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Draw Fe Fe3C diagram and explain Euctetoid, Eutectic and Peritectic transformations in Fe Fe3C.

Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 10M, 8M

Year: May 2014, Dec 2014

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  1. Ferriteis known as α solid solution.

    • It is an interstitial solid solution of a small amount of carbon dissolved in α(BCC) iron.
    • Stable form of iron below 912C
    • The maximum solubility is 0.025 % C at 723Cand it dissolves only 0.008 % C at room temperature.
    • It is the softest structure that appears on the diagram.
  2. Austeniteis an interstitial solid solution of Carbon dissolved in  (FCC) iron.

    • Maximum solubility is 2.0 % C at 1130°C.
    • High formability, most of heat treatments begin with this single phase.
    • It is normally not stable at room temperature. But, under certain conditions it is possible to obtain austenite at room temperature.
  3. Martensite a super-saturated solid solution of carbon in ferrite.

    • It is formed when steel is cooled so rapidly that the change from austenite to pearlite is suppressed.
    • The interstitial carbon atoms distort the BCC ferrite into a BC-tetragonal structure (BCT) responsible for the hardness of quenched steel
  4. Ledeburite is the eutectic mixture of austenite and cementite.

    • It contains 4.3 percent C and is formed at 1147°C
  5. Cementiteor iron carbide is very hard, brittle intermetallic compound of iron & carbon, as Fe3C, contains 6.67 % C.

    • It is the hardest structure that appears on the diagram, exact melting point unknown.
    • Its crystal structure is orthorhombic.
    • It is has low tensile strength (approx. 5,000 psi), but high compressive strength.
  6. Pearliteis the eutectoid mixture containing 0.80 % C and is formed at 723°C on very slow cooling.

    • It is a very fine plate-like or lamellar mixture of ferrite and cementite.
    • The white ferritic background or matrix contains thin plates of cementite (dark).

Transformations

  • Peritectic reaction

    Liquid+Solid1↔Solid2

    L(0.53wt%C)+δ(0.09wt%C)↔γ(0.17wt%C) at 1495°C

    Liquid-18.18wt% +δ-ferrite 81.82 wt%→100wt% γ

  • Eutectic reaction

    Liquid↔Solid1+Solid2

    Liquid (4.3wt%C) ↔ γ(2.11wt%C) + Fe3C (6.67wt%C) at 1147˚C

    Liquid-100 wt% →51.97wt% γ +Fe3C (48.11wt%)

    The phase mixture of austenite and cementite formed at eutectic temperature is called ledeburite.

  • Eutectoid reaction

    Solid1↔Solid2+Solid3

    γ(0.77wt%C) ↔ α(0.0218wt%C) + Fe3C(6.67wt%C) at 727°C

    γ (100 wt%) →α(89 wt% ) +Fe3C(11wt%)

    Typical density

    α ferrite=7.87 gcm-3

    Fe3C=7.7 gcm-3

    Volume ratio of α- ferrite: Fe3C=7.9:1

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