In this research ,the effect of double tempering on microstructure ,impact and tensile properties of a dual phase structural  steel with chemical composition of C 0.17%-Si 0.41%-Mn 1.24% was investigated .For dual phase ferrite-martensite microstructure formation ,intercritical annealing at 780 °C for 45 min and quenching in water was selected. After that, double tempering at 150, 250,350 and 450 °C for 30 min and quenching in still air was performed. Tensile testing was carried at room temperature and impact test at room and sub-zero temperatures. Microstructure evaluation was done using an optical and scanning electron microscope. The results indicate that by increasing the tempering temperature, elongation and reduction of area will increase but ultimate tensile strength will decrease and there is no significant change of yield strength.

Investigation on impact properties shows that the impact energy & lateral expansion at different testing temperatures will increase gradually due to higher tempering temperature. However, there is remarkable increase of impact properties when samples are tempered at 450 °C.

Keywords: Dual phase steel, two stage tempering, mechanical properties, impact energy