2 The Origin of the Two Theories
The nature of the bainite transformation has been a subject of controversy ever since the bainitic microstructure
was identified by Davenport and Bain in 1930 [15]. Figure 3 is a reproduction of the ‘dark-etched,
acicular microstructure’ they observed. It was suggested that such a phase transformation “would occur just
a little in advance of the carbide precipitation”, via a process akin to that of the martensite transformation.
The theory of a displacive mechanism of bainite formation had been proposed in 1926 by Hultgren [23] to
explain observations of what he described as “needles of troostite” (but which was characteristic of a bainitic
microstructure, troostite being the name then given to all fine mixtures of ferrite and cementite). First
referred to as ‘martensite-troostite’ in the 1930 paper due to morphological similarities to martensite (see
Figure 1.1 in [9]), bainite has maintained an association with the martensitic microstructure. Some have