Iron–Carbon Equilibrium Diagram

The iron–carbon (Fe–C) equilibrium diagram shows the phases and microstructures that form in iron–carbon alloys at different carbon contents and temperatures. Understanding this diagram is essential for selecting and controlling the heat treatment of steel.

1. Important Regions and Microstructures

2. Important Phases

Ferrite (α-iron):

• Soft and relatively weak.
• Has very low carbon solubility.
• Provides good ductility and toughness.

Austenite (γ-iron):

• Exists at higher temperatures.
• Can dissolve considerably more carbon than ferrite.
• It is the starting phase for important heat treatments such as quenching and normalising.

Cementite (Fe₃C):

• Iron carbide containing approximately 6.67% carbon.
• Very hard and brittle.
• Increases hardness and wear resistance, but reduces ductility and toughness.

Pearlite:

• A layered mixture of ferrite and cementite.
• Forms when austenite undergoes eutectoid transformation.
• Provides a useful combination of strength, hardness and ductility.

Martensite:

• A very hard, metastable structure formed when austenite is rapidly quenched.
• It provides very high hardness and strength but is relatively brittle.

3. Critical Points of the Fe–C Diagram

Eutectoid Point

The eutectoid point is approximately:

• 0.77% carbon
• 727°C

At this temperature, austenite transforms completely into pearlite during slow cooling:

Austenite → Ferrite + Cementite = Pearlite

This is one of the most important reference points for steel heat treatment.

Eutectic Point

The eutectic point is approximately:

• 4.3% carbon
• 1,147°C

At this point, liquid alloy solidifies directly into:

Liquid → Austenite + Cementite

This point is particularly important in the study and manufacture of cast irons.

Peritectic Point

The peritectic point occurs at approximately:

• 0.16–0.17% carbon
• 1,493°C

At this point:

Liquid + Delta Ferrite → Austenite

4. Significance in Heat Treatment

The Fe–C diagram is essential for determining the appropriate heating and cooling temperatures for different heat treatments.

• Annealing: The steel is heated to the appropriate temperature and then cooled slowly. This allows the microstructure to approach equilibrium, reducing residual stresses and increasing ductility and toughness.
• Normalising: The steel is heated into the austenite region and then cooled in air. It produces a finer microstructure than annealing and generally improves strength and toughness.
• Quenching: The steel is heated to form austenite and then cooled rapidly. Rapid cooling prevents normal carbon diffusion and transforms austenite into martensite, producing very high hardness and strength.
• Tempering: Tempering is carried out after quenching. The steel is reheated to a suitable temperature and then cooled. It reduces the brittleness and internal stresses of martensite while improving toughness and ductility.
• Carburising: Carburising enriches the surface layer with carbon. The carburised surface can then be quenched to form a hard martensitic case, while the lower-carbon core remains relatively tough and ductile.

The carbon content and resulting microstructure have a major influence on the mechanical properties of steel. As carbon content increases, hardness and strength generally increase, while ductility and toughness generally decrease.

1. Hypoeutectoid Steel – 0.02–0.8% C

Hypoeutectoid steels contain ferrite + pearlite.

• Ferrite provides ductility and toughness.
• Pearlite provides increased strength and hardness.
• As carbon content increases within this range, the amount of pearlite increases, resulting in higher strength and hardness.

Example:

A low-carbon steel with approximately 0.2% C, when normalised, produces a ferrite–pearlite structure with good strength, ductility and toughness. Such steels are suitable where good formability and toughness are required.

2. Eutectoid Steel – Approximately 0.77–0.8% C

At approximately 0.77–0.8% carbon, the steel transforms into mainly pearlite during slow cooling.

Pearlite provides a good balance of:

• Hardness
• Strength
• Ductility

If eutectoid steel is quenched, it forms martensite and becomes very hard and strong. However, it also becomes more brittle.

After quenching, tempering is normally carried out to reduce brittleness and improve toughness.

3. Hypereutectoid Steel – 0.8–2.14% C

Hypereutectoid steels contain pearlite + cementite.

The additional cementite increases:

• Hardness
• Strength
• Wear resistance

However, excessive cementite makes the steel more brittle and reduces ductility and toughness.

Example:

A steel containing approximately 1.0% C, when suitably heat treated, can develop high hardness and wear resistance, making it suitable for components such as tools, cutting components and wear-resistant parts.

4. Effect of Quenching and Tempering

A high-carbon or medium-carbon steel can be heated into the austenite region and then quenched.

Austenite → Martensite

This produces:

• Very high hardness.
• High strength.
• Good wear resistance.

However, untempered martensite is brittle and contains high internal stresses.

Therefore, tempering after quenching is used to:

• Reduce brittleness.
• Relieve internal stresses.
• Increase toughness and ductility.
• Retain an appropriate level of hardness.

The tempering temperature can be selected according to the required balance between hardness and toughness.

5. Carburising – Hard Surface with Tough Core

For a low-carbon steel, carburising can be used to increase the carbon content at the surface.

After carburising and quenching:

• The surface becomes high-carbon martensite and therefore very hard and wear-resistant.
• The core remains relatively low in carbon and therefore retains good toughness and ductility.

This is useful for components requiring a hard, wear-resistant surface together with a tough core, such as gears and similar machine components.

ALTERNATE ANSWER:

Different Phases

α-ferrite

Existing at low temperatures and low carbon content, α-ferrite is a solid solution of carbon in BCC Fe. This phase is stable at room temperature. In the graph, it can be seen as a sliver on the left edge with the Y-axis on the left side and A2 on the right. This phase is magnetic below 768°C.

It has a maximum carbon content of 0.022 %, and it will transform to γ-austenite at 912°C, as shown in the graph.

γ-austenite

This phase is a solid solution of carbon in FCC Fe with a maximum solubility of 2.14% C. On further heating, it converts into BCC δ-ferrite at 1395°C. γ-austenite is unstable at temperatures below the eutectic temperature (727°C) unless cooled rapidly. This phase is non-magnetic.

δ-ferrite

This phase has a similar structure to α-ferrite but exists only at high temperatures. The phase can be spotted at the top left corner on the graph. It has a melting point of 1538°C.

Fe3C or cementite

Cementite is a metastable phase of this alloy with a fixed composition of Fe3C. It decomposes extremely slowly at room temperature into iron and carbon (graphite).

This decomposition time is long, and it will take much longer than the service life of the application at room temperature. Some other factors (high temperatures and the addition of certain alloying elements, for instance) can affect this decomposition as they promote graphite formation.

Cementite is hard and brittle, which makes it suitable for strengthening steels. Its mechanical properties are a function of its microstructure, which depends upon how it is mixed with ferrite.

Fe-C liquid solution

Marked on the diagram as ‘L’, it can be seen in the upper region on the diagram. As the name suggests, it is a liquid solution of carbon in iron. As we know that δ-ferrite melts at 1538°C, it is evident that the melting temperature of iron decreases with increasing carbon content.

Significance in the Heat Treatment of Steel

• Austenitizing Foundation: Heat treatments (like annealing, normalizing, and hardening) begin by heating steel into the stable γ-austenite region. The diagram defines the exact minimum temperature (A3​ or A1​ critical lines) required to dissolve carbon and homogenize the microstructure.
• Controlling Phase Transformations: By tracking carbon content and crossing critical boundary lines, metallurgists predict whether slow cooling will yield soft ferrite-pearlite structures (via annealing) or if rapid quenching will trap carbon atoms to form ultra-hard martensite (the non-equilibrium body-centered tetragonal structure essential for hardening).
• Tailoring Mechanical Properties: The relative proportions of soft ferrite, hard cementite layers (pearlite spacing), and interstitial phases dictate the ultimate balance of tensile strength, hardness, and ductility