The clusters of ferrite plates are known as ‘sheaves’ (Fig. 4); each sheaf is itself in the
form of a wedge–shaped plate on a macroscopic scale. The sheaves inevitably nucleate heterogeneously
at austenite grain surfaces. The cementite precipitates from the carbon–enriched
austenite between the ferrite plates; the ferrite itself is free from carbides. Cementite precipitation
from austenite can be prevented by increasing the silicon concentration to about 1.5 wt%;
this works because silicon is insoluble in cementite. Silicon–rich bainitic steels can have very
good toughness because of the absence of brittle cementite.