Granular bainite
Granular bainite is a term frequently used to
describe the bainite that occurs during continuous cooling
transformation. This terminology is used widely in industry,
where most steels undergo nonisothermal heat treatments. A
good example is the energy generation industry where large
Cr-Mo steel components are allowed to cool naturally from
the austenitic state, to generate bainitic microstructures.
Granular bainite cannot readily be distinguished from
ordinary bainite when examined using transmission electron
microscopy, because its mechanism of formation is not
different. However, because the microstructure forms
gradually during cooling, the sheaves of bainite can be
rather coarse. The optical microstructure then gives the
appearance of blocks of bainite and austenite, so that it is
appropriate to use the adjective "granular".
A characteristic (though not unique) feature of granular
bainite is the lack of carbides in the microstructure.
Instead, the carbon that is partitioned from the bainitic
ferrite stabilises the residual austenite, so that the final
microstructure contains both retained austenite and some
high carbon martensite in addition to the bainitic ferrite.