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

Ferrite

i. Ferrite:

          Iron which contains little or no carbon is called Ferrite. This is a soft and ductile phase and is known as alpha iron generally by metallurgists. Ferrite is the name given to pure iron crystals. Ferrite is present to some extent in a great range of steels, particularly those low in carbon content, and it is also present, in soft cast iron. Ferrite has the following mechanical properties: Its KCU = 2.5 MJ/m2 , and HB = 80-100. Below the critical temperature, the slow cooling of low carbon steel produces ferrite structure. Ferrite is very soft and highly magnetic and does not harden when cooled rapidly. It forms smaller crystals when cooled from a bright red heat at a rapid rate.

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.

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.

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.