (Scale for Stress Only)