Definition - What does Bainite mean?

Bainite is a combination of ferrite and cementite in ferrous metals that is harder than pearlite. It is an aggregate of iron carbide and ferrite, formed from austenite below the temperature at which pearlite forms and above that at which martensite forms.

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.

Corrosionpedia explains Bainite

Bainite is an acicular microstructure that forms in steels at temperatures of 480-1020°F (250-550°C), depending on alloy content. It contains needlelike grain structures, and it requires an initial rapid cooling followed by gradual cooling.

Bainite is formed when austenite is cooled rapidly enough that pearlite does not form. Austenite is an allotrope, or form of iron known as gamma iron, that contains carbon and a cubical lattice structure when heated to 1670-2550°F (910°-1400°C).

If the processing of austenite is done correctly and bainitic steel is formed, 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.

Bainite steel is 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.

Variations on the types of bainitic steel produced are often categorized as:

  • Upper bainite - Range is produced during the cooling process at a temperature of 750-1020°F (400-550°C) and resembles a form of steel known as Widmanstatten ferrite
  • Lower bainite - Generally forms at temperatures of 480-750°F (250-400°C). The transformation nucleates, like upper bainite, by partial shear.

The corrosion rate of bainite is slower than that of ferrite and pearlite. The corrosion products of bainite are uniform and fine.

Bainitic steel varieties are used in the power generation industry because of their unique quality of creep resistance. Since nucleated bainite or acicular ferrite has a greater ability to deflect cracks than traditional bainite, they are used in large structural applications that undergo frequent stress, such as oil rigs and bridges.

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