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16. explain various micro constituent of iron carbon alloy?

There are different microscope constituents of iron carbon alloys exist. The study of these micro constituents is essential in order to understand iron carbide (Fe-C) equilibrium phase diagram

Various micro constituents of iron carbon alloys are:1. Ferrite2. Austenite3. Cementite4. Pearlite5. Ledeburite6. Martensite7. Troostite8. Sorbite9. Bainite

Now we shall present a brief note on the above constituents in the following sections

1. FERRITE (OR α - IRON): Ferrite is a primary solid solution based on α iron having BCC structure It is nothing but the interstitial solid solution of carbon in iron Maximum solubility of carbon in iron is 0.025% carbon at 723˚C, while its solubility at room

temperature is only about 0.008% Ferrite is soft, ductile, and highly magnetic It can undergo extensive cold working

2. AUSTENSITE (OR γ - IRON): Austenite is a primary solid solution based on γ iron having FCC structure This is also an interstitial solid solution of carbon in iron The maximum solubility of carbon in FCC iron is about 2% at 1140˚C Austenite is normally not stable at room temperature; mostly stable only above the critical

temperature Austenite is soft, tough, and highly ductile. Thus it is more suitable for fabrication processes It is also a non- magnetic Austenite has a greater electrical resistance and coefficient expansion than ferrite

3. CEMENTITE: Cementite is the name given to the carbide of iron (Fe3C) It is the hard, brittle, intermetallic compound of iron with 6.69% of carbon The hardness and brittleness of cast iron is believed to be due to the presence of the cementite It is magnetic below 250˚C

4. PEARLITE: Pearlite is the eutectoid mixture of ferrite (87.5%) and cementite (12.5%) It is formed when austenite decomposes during cooling. It contains 0.8% of carbon It consists of alternate thin layers of ferrite and cementite The name derives from its lustrous appearance when viewed in white light under a micro-scope The properties of pearlite is midway between ferrite and cementite. It is relatively strong, hard and

ductile5. LEDEBURITE:

Ledeburite is the eutectic mixture of austenite (γ iron) and cementite (Fe3C) containing 4.3% carbon In pure iron-carbon alloy, it forms at 1140˚C. most of engineering alloy materials belong to this range

of alloy Pig iron, the most important engineering material, is ledeburite

6. MARTENSITE:

Martensite is the super saturated solid solution of carbon α iron It is formed when a steel is very rapidly cooled from the austenite state It exhibits a characteristic acicular or needle like structure It has very hard, more brittle and low ductility properties There is an increase in specific volume during formaustenite. As a result, internal stresses are set up

in the materials leading to the formation of minute cracks7. TROOSTITE:

Troostite is the mixture of radial lamellae of ferrite and cementite. In fact, it differs from pearlite only in the degree of fineness

This constituent is also known as troostite pearlite It is the microstructure consisting ferrite and finely divided cementite, produced on tempering

martensite below 450˚C It is formed by the decomposition of austenite when cooled at a rate slower than that which will

yield a martensitic structure and faster than that which will produce a sorbitic structure It has hardness intermediate between martensite and sorbite

8. SORBITE: Sorbite is the microstructure consisting ferrite, and finely divided cementite, produced on tempering

martensite above 450˚C This constituent is also known as sorbitic pearlite It is formed by the decomposition of austenite when cooled at a rate slower than that which will

yield a troostitic structure and faster than that which will produce a pearlitic structure Though sorbitic steel is slightly less ductile than pearlite steel, its tensile and yield strengths are high.

Thus sorbitic steels are often known as toughened steels9. BAINITE:

Bainite is a decomposition product of austenite, consisting of an aggregate of ferrite and carbide Bainite obtained by transformation of pearlite at higher temperature is called upper bainite Bainite obtained by low temperature transformation is called lower bainite Lower bainite provides high mechanical properties and that is why it is extensively used for

components of machine and structures Bainite has hardness in between the hardness of pearlite and martensite

17. explain simple crystal structure?

It has been found that most of the common metals possess cubic or hexagonal structures only. The important simple crystal structure are

1. Simple cubic structure (SC)2. Body centred cubic structure (BCC)3. Face centred cubic structure (FCC)4. Hexagonal close packed structure (HCP)

1. SIMPLE CUBIC STRUCTURE (SC): The unit cell of simple cubic structure has one atom at each corner. So there are eight atoms The arrangement of lattice points in a SC cell

2. BODY CENTRED CUBIC STRUCTURE (BCC): The unit cell of BCC has atom at each corner. Also it has one atom at the centre of the body. So we

have eight corner atoms and one centre atom The arrangement of lattice points in a BCC cell EXAMPLES: tungsten, vanadium, molybdenum, chromium

3. FACE CENTRED CUBIC STRUCTURE (FCC): The unit cell of FCC has one atom at each corner. It also has one atom at the centre of the each face.

So it has eight corner atoms and six face centred atoms The arrangement of lattice points in a FCC cell EXAMPLE: copper, silver, gold, aluminium, nickel

4. HEXAGONAL CLOSED PACKED STRUCTURE (HCP): The unit cell HCP contains one atom at each corner of the hexagonal prism, one atom each at the

centre of the hexagonal faces and three more atoms within the body of the cell The arrangement of lattice points in a HCP cell EXAMPLES: magnesium, zinc, titanium, zirconium, beryllium, cadmium