Iron Carbon Equilibrium Diagram

The iron-carbon equilibrium diagram (Fe-C diagram) is a binary phase diagram that illustrates the equilibrium phases formed by iron and carbon under varying temperature and composition conditions. It is regarded as a fundamental reference in metallurgy and materials engineering. This diagram is particularly important for understanding and controlling phase transformations during heat treatments in the production of steel and cast iron.

Representative illustration of the Fe-C (Iron-Carbon) Equilibrium Diagram. (Critical temperatures and carbon percentages are highlighted in yellow.) (Image: Muzaffer Yalçın)

Phases and Microstructures

The iron-carbon system consists of several phases:

• Ferrite (α-Fe): A soft phase with a body-centered cubic (BCC) crystal structure and low carbon solubility. Its maximum carbon solubility is 0.022 wt.% and it is stable below 727 °C.
• Austenite (γ-Fe): A phase with a face-centered cubic (FCC) crystal structure that forms at high temperatures. Its maximum carbon solubility is approximately 2.14 wt.%. It is stable between 912 °C and 1394 °C.
• Cementite (Fe₃C): A hard and brittle iron-carbon compound that is not thermodynamically stable but commonly encountered in metallurgical processes.
• Liquid Phase: The molten metal phase that forms at sufficiently high temperatures (e.g., above 1493 °C).
• Graphite (C): A pure carbon phase that is thermodynamically more stable than cementite; it is observed in cast irons.

Representative microstructural images of the phases in the Iron-Carbon Diagram. (Image: Muzaffer Yalçın)

Key Points and Reactions

Important points and reactions in the iron-carbon equilibrium diagram are as follows:

• Eutectoid Point: Occurs at a composition of 0.76 wt.% C and a temperature of 727 °C, where austenite transforms into ferrite and cementite:
• γ (austenite) → α (ferrite) + Fe₃C (cementite)
• Eutectic Point: Occurs at a composition of 4.3 wt.% C and a temperature of 1147 °C, where the liquid phase simultaneously forms austenite and cementite:
• L → γ (austenite) + Fe₃C
• Peritectic Point: Occurs at a composition of 0.16 wt.% C and a temperature of 1493 °C, where δ-ferrite and the liquid phase transform into austenite:
• L + δ → γ

These reactions form the basis of the microstructural transformations in steels and cast irons.

Stable and Metastable Systems

Two distinct equilibrium systems exist in the Fe-C system:

• Stable system (Fe-C): Includes graphite formation.
• Metastable system (Fe-Fe₃C): The system in which cementite forms instead of graphite. In practice, this system is more commonly used in steel production because cementite forms more rapidly and is easier to control industrially.

Industrial Applications and Technical Guidance

The iron-carbon equilibrium diagram forms the foundation for heat treatment processes (e.g., annealing, quenching, tempering) and materials design. It is also used to control and predict microstructures during production processes such as casting, forming, and welding.