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

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.

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.