The study of impact properties and micro hardness of ferrite & martensite on a tempered dual phase steel

In this research, the effect of double tempering on impact properties and micro hardness of ferrite & martensite on a dual phase 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. Impact testing was carried at room temperature and sub-zero temperatures. Micro & macro hardness testing and microstructure evaluation by optical and scanning electron microscope were done. 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. The results indicate that hardness of ferrite and martensite decrease with increasing the tempering temperature. Although hardness of ferrite is decreasing gradually, there is remarkable decrease of martensite hardness at temperature over 250 °C.