Ultrahigh strength-toughness combination in Bainitic rail steel: The determining role of austenite stability during tempering

We elucidate here the impact of tempering on the stability of retained austenite (RA) in obtaining excellent combination of strength and toughness in an ultrahigh-strength low carbon air-cooled bainitic rail steels. The microstructure of rail steels comprises of bainite, martensite and martensite/austenite (M/A) constituents. The bainitic rail steels exhibit excellent strength and toughness combination after tempering at 280 °C (ultimate tensile strength: 1388 MPa; total elongation: 16%; U-notch impact toughness at 20 °C: 130 J/cm2). The filmy RA in the tempered microstructure has high mechanical stability, which ensures retention of superior mechanical properties of bainitic steels. The study underscores stability of filmy retained austenite obtained via tempering is the underlying reason for superior mechanical properties.