IRON CARBON PHASE DIAGRAM | Iron Graphite Phase Diagram

Iron Cementite Phase Diagram (Fe-Fe3C)


This is also known as a iron carbon equilibrium diagram. The temperature at which the allotropic changes take place in iron is influence by alloying elements that is must important which is carbon. This is done in between pure iron and interstitial compound. Iron carbide (Fe3C) containing 6.67% C by weight. It is called iron cementite diagram.

● It is not equilibrium diagram because equilibrium implies no changes of phase with time. It is however that the compound iron carbide will decompose in to iron and carbon. Iron carbide is called metastable phase.

● The phase diagram indicates the phases present and phase changes that occur during heating and cooling. The relative amounts of the phases that exist at any temperature can be estimated with the help of lever rule.

● The solubility of carbon varies in different from of iron. In Delta iron, maximum solid solubility of carbon is 0.1%. In gamma iron the maximum solid solubility of carbon is 2.03%. There are three reaction which occur in iron cementite phase diagram
Iron cementite phase diagram
Iron cementite phase diagram
1. Peritectic reaction.
2. Eutectic reaction.
3. Eutectoid reaction.

Peritectic Reaction :

In the alloy containing 0.16% carbon, the initial crystals of Delta solid solution and the whole of liquid phase is completely transformed to austenite on cooling at 1492℃.
                                 
                                    Cooling
Liquid + delta Sol           =            gamma (austenite)
                                   Heating

Eutectic Reaction :

Alloy with carbon carbon content 4.33% C. The liquid is transformed in to austenite and cementite on cooling at 1147℃. That is

                  Cooling
Liquid            =            austenite +  Fe3C
                  Heating
Where cementite is the compound known as cementite. It contains 6.67% carbon. The eutectic of austenite and cementite is known as ledeburite.

Eutectoid Reaction :

In iron carbon reaction alloy with 0.8% carbon, the austenite is transformed in to ferrite and cementite by eutectoid reaction on cooling at 723℃.
                      Cooling
Austenite            =            ferrite +  Fe3C
                       Heating
The eutectoid of ferrite and cementite is known as pearlite.
● Depending upon carbon content and the reaction occurrs, the iron cementite phase diagram is divided in to two groups :
1. Steel
2. Cast iron.

: STEEL :


● There are three major categories of steels. Carbon range up to 2.03%, but rarely exceeds 1.3 to 1.4%.
• Low carbon steel where carbon max 0.3%.
• Medium carbon steel where C : 0.3 to 0.6%.
• High carbon steel where C : 0.6 to more.

● Steels with carbon content from 0.025% to 0.8% are called hypoeutectoid steels. Steel with 0.8% is known as eutectoid steel. Then steel with carbon greater than 0.8% are called hypereutectoid steel.

Transformation of Hypoeutectoid Steels :

● In hypoeutectoid steel, transformation of gamma iron to alpha iron and the decomposition of austenite. Here line GS in the figure shows the temperature where transformation of austenite to ferrite starts, below this ferrite is separated out of the austenite. The critical point in line GS, where austenite begins to transform to ferrite during cooling and ferrite to austenite during heating.
Eutectoid phase in iron carbon phase diagram
Eutectoid phase in fe-fe3c diagram

● In line ES the variation in the solubility of carbon in austenite with the temperature and it transforms to cementite in cooling.  In Point S (0.8% C) called the eutectoid point where the degree of freedom is zero and the austenite decomposes to 100% lamellar pearlite at 723℃.

● The line PQ shows the variation in solubility of carbon in alpha iron with the temperature, and in cooling it starts precipitation of surplus cementite out of ferrite. In 0.025% C, no pearlite will be formed. The alloy will contain only ferrite grains. Steel containing carbon between 0.025% to 0.8% would contain varying amount of ferrite and pearlite and their relative proportions depends on carbon content. At eutectoid temperature 723℃, the carbon content in the ferrite iron is 0.025% and that of austenite 0.8%. The austenite of eutectoid composition decompose to pearlite.

Austenite.    =           Ferrite     +      Cementite
   (0.8%C)                  (0.025%)            (6.67% C)

Transformation in Hypereutectoid Steel :

At eutectoid temperature the composition of austenite is 0.8% carbon. On further cooling, entire amount of austenite will transform to pearlite. Hence, the final microstructure consists of pearlite and proeutectoid cementite.

Transformation in Eutectoid Steel :

At 0.8%C and 723℃ all austenite will transform in to 100% pearlite. So the microstructure at room temperature will reveal alternate layers of ferrite and cementite called pearlite.

: CAST IRON :


Cast iron can be divided in to two main groups on the basis of the nature of carbon present in them. They are

1. Grey cast irons : A large portion of its carbon is present in free state as graphite flakes. The typical grey cast iron contains C : 2.5 - 3.5%, Si : 1.4 - 2.8%, Mn : 0.5 - 0.8%, P : 0.1 - 0.9%, S : 0.06 - 0.12%. Fractured surface of grey cast iron appears grey because of the presence of graphite.

2. White cast iron : In white cast iron carbon is present in combined formed, that is in the form of cementite. Fractured surface of broken piece of this type of casting has white appearance. Cementite is present as a continuous interdendritic network. This makes the cast iron hard and wear resistant but brittle. As machinability is very poor it has very limitated application.

● Under normal conditions, carbon has a tendency to combine with iron to form cementite. Under very slow rate of cooling carbon atoms get sufficient time to separate out in pure form as graphite. Based on the Fe-Fe3C phase diagram, cast irons can be classified into three groups I.e. hypereutectic, eutectic, hypoeutectic cast irons. Eutectic cast iron contain 4.3% carbon.

Transformation in Hypoeutectic Cast Iron :

A structure just below 1147℃ consist of proeutectic austenite and ledeburite. On further cooling in the temperature range 1147℃ to 723℃, excess carbon comes out as cementite from proeutectic and eutectic austenite. Therefore at the eutectoid temperature both eutectic and proeutectic austenite would contain 0.8% carbon and would decompose by the eutectoid reaction to pearlite.

Transformation in Hypereutectic Cast Iron :

In this structure just below 1147℃ consists of proeutectic cementite and ledeburite. On further further cooling in the temperature range 1147℃ to 723℃, excess carbon comes out as cementite from the eutectic austenite. Austenite from eutectic rejects excess carbon as cementite. At 723℃, the austenite contains 0.8 percent carbon. At 723℃, eutectic austenite transforms in to pearlite.

Transformation in Eutectic Cast Iron :

● This type of cast iron solidify at 1147℃. In the temperature range 1147 to 723 the solid alloy consists of ledeburite eutectic I.e austenite and cementite. As the temperature decrease the solubility of carbon in austenite iron decrease as indicate the cementite line.

● On further cooling, proeutectoid cementite separate out of the austenite, and at 723℃ austenite containing 0.8% C transforms to pearlite by eutectoid reaction.The reaction during transformation in eutectic cast iron on cooling :

Liquid          =         austenite       +         cementite
4.3%C      1140℃.     2.03%C                      6.67% C

Austenite.   =             Ferrite       +          cementite
0.8%C        723℃.     0.025%C                    6.67% C

: Iron Graphite Phase Diagram :


The process by which the stable phase graphite is formed in cast iron or steel is known as graphitization. At all temperature the reaction is
                         Cooling
Cementite            =            3Fe  +   C (graphite)

● At low temperature the reaction occurs so slowly by which cementite remains and the graphitization of the iron carbide occurs at higher temperature. Due to presence of silicon, graphitization becomes faster, aluminium and nickel which are also graphitizers for crystallization of graphite flakes. So the stable phase graphite forms either by separating out of liquid or solid solution, or as a result of decomposition of metastable cementite.

Iron graphite phase diagram
Iron graphite phase diagram

● As the liquid alloy of 3.2%C cool to 1153℃, dendrites of austenite phase starts forming in the liquid. At 1153℃ the liquid reaches eutectic composition and solidify as a eutectic mixture of austenite and graphite. This is called primary state of graphitization.

● In this stage major amount of graphite precipitate. Under slow cooling additional graphite forms from the austenite and eutectoid graphite is formed in the temperature interval from 738℃ to 723℃.

● Separation of secondary graphite form of the austenite is called intermediate stage of graphitization. Formation of eutectoid graphite and decomposition of eutectoid cementite into graphite and ferrite, is called secondary stage of graphitization.

● In this stage the additional graphite merely adds to the pre existing graphite flakes and increases it's size. At 723℃ supercooled austenite decomposes with the separation of a ferrite cementite mixture. The heating at high temperature of cast iron containing carbon in combined formed which also leads to graphitization at below 738℃ or in to graphite and austenite at high temperature.

● Composition and cooling rate are two factors which determine the type of structure formed in cast iron. Rapid cooling inhibits precipitation of graphite and promotes formation of cementite. If liquid cast iron is supercooled below 1147℃ cementite is precipitated and graphite phase is possible only at very slow cooling rates, when ℃ of supercooling doesn't exceed 5℃.

● So the cast iron contain coarse graphite flakes has low strength. A finer flakes size can also produced by adding inoculants like ferrosilicon and calcium silicon which promote graphitization. The rapid cooling prevent the graphitization of cementite in white cast iron, but if the casting is reheated to about 875℃ and held there for long time, then graphite is slowly produced in the form of temper carbon.This is called malleable cast iron.







References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.
Metallurgist.

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