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 ferrite grains with internal fine cementite precipitates lying on one kind of planes (probably stacking fault planes). This behaviour of cementite is different from that of the tempered martensite, and moreover in the ferrite there are hardly seen any traces of the twin faults which are usually found in the martensite. On the other hand, in the upper bainite, the cementite plate is large and situated by the ferrite plate which contains a large number of dislocations. The electron diffraction patterns show that both the upper and lower bainites have nearly the same orientation relationships between the ferrite and the cementite. That is,(011)cem⁄⁄(011)a,[100]cem⁄⁄[01\bar1]a.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.