Abstract: The isothermal bainitic transformations at temperatures from 550℃ to 850℃ in high purity Fe-0.064％C-1.80％Mo and Fe-0.19%C-1.81%Mo alloys are studied. The overall transformation kinetics were determined by quantitative metallography. Transmission electron microscopy was employed to identify the carbides present and the fine-scale bainite morphology. Contrary to conventional wisdom, austenite is completely transformed to bainite. This transformation occurs at least in these two steels, even at overall isothermal transformation temperatures from 550℃ to 850℃. The bainite transformation stops completely for a finite amount of time at 585℃ or 600℃before resuming and going to completion. Both of these two steels show a significant bay in the TTT diagram for the initialization of transformation. There is a distinct change in ferrite morphology as the reaction temperature is decreased below the bay temperature (θb). The transition with temperature from ferrite grain boundary allotriomorphs above θb to both intergranular and intragranular ferrite below θb is remarkably sharp. Especially at higher isothermal transformation temperatures, the decomposition of the austenite commences at grain boundaried. Interphase boundary Mo2C precipitation in sheets was paralled to ferrite-austenite boundaried.

Key words: Bainite, Complete transformation, Transformation stasis