Ferrite is one of the most common phases in steel. Its name comes from the Latin word ferrum, which means iron. Micrography low carbon steel with a fully ferritic microstructure is shown in the figure 1.
Picture 1 – Microstructure ferrite in carbon steel 0,02 %
This microstructure is composed of hundreds of individual ferrite grains., which are separated by grain boundaries. Grain borders are visible as dark borders around each grain. Each of these grains is a separate ferrite crystal. Corn or the ferrite crystal has body-centered cubic (OCK) crystal structure. Grain boundaries separate a ferrite grain of one crystal orientation from a ferrite grain of a different orientation.. Microstructure, which is shown in the figure 1, typical of very low carbon steel.
This microstructure or morphology is equiaxed, in that sense, that the grain sizes are approximately the same in all directions. This ferrite is also called polygonal ferrite..
Ferrite can have other morphological forms.. Ferrite is present as a phase in such microstructure components as perlite and bainite. Another morphological form of ferrite is Widmanstätt ferrite. This form of ferrite was first discovered in iron meteorites.. On two-dimensional thin sections, it is a needle-like structure, in three-dimensional - real - form, it looks like plates or rails (picture 2).
In steels, ferrite usually forms at the primary austenite grain boundaries when cooled from the austenitic state.. This form of ferrite is called pro-eutectoid ferrite and is formed in hypoeutectoid steels. On the picture 3 shows pro-eutectoid ferrite - white phase along grain boundaries - in ordinary carbon steel 1060 (0,60 % carbon). Dark fields are perlite.
There are two morphological types of ferrite along grain boundaries - equiaxial and acicular. Equiaxial ferrite was shown above in the figure 2, and needle-shaped ferrite along grain boundaries is shown in the figure 4. In this microstructure, the white acicular phase is ferrite, and the gray fields are martensite.