MICRO-Constituents of iron & steel

MICRO-Constituents of Iron & Steel:

          We can observe the various constituents of Fe and Steel produced due to decomposition of austenite under a microscope. The following are the important micro-constituents of Iron and Steel:

     1. Ferrite

     2. Cementite

     3. Pearlite

     4. Bianite

     5. Martensite

     6. Austenite

     7. Troosite, and

     8. Sorbite

Cementite

ii. Cementite:

          This is essentially an iron carbide Fe3C (of almost constant composition). It is extremely hard in nature and brittle, being harder than ordinary hardened steel or glass. Cementite increases with the proportion of carbon present, and the hardness and also the brittleness of cast iron is believed to be due to the presence of cementite in it.

          It contains 6.69% C and has a complex rhombic lattice. It is weakly ferromagnetic below 25°C, but loses this property on heating to 210°C. The melting temperature of cementite is difficult to determine, since cementite decomposes on heating. In experiments with laser-beam heating, its melting point has been measured to be 1260°C.

Pearlite

iii. Pearlite:

          Pearlite is the name given to a mechanical mixture of about 87% ferrite and 13% cementite having a two phase micro-structure and found in some steels and cast irons. Pearlite results from the transformation of austenite of eutectoid composition and consists of alternating layers (or lamellae) of ferrite and cementite. When e seen in the microscope the surface of appears like mother of pearl, hence it is give name as pearlite. The thickness of alternate plates and the distance between them is governed by the rate of cooling. Slow cooling produces a coarse structure than rapid cooling.

          A steel with 0.8% carbon is wholly pearlite, with less than 0.8% carbon is hypoeutectoid and with more than 0.8% carbon is hypereutectoid steel. The former contains ferrite and pearlite and is soft while the latter contains pearlite and cementite which are hard and brittle.

Bainite

iv. Bainite:

          This is a ferrite cementite aggregate, i.e., an austenitic transformation product found in some steels and cast irons. This is formed by the growth of a ferrite nucleus. It forms at temperatures between those at which pearlite and martensite transformations occur. It is the product of isothermal decomposition of austenite. The micro-structure consists of ferrite and a fine dispersion of cementite. Bainite is present in two forms; the feathery bainite obtained in the upper part of the temperature range and needle-like or accicular bainite produced by lower reaction temperature.