and can be of either plate or lath morphology.
The subunits in lower bainite tend to be coarser than those in upper bainite, but the two morphologies
are similar as far as microstructure and crystallography are concerned. However, in lower bainite, the
individual ferrite subunits also contain a fine distribution of carbide particles in addition to the interplatelet
carbides.
Figure 1 illustrates the essential differences between the two microstructures, and micrographs of upper
and lower bainite subunits are shown in Figure 2. Upper bainite forms at higher temperatures
(550-400◦C) than lower bainite (400-250◦C) in the same steel. The transition is also determined to an
extent by the carbon content of the steel [12]. Mixtures of upper and lower bainite may be obtained
by isothermal transformation.
2. The bainite start temperature
It can be demonstrated for many steels that there is a temperature BS above which no bainite will
form [19]. In some steels this corresponds to the bay in the TTT diagram between the overlapping
curves of pearlite and bainite [20]. This temperature is well below that at which pro-eutectoid ferrite
formation becomes thermodynamically possible.
3. The incomplete-reaction phenomenon
As the transformation temperature is reduced below BS, the fraction of austenite that transforms to
bainite is progressively increased [21]. However, during isothermal transformation, a limit is reached
beyond which austenite will no longer transform to bainite, despite a significant quantity remaining
untransformed. The reaction is said to be ‘incomplete’ since it is halted before the austenite achieves the
equilibrium composition. The extent of the incomplete transformation in a given steel is compositiondependent:
higher carbon contents will reduce the degree of transformation at a given temperature
[22], although the phenomenon is difficult to detect in plain carbon steels.