Bainite is a microstructural crystalline pattern that forms in steel during heating. It is named after Edgar C. Bain, a US metallurgist who worked on the alloying and heat treatment of steel in Pittsburgh, Pennsylvania, in the United States. Bainite is formed when austenite is cooled rapidly. Austenite is a allotrope, or form of iron known as gamma iron, that contains carbon and a cubical lattice structure when between 1,670° to 2,552° Fahrenheit (910° to 1,400° Celsius).
Two unique temperature conditions have to exist for the bainite microstructure to form. Austenite must be cooled rapidly enough so that pearlite does not form. Pearlite is a alternating layered structure in steel of ferrite and cementite that forms when the steel is slowly cooled, and falls below a temperature of 1,341° Fahrenheit (727° Celsius). Cooling in the austenite must also be delayed long enough to prevent martensite from forming. Martensite is a very hard, brittle crystalline byproduct of austenite production.
If the processing of austenite is done correctly and bainitic steel is formed, it displays some of the characteristics of both pearlite and martensite. It possesses some of the extreme hardness of martensite, as well as the tough structure of pearlite. The bainitic microstructure consists of ferrite, like in pearlite, and a minute dispersion of cementite also.
Uses for bainitic steel varieties are included in the power generation industry because of their unique quality of creep resistance. They are less likely to deform under stress than other steel types. This quality is enhanced by alloying the steel with chromium and molybdenum to increase hardness.
Another variation on bainitic steel manufacturing is to infuse it with nonmetallic particles, which produce a more disorganized microstructure. This is called nucleated bainite, or acicular ferrite, and it has a greater ability to deflect cracks than traditional bainite. Uses for this variety include in large structural applications that undergo frequent stress, such as oil rigs and bridges.
Variations on the types of bainitic steel produced are often categorized as upper bainite or lower bainite. The upper range is produced during the cooling process at a temperature of between 1,022° to 752° Fahrenheit (550° to 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 the acicular morphology. Though lower bainite is not specifically nucleated bainite, it is somewhere between upper bainite and martensite structures in composition.
