Iron Carbon Equilibrium Diagram with Clarification [Phase

iron carbon equilibrium diagram

On this article we are going to talk about in regards to the Iron Carbon Equilibrium diagram, makes use of of equilibrium diagram, varieties of iron carbon equilibrium diagram and numerous phases and phrases related to the iron carbon part diagram.

What are Part Diagrams ?

Part diagrams are graphical representations of the phases current in an alloy beneath numerous temperature, strain, and chemical composition situations.

The solidification of metallic alloys is clearly understood via equilibrium diagrams. These are graphic representations of adjustments in state attributable to variations in temperature and focus. Since this diagram signifies the character and structure of alloys, and the quantity and composition of phases in a given system, it’s also generally known as structure diagram or part diagram.

Equilibrium Diagram Traits and Makes use of

Equilibrium implies that adjustments occurring in a system because of course of continuing in a single route are absolutely compensated by adjustments because of the reversal of the method within the system. So it’s thought of as a dynamic situation of steadiness between atomic actions the place the resultant is zero.

The charges of adjustments of temperature or of composition have been extraordinarily gradual throughout the experimental work, in order that the alloy would “come to relaxation” earlier than a variable resembling temperature, had been once more modified. The situation, due to this fact, is certainly one of relaxation somewhat than change.

Equilibrium diagram point out the next:

1. Temperature at which the stable alloy will begin melting and end melting.

Attainable part adjustments which can happen as the results of altering the composition or temperature.

What Equilibrium Diagram Characterize ?

The diagram describes the acceptable situations for 2 or extra phases to exist in equilibrium. For instance, the water part diagram describes some extent (triple level) the place water can coexist in three completely different phases on the similar time. This occurs at simply above the freezing temperature (0.01°C) and 0.006 atm.

Makes use of of Equilibrium Diagram in Metallurgy

  • Improvement of the brand new alloys on the premise of utility necessities.
  • Manufacturing of those alloys.
  • Improvement and implementation of applicable warmth therapy procedures to enhance the chemical, bodily, and mechanical properties of those new alloys.
  • Troubleshooting points that come up throughout using these new alloys, in the end enhancing product predictability.

Iron Carbon Equilibrium Diagram

The Iron carbon equilibrium diagram (additionally known as the iron carbon part diagram) is a graphic illustration of the respective microstructure states of the alloy iron – carbon (Fe-C) relying on temperature and carbon content material.

The iron carbon part diagram is often used to totally perceive the varied phases of metal and forged iron. Metal and forged iron are each iron and carbon alloys. As well as, each alloys comprise hint parts in small quantities.

The graph is sort of advanced, however as a result of we’re limiting our investigation to Fe3C, we are going to solely be taking a look at as much as 6.67 weight % carbon.

Sorts of Iron Carbon Equilibrium Diagram

The binary iron carbon equilibrium diagram is the premise of metal and forged iron. It considerations transformations that happen in alloys having compositions from pure iron to cementite (6.67 per cent carbon). There are two variations of iron carbon equilibrium diagram :

1. Iron-cementite system. 2. Iron-graphite system.

These two techniques depend upon the speed of cooling. Speedy cooling produces cementite and the system is named Iron-cementite system. On this system, the constructions fashioned within the solidified phases don’t attain sufficiently full equilibrium. So Iron-cementite system is a metastable one.

Whereas gradual cooling produces graphite and the system is generally known as Iron-graphite system. The constructions which might be fashioned within the solidified part attain sufficiently full equilibrium. So this can be a steady one.

Sorts of Ferrous Alloy on Iron Carbon Equilibrium Diagram

The load proportion scale on the X-axis of the iron carbon part diagram ranges from 0% to 6.67% Carbon. The metallic is solely generally known as iron or pure iron as much as a most carbon content material of 0.008 % weight of carbon. At room temperature, it exists within the ferrite state.

Metal is an iron carbon alloy with a carbon content material starting from 0.008 to 2.14 %. Metal grades inside this vary are generally known as low carbon metal (or gentle metal), medium carbon metal, and excessive carbon metal.

When the carbon content material exceeds 2.14 %, we attain the forged iron stage. Forged iron is extraordinarily arduous, however its brittleness severely limits its purposes and forming strategies.


Iron carbon equilibrium diagram

If a collection of time-temperature heating curves are made for steels of various carbon contents and the corresponding important factors plotted a diagram just like Fig. 2.14 can be obtained. This diagram, which applies solely beneath gradual cooling situations, is named a partial Iron carbon part diagram. By referring to this diagram one might readily observe the correct quenching temperatures for any carbon metal, The important factors in Fig. 2.14 on the road PSK are denoted by A1, these on line GS by A3, and people of line SE by Acm

Iron Carbon Equilibrium Diagram with Clarification


Austenite, stable resolution of carbon and different constituents in a specific type of iron generally known as γ (gamma) iron. Allow us to take the instance of a bit of 0.20 per cent carbon metal which has been heated to a temperature round 850°C. Above Ar3, level (GS line) this metal is a stable resolution (interstitial kind) of carbon in gamma iron and is known as austenite. It has a face-centered cubic lattice and is nonmagnetic.

Plain austenite might comprise as much as about 2 per cent carbon at a temperature of 1130°C. Upon cooling this metal the iron atoms begin to type body-centered cubic lattice under the purpose Ar3 (GS line). This new construction that’s being fashioned is known as ferrite or alpha iron and is stable resolution of carbon in alpha iron containing as much as 0.008 per cent carbon at room temperature.

Because the metal is cooled to Ar1, (PSK line), further ferrite is fashioned. On the Arı line the austenite that continues to be is remodeled to a brand new construction known as pearlite. The identify pearlite is because of its pearly luster. It consists of alternate plates of ferrite and cementite and accommodates about 87 per cent ferrite. Pearlite could also be both fine-to-coarse lamellar or granular construction. This can be a sturdy substance and could also be reduce fairly effectively with reducing instruments, i.e., the pearlite constituent in metal is machinable.

Eutectoid Metal

Because the carbon content material of the metal will increase above 0.20 per cent, the temperature at which the ferrite is first rejected from the austenite drops till, at about 0.80 per cent carbon (level S), no free ferrite is rejected from the austenite. This metal is known as eutectoid metal and is 100% pearlite.

Eutectoid Level

What’s Eutectoid Level ?

The eutectoid level in any metallic, as stated earlier, is the bottom temperature at which adjustments happen in a stable resolution.

If the carbon content material of the metal is larger than eutectoid (0.8 per cent carbon), a brand new line is noticed within the iron carbon part diagram denoted by Acm (S line). The road denotes the temperature at which iron carbide is first rejected from the austenite as a substitute of ferrite.

The iron carbide (Fe3C) is named cementite. This can be very arduous, brittle and seems as parallel plates (lamellar layers), as rounded particles (spheroids) or as envelopes across the pearlite grains. At level C, the eutectic combination containing 4.3 per cent carbon is named ledeburite. That is not often seen in slowly cooled alloys because it breaks down, attributable to its unstable nature, to different phases throughout cooling after solidification.

Hypoeutectoid and Hypereutectoid

Steels containing lower than 0.80 per cent carbon are known as hypoeutectoid and people which comprise greater than 0.8 per cent carbon are known as hypereutectoid steels. This terminology applies solely to plain and low alloys steels. With excessive alloy steels the eutectoid composition is altered and the construction might not even exist.

It ought to first be identified that the traditional equilibrium diagram actually represents the metastable equilibrium between iron and iron carbide (cementite). Cementite is metastable, and the true equilibrium ought to be between iron and graphite.

Though graphite happens extensively in solid irons (2-4 wt% C), it’s often troublesome to acquire this equilibrium part in steels (0.03-1.5 wtpercentC). Due to this fact, the metastable equilibrium between iron and iron carbide ought to be thought of, as a result of it’s related to the conduct of most steels in observe.

Iron Graphite System


It has already been stated that iron carbide or cementite is a metastable, though beneath regular situations, it tends to persist indefinitely. When cementite does decompose it does in accordance with the response :

Fe3C <——–> 3Fe + C

Within the steady part, free carbon or graphite happens as a substitute of the part generally known as cementite. Upon small diploma of supercooling, graphite is fashioned when forged iron solidifies from the liquid state. Gradual cooling promotes graphitisation. Speedy cooling partly or fully suppresses graphitisation and results in the formation of cementite.

An iron-graphite system (as dotted line) is proven in Fig. 2.14. The case of a carbon alloy containing 3.5 per cent carbon by weight is taken as an illustration

At level 1 the alloy is within the liquid state . At level 2 on the cooling line the response that happens may be expressed as:


> Between factors 2 and three, the surplus carbon within the austenite is precipitated out as free graphite and never as cementite. At level 3, the eutectoid response happens. That is expressed as:


The mechanism of eutectoid transformation should remodel a single stable part into two others, each with compositions which differ from the unique.

Taking the eutectoid decomposition of iron for example, austenite containing 0.8% C adjustments into ferrite (iron containing nearly no carbon) and cementite (Fe3C, containing 25 at% carbon). Therefore carbon atoms should diffuse collectively to type Fe3C, leaving ferrite. Nuclei of small plates of ferrite and cementite type on the grain boundaries of the austenite, and carbon diffusion takes place on a really native scale simply forward of the interface (schematic under).

Thus the plates develop, consuming the austenite as they go, to type pearlite. The method of graphitisation is managed by various the speed of cooling and by correct alloying of the metallic matrix.

Phrases utilized in Iron Carbon Equilibrium Diagram

Eutectic Level

The eutectoid level in any metallic is the bottom temperature at which adjustments happen in a stable options.

Eutectic reactions happen at these factors, the place a liquid part freezes into a combination of two stable phases. This happens when a liquid alloy of eutectic composition is cooled all the best way to its eutectic temperature.

Eutectic alloys are the alloys that type at this level. Alloys on the left and proper sides of this level are generally known as hypoeutectic alloys and hypereutectic alloys (‘hypo’ in Greek means lower than, ‘hyper’ means higher than).


Austenite, stable resolution of carbon and different constituents in a specific type of iron generally known as γ (gamma) iron.

This part is a stable resolution of carbon in FCC Fe with a most solubility of two.14% C. On additional heating, it converts into BCC ferrite at 1395°C. γ-austenite is unstable at temperatures under eutectic temperature (727°C) until cooled quickly.

Alpha Iron or Ferrite

Current at low temperatures and low carbon content material, α-ferrite is a stable resolution of carbon in BCC Fe. This part is steady at room temperature. Within the graph, it may be seen as a sliver on the left edge with Y-axis on the left aspect and A2 on the suitable. This part is magnetic under 768°C.

It has a most carbon content material of 0.022 % and it’ll remodel to γ-austenite at 912°C as proven within the graph.


Cementite, a metastable part of this alloy with a set composition of Fe3C, is a metastable part of this alloy. At room temperature, it decomposes extraordinarily slowly into iron and carbon (graphite).

This decomposition time is lengthy, and it’ll take for much longer than the appliance’s service life at room temperature. Different elements, resembling excessive temperatures and the addition of sure alloying parts, can affect this decomposition by selling graphite formation.

Cementite is difficult and brittle, making it ideally suited for metal reinforcement. Its mechanical properties are decided by its microstructure, which is set by how it’s combined with ferrite.

Now we have tried to cowl all of the phrases associated to iron carbon part diagram together with the varied phases and phrases utilized in it to know in a greater method. Hope you favored this text about iron carbon equilibrium diagram. Please give your suggestions within the remark under.

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