Comparison of mechanical properties of martensite/ferrite and bainite/ferrite dual phase 4340 steels

Different microstructures were produced by heat treatment of 4340 steel. These microstructures are bainite, martensite, ferrite–martensite and ferrite–bainite. Mechanical tests were carried out at room temperature. The results showed that steel with bainite–ferrite microstructure has better ductility and charpy impact energy than steels with martensite–ferrite and full bainite microstructures. But yield and tensile strengths of this steel are less than the yield and tensile strengths of the other two steels. Hardness measurements showed that their hardness is the same. Fracture surface observations of tensile specimens showed increase in toughness of bainite–ferrite in comparison to martensite–ferrite and full bainite microstructures.