The Iron-carbon Phase Diagram

Of all binary alloy systems the one that is possibly the most important for a metallurgist is the iron and carbon system. We know that both steel and cast iron play a important role in structural applications and they both are iron-carbon system.

Pure iron, upon heating, experiences changes in its crystal structure until 1538°C and melts there. At room temperature, it is present in a stable form called ferrite (α), which has a BCC crystal structure. The ferrite transforms to austenite (γ) at 912°C, which has an FCC crystal structure. At 1394°C, it again undergoes a phase transformation to δ-ferrite, which has a BCC crystal structure. Pure iron finally melts at 1538°C. All these changes can be seen on the left vertical axis of the Fe-C phase diagram.

In the composition axis at 6.7% formation of a intermediate compound known as Fe₃C(Cementite) or iron carbide is observed. In real world all steels and cast iron have carbon content less than 6.7%. Thus in the Fe-C system consideration only upto 6.7% C is taken. Here, 6.7% C is considered as 100% Fe₃C. Between 0.02-0.76% C it is considered as Hypo eutectoid steel and between 0.76-2.14% C it is considered as Hypereutectoid steel.

Carbon is a interstitial compound here and forms solid solution with α,δ and ט. In BCC α ferrite only small concentration of carbon is soluble about 0.02% at 727°C. The limited solubility is explained by the size and shape of BCC interstitial position. The maximum solubility of carbon in ט is about 2.14% at 1147°C. The δ-ferrite is same as α except for the fact that it occurs at higher temperature.

Eutectic reaction for Fe-C system(1147°C):

L ⇌ ט + Fe₃C

Eutectoid reaction for Fe-C system(727°C):

ט ⇌ α + Fe₃C

Peritectic reaction for Fe-C system(1493°C):

δ + L ⇌ ט