Bainite is commonly thought of as a structure of carbides in ferrite and, in fact, is so defined. The carbides are of the M3C type. Some recent reviews of the bainitic transformation confirm this point of view (1). As reported some years ago in several publications (2,3, 4, 5, 6, 7, 8, 9), upper and lower bainite are differentiated in electron metallography by the orientation of the carbide particles with respect to the axis of the ferrite needle. In pure lower bainite, the carbides are parallel and lie at a constant angle with the direction of the bainitic needle (or lens). In pure upper bainite, a feather-like product, the carbides are parallel to the longest direction of the plates — one of the growth directions. These structures are exhibited in Fig. 1, a Cr-V steel, and in Fig. 2, an eutectoid carbon steel. Usually, these two structures appear in all steels containing 0.5% carbon and more, with and without other alloying elements. Here, the pearlitic transformation overlaps the upper portion of the bainitic range. The coexistence of these two structures is especially noticed in anisothermal transformation. In isothermal transformation, it is relatively easy to separate them.
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