Microstructure of steel

Microscopic (microstructure) examination is the study of the internal structure of a metal under a microscope. A small sample is cut, ground, polished to a mirror finish, etched with a mild acid (e.g. nital, 2% nitric acid in alcohol) and viewed at 100–1000× magnification. It shows the phases, grain size and shape, which control the strength and hardness of the steel.

Microstructures of steel

1. Ferrite: Almost pure iron (about 0.02% C) in BCC form. Soft, ductile, weak, magnetic. Appears as light, white grains.
2. Cementite ($Fe_3C$): Iron carbide with 6.67% C. Very hard and brittle. Appears as white needles or plates.
3. Pearlite: A lamellar (layered) mixture of ferrite and cementite, with 0.8% C. Appears dark in an etched sample, like mother-of-pearl. Fairly strong and tough.
4. Austenite: Solid solution of carbon in FCC iron, stable above about 723 °C. Non-magnetic, soft, tough. Exists at room temperature only in special (stainless) steels.
5. Martensite: Supersaturated carbon in BCT iron, formed by rapid quenching of austenite. Needle-like, very hard and brittle.
6. Bainite: Fine mixture of ferrite and cementite formed by cooling at an intermediate rate. Harder than pearlite and tougher than martensite.
7. Troostite and sorbite: Tempered martensite products. Troostite is harder and finer; sorbite is tougher and less hard.

Low-carbon steel has ferrite + pearlite; 0.8% C steel is fully pearlite; high-carbon steel has pearlite + cementite.