The Iron-Carbon Phase Diagram and Its Significance

Aygün Karaköse

1 Jul, 2025 08:44 AM

The iron-carbon (Fe-C) phase diagram is one of the most fundamental metallurgical diagrams, illustrating the phase transformations, microstructures, and thermodynamic equilibria of iron and carbon alloys at various compositions. It is indispensable for understanding the behavior of steel and cast iron, which are among the most widely used materials in industry.

1. Step 1: General Features of the Diagram

   The diagram plots carbon content (in weight percent) on the horizontal axis and temperature (°C) on the vertical axis. The carbon content ranges from 0 to 6.67 wt%, where 6.67 wt% corresponds to the theoretical maximum carbon content of cementite (Fe₃C).

   The diagram shows which phases are stable at given temperatures and carbon concentrations. The main phases include:

   
   

   • Ferrite (α): A body-centered cubic (BCC) phase with low carbon solubility, magnetic and relatively soft.
   • Austenite (γ): A face-centered cubic (FCC) phase stable at intermediate temperatures, with higher carbon solubility.
   • Cementite (Fe₃C): A hard, brittle intermetallic compound rich in carbon.
   • Pearlite: A lamellar mixture of ferrite and cementite formed during eutectoid transformation.
   • Ledeburite: A eutectic mixture of austenite and cementite present in high-carbon alloys.

   
   

2. Step 2: Critical Points and Curves

   Key transformation temperatures and carbon concentrations in the diagram play pivotal roles:

   • A1 (Lower Eutectoid Temperature, ~723 °C): The temperature at which austenite transforms into pearlite during cooling.
   • A3 Line: The boundary between ferrite and austenite phases, decreasing in temperature with increasing carbon content.
   • Acm Line: The boundary between austenite and cementite phases.
   • Eutectoid Point (~0.76 wt% C): Carbon concentration at which austenite decomposes fully into pearlite at the A1 temperature.
   • Eutectic Point (~4.3 wt% C, 1147 °C): The temperature and composition at which liquid transforms directly into austenite and cementite.

3. Step 3: Phase Regions and Microstructures

   
   

   Based on the temperature and carbon content, the alloy’s microstructure varies as follows:

   
   

   • Low-carbon steels (0-0.2 wt% C): Mainly ferrite with small amounts of pearlite; soft and ductile.
   • Medium-carbon steels (~0.3-0.8 wt% C): Mixture of pearlite and ferrite; harder and stronger.
   • High-carbon steels (>0.8 wt% C): Increased pearlite and cementite content; hard and brittle.
   • Cast irons (>2 wt% C): Dominated by ledeburite and cementite phases; hard but brittle.

   Applications and Importance

   The iron-carbon phase diagram serves as a critical reference in materials engineering, metallurgy, and manufacturing technologies for designing steels and cast irons, controlling heat treatments, and optimizing mechanical properties. During thermal processes such as annealing, quenching, and normalizing, this diagram helps predict resulting microstructures and properties.

   Conclusion

   The iron-carbon phase diagram is an essential tool for understanding microstructural development and phase transformations in steels and cast irons. Its critical points and phase regions provide fundamental guidance for material design and heat treatment. Today, the Fe-C diagram remains a cornerstone in both scientific research and industrial applications in metallurgy.