Bainite is a microstructural crystal lattice that forms in steel during rapid cooling of austenite. It possesses the hardness of martensite and the texture of pearlite. Bainitic steel is used in power generation and structural applications, and can be infused with non-metallic particles to produce nucleated bainite. Variations include upper and lower bainite.
Bainite is a microstructural crystal lattice that forms in steel during heating. It is named after Edgar C. Bain, an American metallurgist who worked on the alloying and heat treatment of steel in Pittsburgh, Pennsylvania, USA. Bainite is formed when austenite cools rapidly. Austenite is an allotrope, or a form of iron known as gamma iron, that contains carbon and a cubic lattice structure when it is between 1,670° and 2,552° Fahrenheit (910° to 1,400° Celsius).
Two unique temperature conditions must exist for the microstructure of bainite to form. Austenite must be cooled quickly enough so that pearlite does not form. Pearlite is a steel layered structure of alternating ferrite and cementite that forms when the steel is slowly cooled below a temperature of 1,341° Fahrenheit (727° Celsius). Cooling in austenite must also be delayed long enough to avoid martensite formation. Martensite is a very hard and brittle crystalline by-product of austenite production.
If the machining of austenite is done correctly and bainitic steel is formed, it exhibits some of the characteristics of both pearlite and martensite. It possesses some of the extreme hardness of martensite, as well as the hard texture of pearlite. The bainitic microstructure consists of ferrite, as in pearlite, and a tiny dispersion of cementite as well.
The uses for bainitic steel varieties are included in the power generation industry due to their unique creep resistance quality. They are less likely to deform under stress than other types of steel. This quality is enhanced by alloying the steel with chromium and molybdenum to increase hardness.
Another variation on making bainitic steel is to infuse it with non-metallic particles, which produce a more disorganized microstructure. This is called nucleated bainite, or acicular ferrite, and has a greater ability to deflect cracks than traditional bainite. Uses for this variety include in large structural applications subject to frequent stresses, such as oil rigs and bridges.
Variations on the types of bainitic steels produced are often classified as upper bainite or lower bainite. The upper range is produced during the cooling process at a temperature between 1,022° and 752° Fahrenheit (550° and 400° Celsius) and resembles a form of steel known as Widmanstatten ferrite. Lower bainitic steel is produced at a temperature cooling level of 752° to 482° Fahrenheit (400° to 250° Celsius), where it resembles acicular morphology. While lower bainite is not specifically nucleated bainite, it is somewhere between upper bainite and martensite structures in composition.
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