Carbon steels by far find the maximum use in industries and daily life. Referring to the iron-carbon phase diagram, shown in Figure 1, compositions with C content less than 2.1 wt% C fall under the purview of steel and all compositions beyond 2.1 wt% to 6.7 wt% C is considered cast iron. The right extreme (6.7 wt% C) forms cementite, which is an intermetallic compounds orthorhombic crystal structure.


Figure 1. The Iron Carbon phase diagram

In the Fe-rich portion of the iron-C phase diagram (Figure 2), we see several phases- viz. Austenite, Ferrite and Cementite. Austenite is a Face-centered cubic (FCC) structure that forms minima at 0.76 wt% C and 723⁰C in its single-phase form. Austenite has a high solubility of carbon (max. 2.11wt% at about 1150⁰C).
On cooling such an alloy below 723⁰C (for the composition 0.8 C wt%), the alloy undergoes eutectoid transformation, forming two phases, ferrite(α) and cementite, simultaneously. This transformation is called the Eutectoid reaction. Because of the diffusion-driven process, the two phases form a lamellar structure of ferrite and cementite, known as the pearlite colony.

Figure 2. The zoomed-out view of steel part of iron carbon phase diagram with schematic microstructures.

a. b. c.
Figure 3. Optical micrograph of (a) Hypoeutectoid, (b) Eutectoid, and (c) Hypereutectoid compositions taken at 500 X.

Steels having composition less than 0.8 wt% C (known as hypoeutectoid steels) have primary α and pearlite in the microstructure. On cooling such a steel from γ (austenitic region), it forms α (carbon deficient composition) first until the composition of the remaining γ reaches 0.8 wt%. As it reaches a temperature of 723⁰C, this austenite undergoes eutectoid transformation. Therefore, microstructure of hypoeutectoid steels consists of proeutectoid α with lamellar pearlite. Similarly, Fe-C alloy compositions with C content greater than 0.8wt% are termed as hypereutectoid steels. The microstructure of such steels consists of pro eutectoid Fe₃C with lamellar pearlite.