What is the Iron Carbon Phase Diagram? (PDF)

To study the various phases of steel and cast Iron, there is no alternative to the Iron Carbon phase diagram. We all know that a phase diagram is a graphical representation that shows the relationship between temperature, composition, and phases that exist in a particular alloy system in equilibrium. As Steel and Cast iron are an alloy of iron and carbon (including other trace elements) iron-carbon phase diagram is constructed to study and understand different phases. In this article, we will learn about the various phases of Steel alloy using the iron-carbon diagram.

What is the Iron Carbon Phase Diagram?

The iron-carbon phase diagram can be defined as a 2-D curve that explains different phase changes that occur on slow heating and cooling with respect to its carbon content. It has temperature on the Y-axis and carbon content (weight %) on the X-axis. The iron-carbon phase diagram is also known as the Fe-C diagram or Iron-Fe3C diagram. Refer to Fig. 1 which shows the iron-carbon phase diagram.

Iron-Carbon Phase Diagram
Fig. 1: Iron-Carbon Phase Diagram

As we can see the graphical plot of the iron-carbon phase diagram is quite complex. In the following sections, we will explore the curve with a focus up to 6.67 weight percent of carbon.

Decoding the Fe-C Diagram

As we can see in Fig. 1, there are three horizontal lines that represent isothermal reactions.

  • From the top, the first horizontal line is at 1493°C where the peritectic reaction takes place:
    • Liquid + δ ↔ austenite
  • The second horizontal line is at around (1130°C to 1147°C), where the eutectic reaction takes place:
    • liquid ↔ austenite + cementite
  • And the third horizontal line is at 723°C, where the eutectoid reaction takes place:
    • austenite ↔ pearlite (mixture of ferrite & cementite)

Phases in the Iron Carbon Phase Diagram

There are 7 phases in the iron-carbon phase diagram. They are:

  1. α-ferrite
  2. γ-austenite
  3. δ-ferrite
  4. Cementite( Fe3C)
  5. Pearlite
  6. Ledeburite, and
  7. Martensite

α-ferrite

α-ferrite is an interstitial solid solution of Carbon in BCC iron (Fe). This phase is stable up to a temperature of 912°C. In this phase, the maximum carbon solubility is 0.022 wt% at 727°C. At 912°C, α-ferrite transforms to FCC γ-austenite phase. At room temperature, it dissolves only 0.008 % C. α-ferrite is ductile and highly magnetic, and it has a low tensile strength of approximately 2800 Kg/cm2.

γ-austenite

γ-austenite is an interstitial solid solution of Carbon in FCC Fe. In this phase, the maximum carbon solubility is 2.14 wt % which occurs at 1147°C. At 1395 °C temperature, γ-austenite transforms to BCC δ-ferrite. As you can see from Fig. 1, γ-austenite is stable above the eutectic temperature (727°C).

In the phase transformations of steels, the γ-austenite phase plays an important role. Most of the heat treatment processes start from this phase. The FCC γ-austenite is generally soft, ductile, non-magnetic, and denser than ferrite.

Depending on the cooling rate, there are three transformations of austenite as listed below:

  • On slow Cooling: Austenite transforms to Pearlite (α + Fe3C).
  • On moderate cooling: Austenite transforms to Bainite (α + Fe3C).
  • On rapid cooling (quenching): Austenite transforms to martensite (BCT phase).

δ-ferrite

δ-ferrite is a solid solution of Carbon in BCC Fe. It has the same structure as the α-ferrite but it is only stable at high temperatures, above 1395 °C. This paramagnetic phase has a maximum carbon solubility of 0.09-0.10 wt.%.

Cementite(Fe3C)

Cementite is an intermetallic metastable phase having 6.67% carbon and 93.33% iron by weight. Fe3C phase is found in steel containing over 0.8% carbon when it cools. With an increase in the percentage of carbon in iron, the amount of cementite increases.

Cementite is a very hard and brittle intermetallic compound of iron & carbon. The presence of this phase strengthens steels. With an orthorhombic crystal structure, cementite has high compressive strength but low tensile strength.

Pearlite

The Pearlite phase consists of ferrite and cementite in the proportion 87:13 by weight. It is formed from austenite at a eutectoid temperature of 727°C upon very slow cooling. Having a lamellar mixture of ferrite and cementite, the Pearlite phase contains 0.80 % C. This phase is quite strong having a tensile strength of around 8750 Kg/cm2.

Ledeburite

Ledeburite is the eutectic mixture of austenite and cementite containing 4.3 percent Carbon and is formed at 1130°C.

Martensite

Martensite is a super-saturated solid solution of carbon in ferrite. This phase is formed during the rapid cooling of steel such that austenite to pearlite transformation is suppressed.

Reactions in Iron Carbon or Iron-Iron Carbide Phase Diagram

There are three important phase reactions that are found in the Iron Carbon Equilibrium phase diagram. Those are:

Peritectic Reaction:

A peritectic reaction occurs at 1493°C with the peritectic composition at 0.18% carbon.

L + δ ↔ γ.

This reaction usually has no engineering importance.

Eutectic Reaction

A eutectic reaction occurs at 1147°C with the eutectic composition containing 4.3% carbon.

L ↔ γ + Fe3C

The alloys in eutectic reactions are called cast irons.

Eutectoid reactions

A eutectoid reaction occurs at 727°C with the eutectoid composition at 0.8% carbon where austenite decomposes into ferrite and cementite.

γ ↔ α + Fe3C

The alloys in eutectoid reactions are usually steels containing parallel plates of ferrite and cementite. The alloy compositions to the left of the eutectoid composition (0.022 – 0.76 wt % C) are known as hypo eutectoid steels. Similarly, the alloy compositions to the right of the eutectoid composition (0.76 – 2.14 wt % C) are known as hypereutectoid alloys. The alloy containing more than 2.14 wt% Carbon is known as cast iron.

Applications of Iron Carbon Equilibrium Phase Diagram

For the proper study of Steel and cast iron, the Iron Carbon Phase Diagram provides a strong foundation to start. The curve provides basic knowledge of various heat treatment processes applicable to steel. The Fe-Fe3C diagram provides an understanding of the compositions and properties its various alloys may have. For the aspirants of material science and metallurgy, understanding the iron-carbon diagram is a must.

Limitations of Iron Carbon equilibrium phase diagram

There is certain information that is not available in the Fe-Fe3C phase diagram. Those are listed below:

  • The iron-iron carbide equilibrium diagram does not give any information about other metastable phases; like bainite, martensite, etc.
  • The possibilities of suppression (reduced ) of pro-eutectoid phase separation are not indicated in the diagram.
  • The curve does not provide information about the kinetic energy, size, or exact properties of the alloy.

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Anup Kumar Dey

A Mechanical Engineer with 19 years of working experience in various renowned MNCs.

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