Describe micro constituents (microstructures) of steel

Type micro constituents (microstructures) of steel

Various Microconstituents/microstructures are given below.

(i) Ferrite 

(ii) Pearlite 

(iii) Martensite, 

(iv) Cementite, 

(v) Austenite 

(vi) Troostite, 

(vii) Sorbite, 

(viii) Allotropic Forms 

(ix) Spheroidite. 

(i) Ferrite: 

Ferrite crystals are made of solid solution of carbon iii alpha-iron. Solubility of carbon in ferrite is 0.025 at 723° C. Ferrite is present in low carbon steels and soft cast iron. It does not harden when cooled rapidly. It is very soft, ductile and highly magnetic.

(ii) Cementite: 

Cementite is formed when iron and carbon combine chemically to form iron carbide (Fe3C). It has no ductility. Cementite increases generally with increase in Carbon percentage. Its presence in iron and steel decreases the ductility and tensile strength but increases the hardness and cutting ability. It contains 6.67 percent carbon. Cementite occurs either in the form of a network or in globular form or in massive form depending upon heat treatment process.

micro constituents (microstructures) of steel

(iii) Pearlite: 

It is a mixture of 87.5% Ferrite and 12.5% cementite. It comprises of alternating plates or layers of ferrite and cementite. It has a lustrous pearly appearance. That is why it is known as pearlite. Soft steels are composed of ferrite and pearlite. Steels having 0.83% carbon are composed of 100% pearlite. Hard steels are made of pearlite and cementite.

micro constituents (microstructures) of steel

(iv) Austenite: 

It is a solid solution of iron carbide (Fe3C) in gama-iron. It is non magnetic. It occurs above 723°C. It is soft and ductile than ferrite. Upon cooling below 723°C it is completely transformed into 

(1). ferrite +pcarlite for steels having less than 0.83% carbon 

(2) pearlite for eutectoide steel steels having exactly 0.83% carbon 

(3) pearlite +cementite for steels having more than 0.83% carbon. 

(v) Martensite: 

Martensite is a mass of needle like structure. It is obtained when austenite is cooled rapidly from higher critical temperature. Formation of martensite starts by the decomposition of austenite below 320°C. It is the main constituent of hardened steel. It is extremely hard, brittle and magnetic. 

(vi) Troostite: 

It is a very fine pearlite. Like pearlite it also has alternate layers of ferrite and cementite. It is stronger than pearlite. It is softer and less brittle than martensite and harder than “Sorbite”. It is obtained 

1. When austenite is cooled at a slow rate than required for martensite (i.e. cooled in oil). 

2. when martensite is tempered between 250 to 450° C. 

(vii) Sorbite: 

Depending upon the chemical .mnipositioji, size of the job and degree of hardening, the sorbite, begins to form when tempered above 400°C and upto 680°C. Layers of cementite contract to form granules as shown in figure. Sorbite is less ductite than pearlite but its tensile strength is higher. It is softer and less hard than “troostite” but more ductile than troostite. Sorbite may also be obtained when austenite is cooled at a rate faster than required for pearlite and cooled at a rate slower than than required or “troostite”.

(vii) Spheroidite: 

To soften air hardened stels and carry out machining operations, the steels are heated just below the lower critical temperature (i.e between 680° C and lower critical temp 723° C ) cementite is converted into small rounded speroids.. Granules of comentite are converted into gladoules of cementite, which is softer than “Sorbite”. 

Allotropic Forms of Iron

Allotropic Forms of Iron: 

Iron and steel also possesses allotropy. Its various allotropic form are: alpha-iron, gamma-iron and S iron 

1. Alpha Iron: It has been structure and occurs in two forms. 

(i) Ferromagretite alpha-iron: 

It is magnetic from room temperature to 768°C. It has bcc structure and strong. 

(ii) Para magnetic alpha-Iron: 

It also has bcc structure. It is magnetic and found between 768 to 910°C. 

2. Gamma Iron : (Iron) or Austenite: It is found between 910 and 1410°C and fcc structure. It is non-magnetic in nature. It is very ductile and soft. It is known as austenite. 

3. Delt Iron (S-Iron): This form is stable between 1410°C and 1539°C and has a bcc crystal lattice.

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