Unknown secrets of Iron-Cementite equilibrium diagram !!
See, the problem is that once we learn this diagram we believe that it's all that is there to know. But in reality we only know the tip of the iceberg.
This post answers and reveals most of the unknown or unanswered questions of all time.
The diagram shown below in Fig.1 is the one which we all have learnt and is given in many textbooks, but it is actually hard to read because of two reasons :
1) it is the superposition of the stable and metastable phase diagram
2) only the Fe-rich part is shown so it is not obvious which phases are connected by the tie lines.
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| Fig.1 : Superposition of Fe-C & Fe-Fe3C diagrams |
At this point you might be thinking that, "well, what do we mean by stable & metastable phase diagrams ?" And how these terms affects the phase diagrams ?
1) Stability, Instability & Metastability
A general approach to describe all 3 terms is based on Gibbs free energy. If the free energy ( ΔG ) of a system is plotted as a function of possible arrangements of atoms, the presence of the maxima and minima is generally observed , as schematically shown in Fig.2
- The maxima ( here, point B ) corresponds to unstable state, where the free energy can be easily lowered by localized homophase atomic rearrangements.
- The decrease of free energy drives the system towards a minimum. The absolute minimun ( here, point C) corresponds to the stable state and, in order to transform the system, an external energy input is necessary.
- Relative minima ( here, point A ) correspond to metastable states, where the system is in internal equilibrium, though its free energy is above that of the stable state.
Eg. In an iron-carbon system, graphite ( Cgraph ) is a stable form of carbon where else cementite ( Fe3C ) is a metastable phase.
This means that cementite will decompose into graphite with time. But, then why do we call it a iron-cementite equilibrium diagram ? This is because cementite takes years to decompose, so we can consider the system as an equilibrium.
2) Stable & Metastable diagrams of Fe-C system
As we now know there exists two diagrams for Fe-C system
Fe-C = Stable phase diagram ( Fig.3 )
Fe-Fe3C = Metastable phase diagram ( Fig.4 )
Both the diagrams given above are of 100% atomic composition of components but in reality we do not require the whole diagram since we are only interested in Fe-rich products. That's why we only study upto 30% atomic composition of C.
This means that cementite will decompose into graphite with time. But, then why do we call it a iron-cementite equilibrium diagram ? This is because cementite takes years to decompose, so we can consider the system as an equilibrium.
2) Stable & Metastable diagrams of Fe-C system
As we now know there exists two diagrams for Fe-C system
Fe-C = Stable phase diagram ( Fig.3 )
Fe-Fe3C = Metastable phase diagram ( Fig.4 )
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| Fig.3 : Fe-C equilibrium diagram |
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| Fig.4 : Fe-Fe3C equilibrium diagram |
Both the diagrams given above are of 100% atomic composition of components but in reality we do not require the whole diagram since we are only interested in Fe-rich products. That's why we only study upto 30% atomic composition of C.
Magnified view of Fig.3 and Fig.4 as per our requirement is shown below as Fig.5 and Fig.6 respectively.
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| Fig.5 : Magnified view of Fe-C diagram |
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| Fig.6 : Magnified view of Fe-Fe3C diagram |
So now you can understand why Fig.1 as discussed in beginning is a superposition of both stable & metastable phase diagrams. During slow cooling we get metastable phase instead of the stable one. That's we usually we study & use metastable diagrams in practice. But bot diagrams slightly differ in Fe-rich region both in temperature and composition.
3) Important Points
The number of experimental and theoretical publications on the Fe-C phase diagrams and related subjects is virtually unlimited because of the unquestionable importance of Fe-C alloys in all aspects of human activities.
The details of stable & metastable Fe-C system, especially in Fe-rich region are known much better than any other binary systems with similar complexity.
In the present evaluation, the assessed stable Fe-C ( graphite ) and metastable Fe-Fe3C ( cementite ) equilibrium phase diagrams for 0 to 25 at.% C are based on thermodynamic calculations reported by [79Sch1] and [84Oht].
Comparison of these calculated results with experimental data indicates that the uncertainity of the diagram is approximately ± 2°C and ± 0.1 at.%. One of the reasons for this uncertainty is that even the transition temperature of pure Fe are not well determined. Other reasons are unknown impurities, cooling/heating rate effect, uncertainty in the chemical composition of specimens.
The stable equilibrium phases of the Fe-C system at ambient pressure are
- the gas ( g )
- the liquid ( L )
- BCC ( δ-ferrite )
- FCC ( γ-austenite )
- BCC ( α-ferrite )
- hexagonal C ( graphite )
- orthorhombic Fe-3C ( cementite )
Stable Fe-C ( graphite ) system
1) g ↔ L
Boiling | 2862 °C
2) L ↔ δ-ferrite
Melting | 1536 °C
3) δ-ferrite ↔ γ-austenite
Allotropic | 1392 °C
4) γ-austenite ↔ α-ferrite
Allotropic | 911 °C
5) L + δ-ferrite ↔ γ-austenite
Peritectic | 1493 °C
6) L ↔ γ-austenite + C
Eutectic | 1153 °C
7) γ-austenite ↔ α-ferrite + C
Eutectoid | 740 °C
8) g ↔ C ( graphite )
Sublimation | 3826 °C
Metastable Fe-Fe3C ( cementite ) system
1) L ↔ Fe3C
1) L ↔ Fe3C
Congurant | 1252 °C
2) L + δ-ferrite ↔ γ-austenite
2) L + δ-ferrite ↔ γ-austenite
Peritectic | 1493 °C
3) γ-austenite ↔ α-ferrite + Fe3C
3) γ-austenite ↔ α-ferrite + Fe3C
Eutectoid | 727 °C
4) L ↔ γ-austenite + Fe3C
4) L ↔ γ-austenite + Fe3C
Eutectic | 1147 °C