5. BETTER BAINITE
Because of the difference in the mechanism of transformation, bainitic steels have always been second-
best when compared with tempered martensite. The lack of toughness can in principle be eliminated
by using steels with a high silicon concentration (e.g. 1.5 wt%). Silicon h& a negligible solubility in
cementite and hence greatly retards its precipitation.
The transformation stops before the final stage illustrated in Fig. 3 is reached, leaving at the isother-
mal transformation temperature, plates of bainitic ferrite separated by films of carbon-enriched austenite
(Fig. 4a). There are no cementite particIes to nucleate cleavage cracks or voids; the bainitic ferrite has a
low concentration of dissolved carbon; strengthening and toughening is achieved by the very fine ferrite
plates (a natural consequence of the transformation mechanism); there are intimately-mixed ductile films
of austenite to blunt any cracks and perhaps to toughen via a TRIP effect; the austenite also impedes
penetration of the steel by hydrogen. Evidently, a dream microstructure.