Micro Constituents of Iron and Steel

Some of the micro constituents of Iron and Steel are Austenite, Ferrite, Cementide, Pearlite, Bainite, Martenite, Troostite, Sorbite and Ledeburite.

Austenite (γ-iron): It is solid solution of ferrite iron carbide in gamma iron which is formed when steel contains carbon up to 1.8% at 1130°C. On cooling below 723°C it starts transforming into pearlite and ferrite. Austenite is non-magnetic and soft. It exists in FCC crystal structure.

Ferrite: It is a BCC iron phase with very limited solubility of carbon. The solubility of carbon in ferrite is 0.08% at 723°C. Ferrite does not harden when cooled rapidly. It is very soft and highly magnetic. At room temperature ferrite contains maximum 0.0025% C only.

Cementide: Cementide is actually Fe3C, which contains 6.67%C by weight, which is extremely hard and brittle in nature. Cementide increases gradually with increase in carbon percentage. it is magnetic at below 200°C. Cementide contains orthorhombic crystal structure.

Pearlite: Pearlite is a combination of ferrite and 13% of Cementide. Steel with 0.8% carbon is wholly Pearlite, less than 0.8% carbon is wholly Pearlite, less than 0.8% is hypo eutectoid contains ferrite and Pearlite and is soft. More than 0.8% is hyper eutectoid steel which contains Pearlite and Cementide which is hard and brittle. It is having a pearl like lusture when viewed through microscope.

Bainite: Bainite is the product of isothermal decomposition of Austenite and it cannot be produced by continuous cooling Banite is aggregated of ferrite and carbide. Also it is tougher.

Martenite: This is obtained by rapid cooling of Austenite. It is extremely hard and posses articular needle like structure. It is magnetic and has carbon content up to 2%. It is extremely hard and brittle. The decomposition of Austenite below 320°C starts the formation of Martensite.

Troostite: Troostite differs from Pearlite only in the degree of fitness of structure and carbon content. It is produced by transformation of tempered Martensite. Troosite is weaker than Martensite.

Sorbite: Sorbite micro structure constitute a mixture of ferrite and finely divided cementide produced on tempering martensite above 450°C. Pearlite, Troostite and Sorbite all are ferrite cementide mixture having a lamellar structure.

Ledeburite: Ledeburite is the product of eutectic reaction. Thus Ledebruite is a euctectic mixture; consists of alternative layers of Austenite and Cementide. It contains 4.3% carbon and is formed at about 1130°C.

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