Effects of Tempering on the Microstructure and Properties of a High-Strength Bainite Rail Steel with Good Toughness

Abstract

An advanced bainite rail with high strength–toughness combination was produced in a steel mill and the effects of tempering on the microstructure and properties of the bainite rail steel were investigated by optical microscopy, transmission electron microscopy, electron back-scattering diffraction and X-ray diffraction. Results indicate that the tensile strength, elongation and impact toughness were about 1470 MPa, 14.5% and 83 J/cm2, respectively, after tempering at 400 °C for 200 min. Therefore, a high-strength bainite rail steel with good toughness was developed. In addition, the amount of retained austenite (RA) decreased due to bainite transformation after low-temperature tempering (300 °C) and RA almost disappeared after high-temperature tempering (500 °C). Moreover, as the tempering temperature increased, the tensile strength of the rail head first decreased due to the decreased dislocation density and carbon content in bainite ferrite and the coarseness of bainite ferrite, and then increased because of carbide precipitation at high-temperature tempering. Furthermore, RA played a significant role in the toughness of bainite rail. The elongation and toughness of the rail obviously decreased after tempering at 500 °C for 200 min because of the disappearance of RA and appearance of carbides.

Keywords

• bainite rail
• tempering
• retained austenite
• tensile property
• impact toughness