Iron Carbon Diagram

Iron-Carbon diagram, also known as the Iron-Iron Carbide diagram or Fe-C diagram, is a graphical representation that illustrates the phases and microstructures formed in iron-carbon alloys as a function of temperature and carbon content. This diagram is a fundamental tool in metallurgy and materials science, providing valuable insights into the behavior of iron and carbon alloys, particularly steels.

Phases in Iron-Carbon Alloys:

3. Phases in Iron Alloys:
   • Iron exists in different crystal structures or phases at various temperatures. The primary phases are ferrite, austenite, cementite, and delta ferrite.
4. Ferrite (α-iron):
   • Ferrite is a solid solution of carbon in iron, and it is stable at lower temperatures. It has a body-centered cubic (BCC) crystal structure.
5. Austenite (γ-iron):
   • Austenite is the high-temperature phase of iron with a face-centered cubic (FCC) crystal structure. It can dissolve more carbon than ferrite and is stable at elevated temperatures.
6. Cementite (Iron Carbide – Fe3C):
   • Cementite is a hard, brittle intermetallic compound of iron and carbon. It forms at higher carbon concentrations and is an essential component in the Iron-Carbon diagram.
7. Delta Ferrite (δ-iron):
   • Delta ferrite is a phase that exists at high temperatures and is stable in specific alloys. It has a body-centered cubic (BCC) crystal structure.

Uses of Iron Carbon Diagram

The Iron-Carbon diagram is widely used in metallurgy and materials science for designing and understanding the properties of iron-carbon alloys, particularly steels. Here are several applications of the Iron-Carbon diagram:

1. Steel Design:
   • Example: Engineers use the Iron-Carbon diagram to design steel alloys with specific mechanical properties. For instance, if a high-strength steel is needed for a structural application, the diagram guides the selection of alloying elements and heat treatment processes to achieve the desired properties.
2. Heat Treatment Planning:
   • Example: The diagram is crucial in planning heat treatment processes such as annealing, normalizing, and quenching. It helps determine the appropriate temperatures and cooling rates to achieve the desired microstructure and mechanical properties in the final steel product.
3. Understanding Phase Transformations:
   • Example: During the eutectoid reaction, austenite transforms into a mixture of ferrite and cementite. This transformation, depicted in the Iron-Carbon diagram, is essential in understanding how different microstructures form at specific temperatures and carbon contents.
4. Prediction of Microstructures:
   • Example: Knowing the carbon content and cooling rate, one can predict the microstructure that will form in a steel alloy using the Iron-Carbon diagram. For instance, rapid quenching from the austenite phase can result in the formation of martensite, a hard and brittle microstructure.
5. Selection of Alloying Elements:
   • Example: Alloying elements such as chromium, manganese, and nickel can alter the Iron-Carbon diagram. Engineers use the diagram to understand how these elements affect the phase transformations and properties of the resulting steel alloy.
6. Welding Considerations:
   • Example: The Iron-Carbon diagram is crucial in understanding the effects of welding on steel. Rapid heating and cooling during welding can influence the microstructure, and knowledge of the diagram helps in managing these effects.
7. Quality Control in Manufacturing:
   • Example: Manufacturers use the Iron-Carbon diagram to ensure the quality and consistency of steel products. By understanding how different heat treatment processes influence microstructures, they can control the final properties of the manufactured steel.
8. Development of Advanced Alloys:
   • Example: The Iron-Carbon diagram serves as a foundation for developing advanced steel alloys, including High-Strength Low-Alloy (HSLA) steels. Engineers can manipulate the composition and heat treatment processes to achieve specific performance characteristics.
9. Ternary Phase Diagrams:
   • Example: The Iron-Carbon diagram’s principles extend to more complex alloy systems, where additional elements are present. Engineers use ternary phase diagrams to study and design alloys with three components, building upon the fundamental concepts of the Iron-Carbon diagram.
10. Research and Innovation:
    • Example: Researchers use the Iron-Carbon diagram as a starting point for exploring new materials and manufacturing processes. It guides the development of innovative alloys with enhanced properties, contributing to advancements in materials science.

Temperatures in Iron Carbon Diagram

The Iron-Carbon diagram illustrates the phases present in iron-carbon alloys at different temperatures and carbon concentrations. The primary phases include ferrite (α-iron), austenite (γ-iron), cementite (Fe₃C), and delta ferrite (δ-iron). The temperatures associated with these phases are approximate and can vary based on the specific alloy composition. Here are the general temperature ranges for the main phases:

1. Ferrite (α-iron):
   • Temperature Range: Up to about 912°C (1674°F)
   • Structure: Body-Centered Cubic (BCC)
2. Austenite (γ-iron):
   • Temperature Range: Above 912°C (1674°F)
   • Structure: Face-Centered Cubic (FCC)
3. Cementite (Fe₃C):
   • Temperature Range: Cementite is a compound and doesn’t have a distinct temperature range. However, it often forms during the eutectoid reaction at 727°C.
   • Structure: Orthorhombic crystal structure
4. Delta Ferrite (δ-iron):
   • Temperature Range: Commonly found in high-temperature alloys.
   • Structure: Body-Centered Cubic (BCC)

Eutectoid Reaction:

• The eutectoid reaction occurs at a specific temperature and carbon composition. For iron-carbon alloys, it takes place at 727°C (1341°F) with a carbon content of 0.76%. During this reaction, austenite transforms into a mixture of ferrite and cementite.

Other Transformations:

• The temperature at which other transformations occur, such as the formation of pearlite, bainite, or martensite, depends on factors like alloy composition and cooling rate during heat treatment.

It’s important to note that the temperatures mentioned are general approximations, and specific alloys may exhibit variations. The Iron-Carbon diagram serves as a guide, providing a macroscopic view of the phase transformations in iron-carbon alloys under equilibrium conditions. In practical applications, factors like cooling rate, alloying elements, and heat treatment processes influence the actual phase transformations and microstructures in a given steel alloy.

each phase or region assigned a distinct color. For example:

• Ferrite: Light gray
• Austenite: Dark gray
• Cementite: Black
• Mixed-phase regions: Different shades of gray

Perlite and cementite

Three important transformations of the iron-carbon equilibrium diagram

Eutectoid Reaction
• At temperature 723° C and carbon composition 0.8% Austenite (y) (S) Ferrite (a-iron) (S) + Carbide (Fe3C) (S) One solid converted into two solids.
Eutectic reaction
• At temperature 1130° C and carbon composition 4.3%. • Liquid Iron (L) Austenite (S) + Cementite (S)
• One liquid converts into two solids.
Peritectic reaction
• At temperature 1498°C and carbon composition 0.09% 8-iron (S) + Liquid iron (L) Austenite (S)
One liquid and one solid converts into another solid.

Martensite – steel is not present in the Iron carbon diagram

carbon content in steel: 0-2%

Eutectoid/Pearlite steel:
A 0.8% carbon steel or eutectoid steel is known as PEARLITE steel.
This is much stronger than ferrite or cementite.
• It is a phase mixture of ferrite and cementite. It contains ferrite 87% and cementite 13%.
Hypo-eutectoid Steel:
Plain carbon steels in which carbon percentage is less than 0.8% are called hypo-eutectoid steel.
Hyper-eutectoid Steel:
Plain carbon steels in which carbon percentage is more than 0.8% are called hyper-eutectoid steel.
High-Speed Steel is a high carbon tool steel, containing a large dose of tungsten. A typical HSS composition is: 18% tungsten, 4% Chromium, 1% Vanadium, 0.7% carbon and the rest, Iron.
• Cast irons have a chemical composition of 2.15-6.7% carbon.

Bainite:
Very fine microstructure of ferrite and carbides (Fe3C)
Properties are like ferrite but with different structures.
Stronger and more ductile than particles at the same hardness level.
Martensite:
• Hardest, brittle, and least ductile constituent of steel.
• Needle-like structure.
Solid solution of non-carbide in a – iron.
• It has a body-central tetragonal structure.
Ferrite:
• There are two forms of ferrite i.e., a – ferrite and 8 – ferrite
• a – ferrite, is a solid solution of body-centered cubic iron.
• It has a maximum solid solubility of 0.022% C at a temperature of 727°C. • ō- ferrite is another form that is stable only at very high temperatures and is of no practical significance in engineering.
• Ferrite is relatively soft and ductile

Cementite:
• Cementite, which is 100% iron carbide (Fe3C), has a carbon content of 6.67%.
• It is also called carbide.
• It is a very hard and brittle intermetallic compound and has a significant influence on the properties of steel.