Evolution and thermal stability of bainite in a low-carbon steel during reheating below A_1 temperature

In-situ tracing metallographic observation, Vickers hardness test, transmission electron microscopy and electron back-scattered diffraction were carried out to investigate the evolution and thermal stability of bainite in a low-carbon steel during reheating in a temperature range of 550 to 675℃. Experimental results show that the bainite evolves into polygonal ferrite through the way of recovery and recrystallization. In this process the granular bainite changes into polygonal ferrite firstly and then the polygonal ferrite consumes bainitic ferrite, so the bainitic ferrite exhibits a higher thermal stability than the granular bainite. During recovery, low degree boundaries between adjoining ferrite laths are partially removed, and the ferrite laths are tilted and merged. There is an evolutionary stable stage between recovery and recrystallization during reheating of bainite, and the duration of the stable stage is significantly prolonged with the drop of temperature.