Ultrahigh impact toughness achieved in high strength bainitic ferrite/retained austenite lamellar steels below-M temperature

Abstract

• Bainite transformation occurs during austempering process below- M f temperature; • Bainite transformation below- M f temperature refines bainitic ferrite laths; • Bainite transformation below- M f temperature reduces the size of blocky M-RA; • The impact toughness of high strength bainitic steel is improved significantly. A novel heat treatment process below- M f for medium carbon steel, with the relationship between the microstructure and mechanical properties was investigated. This study demonstrated that a bainitic ferrite/retained austenite lamellar steel with a tensile strength of 1436 MPa and a Charpy V-notch impact energy of more than 130 J was obtained by an isothermal heat treatment below- M f . This result is considerably higher than that of above- M s (21.3 J at 400 °C) and below- M s (46.0 J at 300 °C). The isothermal heat treatment process below- M f refines the bainitic ferrite laths, reduces the size and amount of blocky martensite/retained austenite, and makes the bainitic ferrite and retained austenite lamellae more uniform. The results demonstrate that the lamellar structure of bainitic ferrite laths and ductile carbon-rich retained austenite significantly improve the impact toughness of bainitic steels.