IRON-CARBON EQUILIBRIUM DIAGRAM:

 

IRON-CARBON EQUILIBRIUM DIAGRAM:

The Iron-Carbon Equilibrium Diagram (also known as the Iron-Carbon Phase Diagram) is a fundamental tool in metallurgy and materials science, particularly for understanding steel and cast iron behavior. It shows the phases that occur in iron-carbon alloys as a function of temperature and carbon content.

Iron-Carbon Phase Diagram Overview

Axes:

• X-axis: Carbon content (from 0% to 6.67% by weight)
• Y-axis: Temperature (up to ~1600°C)

·         Important Points on the Diagram

Feature	Description
Pure Iron	Left side (0% C), exists in different forms: ferrite (α), austenite (γ), and delta-ferrite (δ).
Eutectoid Point	0.76% C at 727°C → Austenite (γ) transforms into Pearlite (α + Fe₃C).
Eutectic Point	4.3% C at 1147°C → Liquid transforms into Austenite + Cementite (γ + Fe₃C).
Cementite (Fe₃C)	Hard and brittle intermetallic compound; appears at 6.67% C.
Ferrite (α-Fe)	Soft, BCC structure, can dissolve up to 0.02% C at 727°C.
Austenite (γ-Fe)	FCC structure, stable at high temperatures, can dissolve up to 2.14% C at 1147°C.
Pearlite	A lamellar mixture of ferrite and cementite formed at the eutectoid point.
Ledeburite	Mixture of austenite and cementite in high-carbon alloys (>4.3%).

 

Phases and Reactions

1. Eutectoid Reaction (Steel)

• γ (Austenite) → α (Ferrite) + Fe₃C (Cementite) at 727°C, 0.76% C

2. Eutectic Reaction (Cast Iron)

• Liquid → γ (Austenite) + Fe₃C at 1147°C, 4.3% C

3. Peritectic Reaction

• Liquid + δ (delta-ferrite) → γ (austenite) at ~1495°C, 0.17% C

·         Applications

Carbon Content	Name	Application
< 0.25%	Low-carbon steel (mild steel)	Ductile, used in construction
0.25–0.6%	Medium-carbon steel	Gears, railway tracks
0.6–1.5%	High-carbon steel	Cutting tools, springs
>2.0%	Cast iron	Engine blocks, pipes

• On the left, you have pure iron, which goes through delta-ferrite → austenite → ferrite as it cools.
• In the middle (0.76% C), you have the eutectoid composition, important for steels.
• On the right (4.3% C), you have the eutectic composition, important for cast irons.
• Beyond 6.67% C, the material is cementite.

1. What is the Iron-Carbon Diagram?

The Iron-Carbon Equilibrium Diagram shows how iron (Fe) and carbon (C) mixtures behave at different temperatures and carbon contents. It tells us what phases (solid, liquid, or mixtures) will exist in an alloy of iron and carbon at equilibrium.

It’s mainly useful in understanding the microstructure of steels and cast irons.

• The diagram considers carbon content up to 6.67%, which is where cementite (Fe₃C) forms.
• In real-life steelmaking, the most important range is up to about 2% carbon, which is where steels fall.
• Alloys with more than 2% carbon are classified as cast irons.

Phase	Symbol	Description
Ferrite	Α	BCC structure, soft and ductile, holds little carbon (max ~0.022% at 727°C). Stable at low temperatures.
Austenite	Γ	FCC structure, can dissolve more carbon (up to ~2.14% at 1147°C), stable at higher temperatures.
Cementite	Fe₃C	A hard, brittle compound (iron carbide). Not a solution, but a chemical compound.
Liquid	L	Molten state of the alloy.
Pearlite	α + Fe₃C	A layered (lamellar) structure formed at the eutectoid point. Mixture of ferrite and cementite.
Ledeburite	γ + Fe₃C or L + Fe₃C	Found in high-carbon alloys (cast irons), also a layered mixture.

Eutectoid Reaction (Most important for steel)

Occurs at:

• Temperature: 727°C
• Composition: 0.76% C

Reaction:

γ→α+Fe3C\gamma \rightarrow \alpha + Fe_3Cγ→α+Fe3​C

(That means austenite transforms into pearlite.)

2. Eutectic Reaction (Important for cast iron)

Occurs at:

• Temperature: 1147°C
• Composition: 4.3% C

Reaction:

L→γ+Fe3CL \rightarrow \gamma + Fe_3CL→γ+Fe3​C

(Liquid turns into austenite and cementite simultaneously.)

3. Peritectic Reaction

Occurs at:

• ~1495°C and 0.17% C

Reaction:

L+δ→γL + \delta \rightarrow \gammaL+δ→γ

(A rare reaction in practical steelwork.)

 

🧱 4. Types of Steel Based on Carbon %

Carbon %	Type of Steel	Properties
< 0.25%	Low-carbon (mild) steel	Very ductile, soft, easy to weld and machine.
0.25 – 0.6%	Medium-carbon steel	Stronger, used for mechanical parts.
0.6 – 1.5%	High-carbon steel	Very hard, good wear resistance (e.g., tools, knives).

 

🧊 5. Phases on Cooling

Let’s take a steel with 0.8% carbon (eutectoid composition):

1. At high temp (~900°C): Fully austenite (γ).
2. Cool to 727°C: Austenite transforms into pearlite (α + Fe₃C).
3. Below 727°C: Entire structure is pearlite.

 

⚙️ 6. Practical Use in Heat Treatment

Understanding the diagram helps in processes like:

• Annealing: Softening steel by slow cooling to form ferrite + pearlite.
• Quenching: Rapid cooling to form martensite (not shown in equilibrium diagram — it’s a non-equilibrium phase).
• Tempering: Reheating quenched steel to adjust hardness and toughness.

 

🧠 Summary of Key Points

Concept	Value
Eutectoid Point	0.76% C at 727°C
Eutectic Point	4.3% C at 1147°C
Cementite (Fe₃C)	Forms at 6.67% C
Max Solubility of C in γ	2.14% at 1147°C
Max Solubility of C in α	0.022% at 727°C

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Phases on Cooling

Let’s take a steel with 0.8% carbon (eutectoid composition):

1. At high temp (~900°C): Fully austenite (γ).
2. Cool to 727°C: Austenite transforms into pearlite (α + Fe₃C).
3. Below 727°C: Entire structure is pearlite.

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⚙️ 6. Practical Use in Heat Treatment

Understanding the diagram helps in processes like:

• Annealing: Softening steel by slow cooling to form ferrite + pearlite.
• Quenching: Rapid cooling to form martensite (not shown in equilibrium diagram — it’s a non-equilibrium phase).
• Tempering: Reheating quenched steel to adjust hardness and toughness.

 

 

 

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