Fe-C equilibrium diagram

—The structural form of pure iron at room temperature is called ferrite or    -iron.

—Ferrite is soft and ductile.

—Since ferrite has a body-centred cubic structure, the inter-atomic spaces are small and pronouncedly oblate, and cannot readily accommodate even a small carbon atom. Therefore, solubility of carbon in ferrite is very low, of the order of 0.006% at room temperature.

—The maximum carbon content in ferrite is 0.05% at 723 °C.

—In addition to carbon, a certain amount of silicon, manganese and phosphorous may be found in ferrite.

—The face-centred modification of iron is called austenite or    -iron. It is the stable form of pure iron at temperatures between 910°C and 1400°C. At its stable temperature austenite is soft and ductile and consequently, is well suited for manufacturing processes.

—The face-centred cubic structure of iron has larger inter-atomic spacing than in ferrite. Even so, in FCC structure the interstices are barely large enough to accommodate carbon atoms, and lattice strains are produced. As a result, not all the interstitial sites can be filled at any one time.

—The maximum solubility is only 2% of carbon at 11 30°C.

—Above 1400°C, austenite is no longer the most stable form of iron, and the crystal structure changes back to a body-centred cubic phase called delta iron. This is the same phase as the    -iron except for its temperature range.

—The solubility of carbon in   -ferrite is small, but it is appreciably larger than In   -ferrite, because of higher temperature. The maximum solubility of carbon in &iron is 0.1% at 1490°C.

—In iron-carbon alloys, carbon in excess of the solubility limit must form a second phase, which is called iron carbide or cementite.

—Iron carbide has the chemical composition of       . This does not mean that iron carbide forms molecules of   but simply that the crystal lattice contains iron and carbon atoms in a three -to- one ratio.

—The compound   has an orthorhombic unit cell with twelve iron atoms and four carbon atoms per cell, and thus has a carbon content of 6.67%.

—As compared to austenite and ferrite, cementite being an inter-metallic compound, is very hard and brittle.

—The presence of iron carbide with ferrite in steel greatly increases the strength of steel.

—In the reaction, the simultaneous formation of ferrite and cementite from austenite results at the temperature of 723°C and composition of 0.80% carbon.

—There are nearly 12% of iron carbide and slightly more than 88% of ferrite in the resulting mixture.

—Since the ferrite and cementite are formed simultaneously, they are intimately mixed. Characteristically, the mixture is lamellar, i.e., it is composed of alternate layers of ferrite and cementite.

—This micro-structure is called pearlite which is very important in iron and steel technology, because it can be formed in almost all steels by means of suitable heat treatments.

—The alloy containing 0.80% of carbon is called the eutectoid steel.

—Upon cooling the eutectoid steel below 723°C, all of the austenite is transformed into pearlite.

—Alloys with less than 0.80% C are called hypo-eutectoid steels and those with higher composition are called hyper-eutectoid steels.