Which statement correctly describes bainite?

• A. A metastable phase formed by diffusion at low temperatures.
• B. A diffusion-controlled transformation that occurs during slow cooling.
• C. A lamellar mixture of ferrite and cementite in alternating layers.
• D. An austenitic transformation product found in some steels and cast irons. It forms at temperatures between those at which pearlite and martensite transformations occur. The microstructure consists of ferrite and a fine dispersion of cementite.

Answer: (D)

Bainite is a steel microstructure that forms when austenite cools through a specific temperature window between the pearlite and martensite ranges. It forms by diffusion of carbon, not by a diffusionless collapse, so the carbon redistributes to create cementite within a ferritic matrix rather than forming a single, instantaneous transformation. The resulting image is ferrite with a fine dispersion of cementite, which explains its characteristic appearance and properties.

This description aligns with bainite being a transformation product from austenite found in steels and cast irons, occurring at temperatures between those where pearlite and martensite form, and yielding a ferrite-plus-cementite microstructure. It’s distinct from pearlite, which shows a lamellar ferrite–cementite structure, and from martensite, which is diffusionless. The nuance of upper versus lower bainite relates to the exact temperature within that window, but the essential idea—diffusion-controlled transformation producing ferrite with finely distributed cementite within a defined temperature range—is captured here.