Effect of Austempering below and above Ms on the Microstructure and Wear Performance of a Low-Carbon Bainitic Steel

The microstructure and wear performance of a low-carbon steel treated by austempering below and above martensite start temperature (Ms) were investigated. The results show that the bainite, fresh martensite (FM) and retained austenite (RA) were observed in samples austempered above Ms. Except for the three above phases, the athermal martensite (AM) was also observed in samples austempered below Ms. The bainite transformation was accelerated and finer bainite was obtained due to the AM formation in samples austempered below Ms. In addition, the strength and hardness were improved with the decrease of the isothermal temperature and time, whereas the total elongation decreased with the increasing isothermal time and the decreasing isothermal temperature. Moreover, the materials austempered below Ms exhibited better wear performance than the ones treated above Ms, which is attributed to the improved impact toughness by the finer bainite and the enhanced hardness by AM. The best wear resistance was obtained in the samples austempered at 300 °C below Ms for 200 s, due to the highest hardness and considerable impact toughness.

Influence of Si and Al contents and isothermal treatment condition on the microstructure and tensile properties in ultra-high strength Fe-0.2C-2.0Mn martensite-bainite complex phase steels

This study investigated the influence of partial replacement of Si by Al on the microstructure and tensile properties of ultra-high strength steels with martensite-bainite complex microstructure produced by austenitization and subsequent isothermal heat treatment around Ms temperature. When the isothermal heat treatment was done below the Ms temperature, the fraction of martensite increased with the lower isothermal temperature, but the fractions of constituent phases in the final microstructure were not significantly affected by the partial replacement of Si by Al. Nevertheless, the increase in Al content in the complex phase steel accelerated the bainite transformation, which is thought to be associated with the increase of the free energy difference between FCC and BCC. The enhancement of the bainite transformation not only effectively suppressed the martensite formation upon final cooling when the isothermal temperature was above the Ms temperature but also helped refine the final microstructure when subjected to isothermal heat treatment below the Ms temperature. The yield strengths of the investigated complex phase steels were little influenced by the partial replacement of Si with Al, as long as the fractions of the constituent phases were comparable. This possibly originates with the solid solution hardening and the microstructure refinement with Al addition.

Study on Kinetics of Transformation in Medium Carbon Steel Bainite at Different Isothermal Temperatures

Ultra-fine carbide-free bainitic (UCFB) steel, also known as nano-bainite (NB) steel, is composed of bainitic ferrite laths with nanoscale thickness and carbon-rich film-like retained austenite located between laths. The bainite transformation kinetic model can accurately describe the bainite transformation kinetics in conventional austempering (CA) processes based on the shear mechanism combined with the dilatometer test. UCFB steels with medium and high carbon composition are designed in this work to systematically study the transformation kinetics of bainite, and the evolution of its microstructure and properties, and reveal the influence of heat treatment processes on the microstructure and properties the UCFB steels. The results show that the activation energy for BF nucleation decreases during the CA process and isothermal transformation temperature decreases. The bainite transformation is first nucleated at the grain boundaries, and then nucleated at the newly formed bainitic ferrite/austenite interface.