Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 10M, 8M

Year: May 2014, Dec 2014

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