Papers by Author: Chao Sun

Abstract: Microstructure evolution in low-carbon bainitic steel during tempering is investigated by hardness measurements and metallographical examinations. It is found that the microstructure evolution and the hardness variation can be divided into four stages when samples were tempered at 600°C and 700°C, and the evolution of bainte is similar to recovery and recrystallization of deformed metals. It is also found that the newly formed ferrite during recrystallization grows more rapidly along the long axis of bainite laths, and there is evidence of composition changing during recrystallization.

Abstract: The dominant microstructures in low carbon bainite steels such as bainitic ferrite are non-equilibrium phases, which will tend to evolve into equilibrium phases when the steels are subjected to thermal disturbance. In-situ observation by optical and scanning electron microscopy was carried out in this investigation to track the evolution when the steels were isothermally held below A1 temperature. It is found that the primary polygonal ferrite grows slowly during isothermal holding, while bainitic ferrite changes rapidly into polygonal ferrite. Self-tempered bainitic ferrite would recover further and recrystallize. The lower the concentration product of carbon and niobium, the faster is the evolution.