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

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.

Troosite

vii. Troosite:

          This is obtained either by tempering a martensite steel at between 250 and 450°C or by quenching steel at a speed insufficient to suppress the thermal change point fully. In comparison to martensite, this is less hard and brittle and also weaker than martensite. At the transformation temperature, the inter lamellar spacing decreases to 1X10-7 -- 2X10-7 m. It has dark appearance on etching.