Bainite in steel

If the steel is cooled from the austenitizing temperature slightly faster, than just outside, then it becomes possible to form such a microstructure of steel as bainite.

Differences between bainite and perlite

Like perlite bainite is formed from austenite and also originates first along the austenite grain boundaries. Therefore, upon rapid cooling of steel, competition arises at the grain boundaries between pearlite and bainite., as shown in the picture 1.

bainite-perlite-1000Picture 1 - Mixed pearlite and bainite structures, which formed along the boundaries of the primary austenite grains (shown with white lines). Accelerated American Steel 1095 (analogue of U9 or U10 steels).
Original magnification 1000x.

Bainite and perlite under a light microscope

Bainite is a lighter structure, which grows from the boundaries of the primary austenite grains. White thin line along old austenite grains is bainite. Like perlite, bainite is a mixture ferrite and cementite. However, the internal structure of bainite is different from pearlite. If in perlite cementite is present in the form of plates, located between ferrite plates, then in bainite it is in the form of a kind of threads or small particles, scattered across the ferrite matrix.

Bainite and Perlite Growth Front

In the picture picture 1 you can see another difference between these two structures - pearlite and bainite. The growth front of pearlite is spherical, and in bainite it looks like a needle-like. However, one must remember, what's the picture in the picture 2 Is a 2D cut through the structure. If the bainite growth front were really needle-like, then these needles would only be visible, if the incision lay parallel to the axes of the needles, which is unlikely. Therefore it is clear, that the bainite growth front here has a plate-like shape and looks like acicular in a two-dimensional image for many orientations of the cut plane along this front. The bainite growth front in this micrograph is the same lamellar, like bainitic structures of most steels.

Maximum magnification of the light microscope

Optical micrograph on the image 2 was taken at a maximum useful magnification of 1000x. At higher magnification, no new structural components can be seen.. The thing is, that such an increase is already beyond the resolution of an optical microscope - 0,2 μm. Lamellar and "thread" structures of cementite in pearlite and bainite are not visible, because both are too small, so that they can be examined through an optical microscope.

After polishing and etching section a steel specimen, cementite plates scatter light and therefore appear dark under an optical microscope. Perlite looks darker than bainite because it scatters light from its plates more efficiently., than bainite from its smallest particles.

Bainite and perlite under an electron microscope

A scanning electron microscope takes images of a surface with resolution, which is much higher than that of a light microscope - a hundred times: 2 nanometer instead 0,2 micrometers.

On the picture 2 and 3 showing micrographs of pearlite and bainite at the same point, as in the micrograph of the figure 1.

perlite-21000Picture 2 - Micrograph of a scanning electron microscope at that point, as in the picture 1. Initial magnification: 21000x

bainite-20000Picture 3 - Micrograph of a scanning electron microscope at that point, as in the picture 1. Initial magnification: 20000x

In both photos, the dark phase is cementite. The difference between these two microstructures can be clearly seen from these two photographs.. With an increase in the cooling rate, the distance between cementite plates in pearlite decreases. On the picture 3 this distance is 0,05 μm is the minimum distance for untempered and undeformed pearlite. As seen in the picture 4 cementite in bainite is mainly present in the form of particles, which are in the form of threads or ribbons. Like the axes of these bainite ribbons, and the planes of pearlite plates are oriented along the growth direction of these structures.

Cm. also review Microstructure of steel

Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007