Impact properties of tempered bainite–ferrite dual phase steels

To improve both strength and toughness of AISI 4340 steel three microstructures, full bainite, bainite–ferrite and tempered bainite–ferrite, were produced by heat treatment of this steel. Tensile, impact and hardness properties of these microstructures were compared. The results showed that with tempering of bainite–34% ferrite dual phase steel, elongation and charpy impact energy increased significantly in comparison to bainite and bainite–ferrite microstructures. Also ductile-brittle transition temperatures of bainite–ferrite and tempered bainite–ferrite steels were measured and confirmed superior toughness properties of this microstructure. Fracture surface analysis of charpy specimens also showed increase in toughness of tempered bainite–ferrite in comparison to bainite–ferrite and full bainite microstructures. Radial marks, shear lip areas and crack initiation site region in fracture surfaces of three mentioned microstructures were also considered and compared.