Bainita

Bainita is a microstructure in steel and cast iron consisting of a mixture of ferrite and cementite that forms when austenite transforms at temperatures below those that produce pearlite but above those that yield martensite. The term bainita (bainite) was named after the American metallurgist Edgar Bain, who described the transformation in the early 20th century.

Formation and microstructure: Bainite forms through the isothermal transformation of austenite by holding steel within the bainitic temperature range after quenching. The process yields a characteristic non-pearlitic structure that provides a sharp contrast to both pearlite and martensite. The resulting ferrite–cementite mixture can take different morphologies depending on the temperature at which transformation occurs.

Upper and lower bainite: There are two principal forms. Upper bainite forms at higher bainitic temperatures and typically shows feathery or lath-like ferrite with elongated cementite particles located between the laths. Lower bainite forms at lower temperatures and yields a finer ferrite lath structure with carbide precipitates forming within the ferrite and more finely distributed cementite. The exact temperature boundaries depend on alloy composition and carbon content.

Properties and applications: Bainitic steels generally offer a favorable combination of strength and toughness, with improved wear resistance relative to pearlite and greater ductility than martensite. The balance of properties makes bainitic microstructures attractive for gears, shafts, bearings, and other components requiring high strength and toughness. Bainite can be produced by controlled cooling or isothermal holding, providing versatility in heat-treatment design for various steel grades.