Micro Constituents of Iron and Steel

Iron and Steel's microconstituents are Austenite, Ferrite, Cementide, Pearlite, Bainite, Martenite, Troostite, Sorbite, and Ledeburite.

Austenite (γ-iron): It is a 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: Ferrite 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 a maximum of 0.0025% C only.

Cementide: Cementide is actually Fe3C, which contains 6.67%C by weight and is extremely hard and brittle. Cementite increases gradually with an increase in carbon percentage. it is magnetic at below 200°C. Clementine contains an 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 and less than 0.8% is hypo eutectoid, contains ferrite and Pearlite, and is soft. More than 0.8% is hypereutectoid steel, which contains Pearlite and Cementide, which is hard and brittle. It has a pearl-like luster when viewed through a microscope.

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

Martenite: This is obtained by the rapid cooling of Austenite. It is extremely hard and possesses an articular needle-like structure. It is magnetic and has a carbon content of 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 the transformation of tempered Martensite. Troosite is weaker than Martensite.

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

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