Bainitt

Bainite is a microstructure found in steel that forms at temperatures between those at which pearlite and martensite are formed. It is characterized by a mixture of ferrite and carbide phases, typically cementite. The formation of bainite involves a diffusion-controlled transformation, but with a shear component that distinguishes it from pearlite formation. There are two main types of bainite: upper bainite and lower bainite.

Upper bainite typically forms at higher temperatures within the bainite transformation range. It consists of lath-shaped ferrite structures with carbide precipitates located between the ferrite laths. The carbides in upper bainite are generally in the form of cementite.

Lower bainite forms at lower temperatures within the bainite range. In lower bainite, the carbide precipitates, also cementite, are found within the ferrite laths, often at an angle to the lath axis. This internal precipitation gives lower bainite distinct properties compared to upper bainite.

The mechanical properties of bainite are intermediate between those of pearlite and martensite. Bainite generally exhibits good strength and toughness, making it a desirable microstructure for various steel applications, including tools, automotive parts, and structural components. The specific properties depend on the exact composition of the steel and the precise transformation conditions. Heat treatment processes are carefully controlled to achieve the desired bainitic microstructure and its associated properties.