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Table of Contents:
Introduction
The Iron-Iron carbide diagram is the most asked question in interviews and competitive exams. It is a very important topic if you want to understand iron and its different compositions for different applications.
What is a Phase diagram?
Phase diagrams are graphical representations of the phases present in an alloy at different conditions of temperature, pressure, or chemical composition.
What is the Iron-Iron carbide diagram?
The iron-iron carbide diagram is a diagram between iron and carbon at different temperatures, compositions, and phases of iron.
In this diagram, the temperature is on the x-axis and the composition of Carbon in iron is on the y-axis. But why Iron carbide in its name? If you consider the vertical y-axis at 0% of carbon the iron at that point is pure iron. Conversely, if you consider the vertical line at the right end of the diagram with 6.67% of carbon, the iron at that point is called iron carbide (Fe3C). There will be different phases of iron in between these two points as the temperature and composition vary.
Hence, the name Iron-Iron Carbide phase diagram.
This diagram does not exceed 6.67% of Carbon because Carbon is no more soluble in Iron.
As you can see in this diagram there are 3 horizontal lines at 727 deg, 910 deg, and 1147 deg Celcius. Each horizontal line represents an isothermal reaction.
These are 3 important reactions that help us understand the composition of iron and its properties.
Peritectic Reaction:
At 1493 deg. Cel., an isothermal reaction occurs. When liquid iron and delta-ferrite cools it gives rise to Austenite.
If you follow the T-line (isothermal line) at 1493 deg. Cel. you can easily follow the Carbon percentages for this reaction to occur. To the extreme right of this line, 0.55% of Carbon is present in the Liquid. Whereas, towards the extreme left 0.1% of Carbon is in delta ferrite, while Austenite is in the middle at 0.18% of Carbon.
Ferrite contains very little or no carbon in iron. It has a BCC lattice structure. It is the name given to pure iron crystals which are soft and ductile. Ferrite does not harden when cooled rapidly. It is highly magnetic.
Austenite is a solid solution of free carbon (ferrite) and iron in gamma iron. It has an FCC lattice structure. Austenite is hard, ductile, and non-magnetic. It is able to dissolve large amounts of carbon. Austenitic steels cannot be hardened by usual heat treatment methods.
Eutectic Reaction:
At 1147 deg. Cel., another isothermal reaction occurs. When the liquid iron is cooled, a mixture of Austenite and Cementite is formed which is called Ledeburite.
If you follow the T-line (isothermal line) at 1147 deg. Cel. you can easily follow the Carbon percentage for this reaction to occur. To the extreme right of this line, 6.67% of Carbon is present in Cementite. Whereas, towards the extreme left 2.03% of Carbon is in Austenite, while the liquid is in the middle at 4.3% of Carbon.
Cementite is a chemical compound of carbon with iron and is known as iron carbide (Fe3C). The structure of cementite is present in cast iron with 6.67% carbon content. Steel with a carbon content greater than 0.83% contains free cementite. It increases with an increase in carbon % as reflected in the Fe-C Equilibrium diagram. In cast iron, cementite is believed to be responsible for its hardness and brittleness. It is magnetic below 200°C.
Eutectoid Reaction:
At 727 deg. Cel., when Austenite is cooled, a mixture of alpha-ferrite and cementite is called pearlite.
If you follow the T-line (isothermal line) at 727 deg. Cel. you can easily follow the Carbon percentage for this reaction to occur. To the extreme right of this line, 6.67% of Carbon is present in Cementite. Whereas, towards the extreme left 0.025% Carbon is in alpha-ferrite, while Austenite is in the middle at 0.8% of Carbon.
Pearlite's hardness increases with the proportion of pearlite in ferrous material. It is built up of alternate light and dark plates. These layers are alternately ferrite and cementite. The surface of pearlite looks like an actual pearl when viewed under a microscope, which explains its name. Hard steels are mixtures of pearlite and cementite while soft steels are mixtures of ferrite and pearlite.
Conclusion
Phase diagrams are graphical representations of the phases present in an alloy at different conditions of temperature, pressure, or chemical composition.
The iron-iron carbide diagram is a diagram between iron and carbon at different temperatures, compositions, and phases of iron.
There are 3 horizontal lines at 727 deg, 910 deg, and 1147 deg Celcius. Each horizontal line represents an isothermal reaction. i.e. Peritecticc, Eutectic, Eutectoid reactions.
At 1493 deg. Cel., liquid iron, and delta-ferrite cool give rise to Austenite. At 1147 deg. Cel., liquid iron cools to give Ledeburite. At 727 deg. Cel., Austenite is cooled to Pearlite.
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