Bainite is a microstructure occuring in Steel. It consists of very small strips of Iron (Ferrite), with tiny needle shaped particles of Iron Carbide (Cementite) stuck in them. They quite often show up stuck in a matrix of another mircostructure type.
Compared to Pearlite, it is harder and more ductile than Pearlite of the same strength, although Pearlite usually doesn't show up as strong as Bainite, so in general Pearlite is more ductile. It's not at hard or as strong as Martensite, but it's a hell of a lot more ductile.
Bainite is made by taking some Austenite, basically any steel above 727C, and cooling it down quickly. It will only form between around 540C and 210C. However, you have to cool it down quick enough that it doesn't start turning into Pearlite, which means you have to get it below 540C in about a second. Usually this is done by quenching it in oil.
There are two forms of Bainite that will form, Upper and Lower. Upper Bainite forms between 300C and 540C, and Lower forms between 210 and 300. Lower Bainite is proportionally much thinner, as well as being a whole load smaller, small enough that you need an electron microscope to look at it.
As seems to generally be the case, the more "Upper" Bainite is, the more ductile it is, but the less strong and hard it is..
As mentioned earlier, Bainite will generally not form on it's own, depending on the Carbon content of the Steel. If the Carbon content of the Steel is less than 0.76%, some of the Austenite will have already transformed into Ferrite before it reaches 727C, and if it's above 0.76% Carbon, some of it will have turned into Cementite.
The Ferrite or Cementite will form in clumps (called grains) that may or may not grow together. The Bainite will then fill the gaps between it.
Of course, if you've got exactly 0.76% Carbon, you can get pure Bainite.
The more carbon, the more Cementite, the less carbon the more Ferrite. At 0% carbon, you'll be getting all ferrite, no Bainite. At 6.70% carbon, you'll be getting all Cementite.
And, if you really want to get funky, you can cool it down below 200C while the Bainite is still only half done forming. You've got to do this quick, because Upper Bainite forms within 10 minutes at most, finishing within 10 seconds at the higher temperature ranges. Lower Bainite can take up to 2.5h to form.
Anyways, when you do this, any remaining Austenite that hasn't tranformed into Bainite turns into Martensite. Cool, huh?
Sources:
William D. Callister, Jr., Materials Science and Engineering An Introduction, 5th Edition. John Wiley & Sons, Inc.
My memory from ENME421,
Mechanical Engineering, Materials I, at the
University of Calgary.