Iron carbon system

Iron carbon Equilibrium Diagram

1. IRON- CARBON SYSTEM

2. Allotropic Forms of pure iron

3. Micro-constituents of Iron- Carbon alloys Ferrite Pearlite Austenite Cementite Ledeburite Martensite Troostite Sorbite Bainite

4. Definition of structures Ferrite is 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 deg.C The maximum solubility is 0.025 % C at 723  C and it dissolves only 0.008 % C at room temperature. It is the softest structure that appears on the diagram.

5. Definition of structures Ferrite Average properties are: Tensile strength = 40,000 psi; Elongation = 40 % in 2 in; Hardness > Rockwell B 90

6. Definition of structures Pearlite is 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).

7. Definition of structures Pearlite Average properties are: Tensile strength = 120,000 psi; Elongation = 20 % in 2 in.; Hardness = Rockwell C 20

8. Definition of structures Austenite is an interstitial solid solution of Carbon dissolved in  (F.C.C.) 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 .

9. Definition of structures Austenite Average properties are: Tensile strength = 150,000 psi; Elongation = 10 percent in 2 in.; Hardness = Rockwell C 40, approx ; and toughness = high

10. Definition of structures Cementite or iron carbide, is very hard, brittle intermetallic compound of iron & carbon, as Fe 3 C, 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.

11. Definition of structures Ledeburite is the eutectic mixture of austenite and cementite. It contains 4.3 percent C and is formed at 1130°C.

12. Definition of structures 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

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14. TRANSFORMATION STRUCTURE IN Steel ( EUTECToid , HYPO EUTECToid AND HYPER EUTECToid steel) TRANSFORMATION OF Eutectoid Steel TRANSFORMATION OF HYPO Eutectoid Steel TRANSFORMATION OF HYPER Eutectoid Steel

15. TRANSFORMATION STRUCTURE IN CAST IRON (EUTECTIC, HYPOEUTECTIC AND HYPER EUTECTIC CAST IRONS) 1) TRANSFORMATION OF EUTECTIC CAST IRON: CAST iron contains 4.3% C Heated to 1175 C Cooled slowly to 1148 C Mixture of austenite + cementite forms

16. 2) TRANSFORMATION OF HYPO EUTECTIC CAST IRON: It is having LESS THAN 4.3% C A 3.4% C cast iron heated up to 1350 C Cooled slowly to 1250 C (liquid metal and austenite crystals forms) At 1148 C 50% austenite and 50% cementite Cooling slowly below 723 C Final structure as cementite , pearlite and transformed ledeburite

17. 3) TRANSFORMATION OF HYPER EUTECTIC CAST IRON CAST IRON HAVING MORE THAN 4.3% C a sample of 4.8% C Heated to 1450 C for sufficient time It very cooled slowly formation of liquid phase Further slow cooling at 1148 C formation of ledeburite takes place. Cooling below 723 C formation of pearlite takes place