Iron-Carbon Diagram Theory 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.
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 Diagram Theory 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 Diagram Theory-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.
- 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 Diagram Theory-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 Diagram Theory-Phase Transformation:
The four Phase Transformations in Fe-C Diagram are as follows.
- Eutectoid Phase Transformation
- Eutectic Phase Transformation
- Peritectic Phase Transformation
- Peritectoid Phase Transformation
The Detailed Explanation of all the above phases is presented below.
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.
In the Iron-Carbon Diagram, the austenite phase(γFe) can undergo a Eutectoid transformation to produce ferrite and cementite called as Pearlite.
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.
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 Diagram Theory 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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