IRON - CARBON SYSTEM
*α-iron - Body Centered Cubic (BCC) - Stable at temperature upto 908°C
*γ-iron - Face Centered Cubic (FCC) - Stable between 908°C and 1388°C
*δ-iron - Body Centered Cubic (FCC) - Stable between 1388°C and 1535°C
1.Ferrite
*Ferrite is a primary solid solution based on α-iron having BCC structure.
*Ferrite is soft, ductile and highly magnetic.
*It is nothing but the interstitial solid solution of carbon in iron.
2. Austenite
*Austenite is a primary solid solution based on γ-iron having FCC structure.
*It is interstitial solid solution of carbon in iron. Austenite is normally not stable at room temperature.
*Austenite is soft, tough and highly ductile. It is non-magnetic.
3. Cementitte
*Cementite is the name given to the carbide of iron. It is hard, brittle with 6.69% iron.
*It is magnetic below 250°C.
4. Pearlite
*Pearlite is the eutectoid mixture of ferrite (87.5%) and cementite(12.5%).
*The properties of pearlite is midway between ferrite and cementite. It is relatively strong, hard and ductile.
5. Leduburite
*Leduburite is the eutectic mixture of austenite and cementite and containing 4.3% carbon.
*In pure iron-carbon alloy, it forms at 1140°C.
6. Martensite
*Martensite is the super saturated solid solution of carbon in α iron.
*It is formed when a steel is very rapidly cooled from the austentic state.
*It has very hard, more brittle and low ductility properties.
7. Troostite
*Troostite is the misture of radial lamellae of ferrite and cementite.
*This constituent is also known as troostite pearlite.
*It has hardness intermediate between martensite and sorbite
8. Sorbite
*Sorbite is the microstructure consisting ferrite and finely divided cementite produced on tempering martensite above 450°C.