Iron iron carbide diagram By Hariprasad

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Text of Iron iron carbide diagram By Hariprasad

  • Hari Prasad

    Presentation by

    Hariprasad (Asst. Professor)

  • Steels


    Hari Prasad



  • Pure iron, upon heating, experiences two changes

    in crystal structure before it melts.

    At room temperature the stable form, called

    ferrite, or iron, has a BCC crystal structure.

    Ferrite experiences a polymorphic transformation

    to FCC austenite, or iron, at 9120C.

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  • This austenite persists to 1394oC, at which

    temperature the FCC austenite reverts back to

    a BCC phase known as ferrite, which finally

    melts at 15380C

    All these changes are apparent along the left

    vertical axis of the phase diagram.

    The -ferrite is virtually the same as -ferrite,except for the range of temperatures over

    which each exists.

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  • (a) Ferrite (90X) (b) Austenite (325X)

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  • Maximum solubility of carbon in various single






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    Room temp solubility

    Stable at only high temp

  • The solubility of carbon varies in different formsof iron.

    In delta iron, maximum solid solubility of carbonis 0.1%

    In gamma iron, the maximum solid solubility ofcarbon is 2.03%

    Austenite is a solid solution of carbon in FCCiron and solute atoms occupy interstitialpositions in this lattice.

    In alpha iron, carbon has a limited solidsolubility of about 0.008% at room temperature

    The maximum solubility of carbon in ferrite is0.025%.

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  • Description of the phases in Iron-Iron Carbide system

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  • Steels


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  • Interstitial solid solution of small amounts carbondissolved in BCC-Iron

    Max. solubility of carbon is 0.025% at 727oC and0.008% at room temp

    Softest among all the phases


    Interstitial solid solution of carbon in FCC iron.

    Max solubility of carbon is 2.1%

    It is not stable below 727oC



    Interstitial solid solution of carbon in BCC iron

    Upon heating Austenite changes to BCC and gets-Ferrite

    Max solubility of carbon is 0.1%-Ferrite

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  • This is also known as Iron Carbide (Fe3C)

    Its a chemical compound because it contains afixed percentage of carbon (6.67%)

    It has an orthorhombic crystal structure (abc)


    It is the eutectic lamellar mixture of austenite(light phase) and cementite (dark phase)

    It is unstable below 727oC and transforms into-ferrite and cementite.


    Austenite containing 0.8%C forms pearlite uponslow cooling below 727oC

    Its lamellar structure of -ferrite and cementite. It is very bright in appearance (like a pearl)

    It has a fingerprint like appearance


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  • There are three reactions which occur in iron

    cementite phase diagram.




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  • Steels


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  • Peritectic reaction: In the alloy containing 0.18%

    carbon, the initial crystals of delta solid solution

    and the whole of liquid phase is completely

    transformed to from austenite on cooling at


    L+ (austenite)

    Eutectic reaction: alloy with carbon content

    4.33%, the liquid is transformed into austenite

    and cementite on cooling at 11470C

    L (austenite)+cementite





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  • The eutectic of austenite and cementite is known as


    Eutectoid reaction: in Iron carbon alloy with

    0.8% carbon, the austenite is transformed into

    ferrite and cementite by eutectoid reaction on

    cooling in 7270C

    Gamma(austenite) alpha+cementite

    The eutectoid of ferrite and cementite is known as




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  • If the carbon % is less than 2.14, that type of alloy

    comes under the category of steels.

    If the carbon % greater than 2.14, that alloy

    comes under the category of cast irons


    There are major categories of steels.

    i. Hypoeutectoid steels

    ii. Hypereutectoid steels

    iii. Eutectoid steels

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  • Steels with carbon content from 0.025% to 0.8%

    are called hypoeutectoid steels.

    Steels with a carbon content of 0.8% is known as

    eutectoid steels

    Steels with a carbon content greater than 0.8%

    are called hypereutectoid steels

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  • Hypoeutectoid steels There are solid state transformations in this steels.

    They are the transformation of gamma iron to alpha

    iron and the decomposition of austenite.

    The limiting composition for getting pearlite is


    With carbon content less than this amount, no pearlite

    will be formed. The alloy will contain only ferrite


    Steels containing carbon between 0.025-0.8% would

    contain varying amount of ferrite and pearlite and

    their relative proportions depend on carbon content

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  • Schematic representations of the microstructures for an

    ironcarbon alloy of hypoeutectoid composition

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  • Point C: At about 8750C, point c, the

    microstructure will consist entirely of grains of

    the -phase, as shown in fig.

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  • Point D: about 775oC, which is within the + phase region, both these phases will coexist as inthe schematic microstructure.

    Most of the small particles will form along the

    original grain boundaries

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  • Micrograph of hypoeutectoid steel (0.34%C)

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  • Hypereutectoid steels

    At eutectoid temperature, the composition of

    austenite is 0.8% carbon

    On further cooling, the entire amount of

    austenite will transform to pearlite

    Hence, the final microstructure consists of

    pearlite and proeutectoid cementite

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  • Schematic representations of themicrostructures for an iron

    carbon alloy of hypereutectoid composition

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  • Micrograph of hypereutectoid steel


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    Pearlite + proeutectoid cementite

  • Eutectoid steel

    On cooling at eutectoid point (0.8%C-7270C), all

    austenite will transform into 100% pearlite.

    So, the microstructure at room temperature will

    reveal alternate layers of ferrite and cementite,

    called pearlite

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  • Schematic representations of themicrostructures for an

    ironcarbon alloy of eutectoidcomposition (0.76 wt% C) above and

    below the eutectoid temperature.

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  • The microstructure for this eutectoid steel that is

    slowly cooled through the eutectoid temperature

    consists of alternating layers or lamellae of the two

    phases ( and Fe3C) tat form simultaneously duringthe transformation

    Point b, is called pearlite because it has the appearance

    of mother of pearl when viewed under the microscope

    at low magnifications

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  • Photomicrograph of a eutectoid

    steel showing the pearlitemicrostructure consisting of Alternating layers of -ferrite (the light phase) and Fe3C (thin layers most of which

    appear dark).

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  • Microstructure of a eutectoid steel The pearlite exists as grains, often termed


    The thick light layers are the ferrite phase,

    and the cementite phase appears as thin

    lamellae most of which appear dark.

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    The evolution of the microstructure of hypoeutectoid and

    hypereutectoid steels during cooling, in relationship to the Fe-Fe3C

    phase diagram

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    Cast iron

  • Cast Irons Hypoeutectic cast iron

    Eutectic cast iron

    Hypereutectic cast iron

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  • Hypoeutectic cast iron

    In this case, a structure below 11470C consists of

    proeutectic austenite and ledeburite (eutectic

    mixture consisting of austenite and cementite).

    On further cooling, in the temperature range

    11470C 7230C, excess carbon comes out as

    cementite from proeutectic and eutectic austenite.

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    Within the temp range of 1147 to 727oC upon cooling

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    Cementite network


    When it cools to room temp

  • Eutectic cast iron (ledeburite

    structure @1147oC)

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    Austenite + Cementite

  • When the same cast iron is cooled to room temp, austenite

    transforms to pearlite with cementite

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    Pearlite formed from austenite Cementite

  • Hypereutectic Cast Iron In this case the structure just below 11470C

    consists of proeutectic cementite and ledeburite.

    On further cooling in the temperature range

    11470C 7270C, excess carbon comes out as

    cementite from the eutectic austenite.

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  • Calculation of relative amounts of

    phases in Fe-Fe3C diagram

    The relative amount of proeutectoid ferrite and

    pearlite in 0.2 percent carbon steel:

    0.025%C 0.8%C0.2%C

    X YZ

    Percent of ferrite:

    = (


    ..)100 =77.4%

    Percent of