Transmission Electron Microscope Observation of the Bainite of Carbon Steel

The structure of the bainite in a 0.7% carbon steel has been studied by means of transmission electron microscopy and selected-area electron diffraction. The lower bainite consists of lens-shaped ferrite grains in which internal fine cementite precipitates are formed on one kind of planes (probably stacking faults planes). This habit of the cementites are different from those in the tempered martensite, and moreover in the ferrite there are hardly seen any traces of the twin faults as found in the martensite. On the other hand, the cementite in the upper bainite has the form of a plate situated by the side of the ferrite plate which contains many dislocation lines.Electron diffraction patterns showed that both the upper and lower bainite have nearly the same orientation relationships between the ferrite and the cementite. That is,(This article is not displayable. Please see full text pdf.) \ oindentwhere relationships (2) agree with[100]cem||[01\bar1]α within experimental error. Referring to these results and the habit of the precipitated cementite, the formation mechanism of the bainite is discussed in connection with the martensite transformation.