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

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.

Martensite

v. Martensite:

          Martensite is magnetic and hard brittle mass of fibrous or needle like structure, it contains carbon upto 2% and it is main constituent of hardened steel. This is a body-centered, tetragonal meta-stable iron phase produced by entraping carbon on decomposition of austenite when the rapid quenching of high carbon steel from a slightly higher temperature than the maximum temperature of critical interval.

          We may note that the decomposition of austenite below 320°C starts the formation of martensite. It is not as tough as austenite. It is differs from austenite in being magnetic.

Austenite

vi. Austenite:

          This is the solid interstitial solution of carbon in gamma iron (FeC), which is stable only within a particular range of composition and temperature. and is non-magnetic. It has a FCC lattice in which the interstices are larger than in the BCC lattice, because of which the solubility of carbon in FeC is much higher and attains 2.14%. Austenite is ductile and has a higher strength than ferrite (HB 160-200) at a temperature 20-25°C. On cooling below 723°C it starts transforming into ferrite and which is magnetic and cementite to form the eutectoid pearlite, together with free ferrite or free cementite, depending on whether the carbon content is less or greater than 0.87% respectively. We may note that the austenite in a eutectoid steel is unstable at all temperatures.