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.
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 possesses 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 a 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 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 Diagram–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 the 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.
True Stress V/s True Strain
Engg. Stress V/s Engg. Strain
Stress-Strain Curve: This is one of the important question asked in most of the interviews and when I had gone to an interview in Hyundai, they asked the same question to me and I had explained as below. In order to understand the topic, you should have to follow the below figure.
The figure represents the relationship between the load and elongation I.e. on the X-axis, elongation is plotted and on the Y-axis, load(F) is plotted.
When the sample is fixed between the grips of Ultimate Tensile Machine(UTM) and the top plate is moved upwards with a certain speed, implies elongation is produced in the sample, implies strain is produced. But the sample will resist the strain with a downward force(F) which is measured by the load cell connected in the UTM.
Therefore, the Load v/s Elongation curve (or) Stress-Strain Curve of Mild Steel is obtained as shown in fig.
Stress-Strain Curve for Mild Steel:
*If the sample is loaded up to the point A, Stress is directly proportional to Strain called as Proportional region ‘OA’.
*If the sample is loaded up to the point B, stress is directly proportional to strain but by removing the elongation load on the sample, it will gain its original shape called as Elastic region ‘OB’.
*The corresponding stress value at the point of B is known as Yield Stress.
*If the sample is loaded beyond the point B, the displacement of atomic planes is slow, which means the plastic strain is produced in the sample but it will bear the maximum load up to the point C.
*The corresponding stress value at the point C is known as Ultimate Tensile Stress and that equals to the tensile strength of the material.
*If the sample is loaded beyond the point C, the displacement of atomic planes is fast and which means the severe plastic strain is produced in the sample ==> a severe increase in length and decrease in cross-section area of the sample takes place and finally it will fail at the point D.
Below figure represents the change in cross-sectional area of a specimen when a force F0, F1 F2 etc.are applied on a specimen.
Difference between True Stress V/s True strain and Engineering Stress V/s Engineering Strain:
True Stress = F0/A0,F1/A1,F2/A2 …….
But during testing time, it is difficult to measure the reduction in the cross-sectional area of the sample(which is very small value).
Therefore,A1,A2,A3…….are not available.
Hence A1=A0,A2=A0,A3=A0…….is substituted.
Engineering Stress = F0/A0,F1/A0,F2/A0,.……..
Actually,A1<A0,A2<A0,A3<A0 implies curve will fall down.
Therefore Engineering Stress V/s Engineering Strain will follow the path of OABCD. whereas, True stress V/s True strain curve will follow the path of OABCD’ (here ‘ =dash).
*The curves will deviate from elastic point Onwards because above the yield stress value, the change in cross-sectional area of the sample is significant.
*If the material is tested at high temperature, the distance between the atoms increases implies binding energy decreases, implies strength decreases.
*Therefore it is always suggested to operate a component at low surrounding temperatures to increase the load-bearing capability of the material.
*If the material is tested at high temperature, the displacement of atomic planes is easy implies plastic strain can be produced easily implies yield stress decreases.
*It is always suggested that heating is one of the processes to shape change the component because at high temperature the yield stress of the material is low, implies shape change will be easy.
- Metal Forming Process: Rolling, Forging, Extrusion, Principle, Applications
- Lathe Machine
- Ultrasonic Machining
This is the detailed explanation of the Stress-Strain Curve in a detailed way. If you have any doubt, you can ask from the comments section. Feel free to share with your friends.