On the picture 1 shown iron-carbon phase diagram, where the region is below the temperature 727 ºС painted in dark color. This area is biphasic. Any steel, which was slowly cooled to a temperature below 727 ºС should consist of a mixture of two phases - ferrite and cementite. The microstructure types of steels in this two-phase region are very diverse.. Perlite is one such type.
Pearlite structure in steel
To understand pearlite structure consider steel with carbon content 0,77 %, eg, steel U8. If you heat it to a temperature 800 ºС and keep at this temperature 1-2 minutes, then in a high-temperature microscope it would look like this, as shown at the top of the picture 1 - structure of fully austenite grains.
After cooling to a temperature below A1 - below 727 ºС - and holding at it for 5-10 minutes, the austenite grains will be completely replaced by completely new grains - pearlite, as shown at the bottom of the picture 1.
Unlike austenite or ferrite grains, pearlite grains are not a single phase.. Moreover, they consist of a mixture of two phases - ferrite and cementite, forming a unique microstructure. To see all the details of this microstructure, a small area within a pearlite grain must be viewed at a very high magnification., as shown in the lower right of the figure 1.
Cementite and ferrite in perlite
The pearlite structure consists of intermittent plates of ferrite and cementite. Ferrite plates are much thicker, than cementite plates. They borrow 90 % total grain volume, compared to the remaining 10 % cementite.
At the boundaries of pearlite grains, there is a sharp change in the orientation of the plates, as can be seen on the microstructure of a real steel sample under an electron microscope at a magnification of x11000 (picture 2).
In this photo, the cementite plates are the light phase., and ferritic ones - dark. Cementite plates are only 0,1 μm - too little, so that they can be seen through a light microscope. Despite, that cementite is brittle, and perlite is not fragile. This is due to the small size of the cementite plates and the large volume fraction of plastic ferrite..
Perlite at room temperature
If now our U8 steel with carbon content 0,77 % chill off 700 ºС to room temperature, as it shown on the picture 1, then the microstructure will not change significantly - no matter how slowly or how quickly it is cooled. Ferrite will remain almost pure iron, and cementite - to remain with carbon content 6,7 % and with the same crystal structure.
According to the phase diagram, when the austenite is cooled below the A1 temperature and held at it for a short time, austenite will be completely replaced by a mixture of ferrite + cementite phases in one form or another. Further cooling of this two-phase mixture to room temperature does not give any changes in the final microstructure of ferrite + cementite..
Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007