Effect of austempering temperature on the microstructure and wear resistance of ultrafine bainitic steel

Ultrafine bainitic steels, which are derived from nanostructured carbide-free bainitic steels, exhibit a remarkable combination of ultra-high strength and toughness together with excellent wear resistance. Their excellent integrated mechanical properties has made ultrafine bainitic steels a popular choice for application as wear-resistant parts. In this study, a 0.7-C low alloy ultrafine bainitic steel was designed, and the effects of different austempering temperatures on the bainitic transformation kinetics, microstructure, and dry sliding wear resistance of ultrafine bainitic steels were studied. Dilatometry, two-body abrasion testing, optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction, laser-scanning confocal microscopy, and energy-dispersive spectrometry were used to study the abovementioned effects. Meanwhile, the wear performance and wear mechanism under two-body abrasion of ultrafine baintic steels with different austempering temperatures were also studied. The results demonstrate that the microstructures of ultrafine bainitic steel produced at different austempering temperatures comprise both lamellar bainitic ferrite and film-like and blocky retained austenite. With increasing austempering temperature, the transformation rate of bainite increases, and the incubation period and phase transformation completion time of bainite significantly reduce; in addition, the bainitic ferrite plates are more coarsened, the volume fraction of retained austenite increases, and the hardness decreases. Moreover, when the ultrafine bainitic steel is subjected to the twobody abrasion test, the wear surface is mainly featured by furrows and grooves, and the predominant wear mechanism is micro-cutting. Furthermore, the wear resistance of ultrafine bainite post austempering at different temperatures is better than that of tempered martensite; this wear resistance increases with decreasing isothermal temperatures. Ultrafine baintic steel post austempering at 250℃ possesses the best wear resistance, and the relative wear resistance is 1.28 times higher than that of tempered martensitic steel; this is attributed to the refined microstructure and the transformation induced plasticity (TRIP) effect of ultrafine bainitic steel.