Microstructure and mechanical properties of high-carbon Si–Al-rich steel by low-temperature austempering

The microstructure and mechanical properties of a high-carbon Si–Al-rich steel austempered at 220–260 °C were studied by optical microscopy, X-ray diffraction, transmission electron microscopy and tension and impact tests. Results show that the nanostructured bainite microstructure composed of 38–57 nm-thickness laths of bainitic ferrite and retained austenite is produced and the excellent combination of mechanical properties is achieved. The high yield strength 1534–1955 MPa and high ultimate tensile strength 2080–2375 MPa are obtained and accompanied by the elongation 6.7–7.8% and the Charpy impact energy 7.8–22.2 J. Fracture surface observations by scanning electron microscopy indicate that both the impact and tension fracture are in a mixed mode of the brittle quasi-cleavage fracture and the ductile dimple fracture, while the ductile dimple fracture is predominant in the tension crack propagation.

► New designed high-carbon Si–Al-rich steel was austempered at 220–260 °C. ► Nanostructured bainite and excellent mechanical properties were achieved. ► Yield strength and tensile strength reached up to 1534–1955 MPa and 2080–2375 MPa. ► Elongation 6.7–7.8% and Charpy impact energy 7.8–22.2 J were obtained. ► Crack propagates in a mixed mode of quasi-cleavage fracture and dimple fracture.