ALL GATE CONCEPTS Material Science

Iron Carbon Phase Diagram with Phase Transformations-GATE 2019

Iron Carbon Phase Diagram with Detailed Explanation: Carbon is added to iron as an interstitial inclusion to improve the strength of iron. The maximum solubility of Carbon(C) in Iron(Fe) is 6.67% called as critical concentration.

If carbon is added to Iron it produces Iron Carbide(Fe3C) phase which is hard and brittle in nature also called as Cementite.

Iron-Carbon Diagram Theory
Iron Carbon Phase Diagram

In this article, I will be explaining all the important concepts which are either underlined or bolded are useful for all types of examinations. Compulsory,2 marks will be given from this Iron-Carbon Diagram Theory in any exam. So, concentrate and read this article completely so that you can understand the different phases in the Iron-Carbon Diagram.

Iron Carbon Phase Diagram with Detailed Explanation:

If the percentage of the carbon is in the range of 0 to 2.11 % then it is called as Steel and if the percentage of carbon is in the range of 2.11 to 6.67% then it is called as Cast iron.

As the carbon content increases, it produces more Iron carbide volume and that phase will exhibit high hardness.

Iron Carbon Phase Diagram-Phases:

The various Phases in Iron-Carbon diagram are as follows.

About 𝛿Fe:(Delta(𝛿) Iron)

The maximum solubility of carbon is 0.1% at 1495 degree centigrade.

    • 𝛿Fe(Delta Iron) possess very low carbon content and thereby it possess low hardness and that indicates it is a highly ductile phase.
    • 𝛿Fe is difficult to produce and most unstable phase because the addition of very small carbon content in Iron(Fe) lattice with uniform distribution is very difficult at high temperature.
    • The structure is BCC.

About γFe:(Gamma(γ)Iron)

    • γFe is also called as “Austenite”
    • The maximum solubility of carbon is 2.11 % at 1145 degree centigrade.
    • By varying carbon content from 0 to 2.11 % variety of Steels in γFe phase can be obtained.
    • By heating or cooling process of γFe phase, the grain size can be modified(heat treatment) so that variety of phases with different strengths can be obtained.
    • The hardness of the γFe phase depends on the percentage of carbon that it possesses.
    • The minimum temperature about which the γFe phase exists is 723-degree centigrade at 0.8 Percentage of Carbon.
    • The structure is FCC.

About αFe:(Alpha Iron)

    • αFe is also called as “Ferrite”.
    • The maximum solubility of carbon is 0.025% at 723-degree centigrade.
    • αFe possess similar properties of pure iron but hardness is slightly high compared to pure Iron.
    • It is magnetic in nature.
    • The structure is BCC.

Iron Carbon Phase Diagram-Why Cast Iron(C.I) is not Heat Treatable?

    • The maximum percentage of carbon that a steel can be produced is 1.5 % only.
    • If the carbon content is 2.11 to 6.67%, above the liquidus line, it exists in the form of molten liquid and below the liquidus line, it exists as a solid.
    • The maximum percentage of carbon in cast Iron can be produced is 5% only because in GE2H region, the melting point is minimum and that implies obtaining molten liquid is easy and thereby casting process is also easy.
    • By the addition of alloying elements, the properties of cast Iron cannot be improved effectively because the carbon content is very high.
    • By either heating or cooling process, modification of grain size in cast iron is difficult because it causes more volume of Iron carbide(Fe3C) and that implies a modification of bonds among the atoms is difficult and that implies Changing of grain size is difficult and hence Cast Iron(C.I) is not heat treatable.

Iron Carbon Phase DiagramPhase Transformation:

The four Phase Transformations in Fe-C Diagram are as follows.

  1. Eutectoid Phase Transformation
  2. Eutectic Phase Transformation
  3. Peritectic Phase Transformation
  4. Peritectoid Phase Transformation

The Detailed Explanation of all the above phases is presented below.

1.Eutectoid Phase Transformation:

1. At ‘E1’ point:(Eutectoid Point)

 S1 ——-> [S2     + S3]

=>γFe ——> [(αFe) + Fe3C] at 0.8%C @723 Degree Centigrade

Note: (αFe) + Fe3C   is called as Pearlite.

Most Important:

In the Iron-Carbon Diagram, the austenite phase(γFe) can undergo a Eutectoid transformation to produce ferrite and cementite called as Pearlite.

2.Eutectic Phase Transformation:

   At ‘E2’ point:(Eutectic Point)

L (Fe+C) ——> [(γFe) + Fe3C] at 4.3%C @1145 Degree Centigrade

             L ——-> [S2     + S3]

A eutectic reaction is a three-phase reaction, by which on cooling, a liquid transforms into two solid phases at the same time.

3.Peritectic Phase Transformation:

At ‘P1’ point:(Peritectic Point)

𝛿Fe+ L ——> (γFe) at 0.18%C @1495 Degree Centigrade

             (S1+L )——-> S2

4.Peritectoid Phase Transformation:

  (S1+S2 )——-> S3


A mixed phase of (γFe + Fe3C) existing from 4.3% to 6.67% between 723 Degree Centigrade to 1145 Degree Centigrade is known as Ledeburate Phase.

So this is the detailed explanation of Iron Carbon Phase Diagram which was explained successfully with the help of important points in bolded and underlined text. If you have any doubts w.r.t. the Iron-Carbon Diagram Theory, you can ask us from the comments section.


There is a Compulsory question from this Iron-Carbon Diagram for either GATE Exam or any competitive exam for 2 Marks.



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