Study on the relationship between the microstructure characteristics, tensile properties and impact toughness of carbide-free bainitic steel

To understand the relationship between microstructure characteristics, tensile properties, and impact toughness of bainite structure, microstructure characterization, μ-DIC and mechanical tests were carried out on bainitic steel subjected to pre-quenching treatment, followed by austenitizing at 800 °C or 880 °C and then by austempering. The resulting microstructure after austempering is composed of bainite, retained austenite and intercritical ferrite and/or fresh martensite. Compared with 880 °C-austenitized specimen, the final microstructure in the 800 °C-austenitized specimen is significantly refined, and the amount of large martensite/austenite (M/A) islands is also decreased. The finer microstructure of the 800 °C-austenitized specimen results in a relatively uniform strain distribution during the tensile process, delaying the occurrence of tensile necking. In contrast, the strain of the 880 °C-austenitized specimen tends to concentrate at the interface between the bainite region and M/A islands, leading to premature fracture and decreased plasticity. In addition, the smaller retained austenite size in the 800 °C-austenitized specimen enhances its stability, leading to persistent work hardening behavior and improved plasticity. Furthermore, the decrease in the amount of M/A islands in the 800 °C-austenitized specimen reduces the initiation of cracks, and finer packets and blocks increase the proportion of high angle grain boundaries, enhancing the resistance to crack propagation. Therefore, the total absorbed energy and toughness of the 800 °C-austenitized specimen are about twice higher than those of the 880 °C-austenitized specimen.