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

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.

Sorbite

viii. Sorbite:

          This is also produced by the transformation of tempered martensite at a temperature of 590-640°C. It is produced when steel is heated at a fairly rapid rate from the temperature of the solid solution to normal room temperature. Its properties are intermediate between those of pearlite and troosite and has a good strength. It is practically pearlite. We may note that troosite and sorbite division of pearlite structure is quite conventional, since the mixture dispersity increases monotonously with decreasing temperature of transformation.