Which statement best describes bainite?

• A. It is a cementite-rich phase identical to pearlite.
• B. It is formed by diffusion-limited growth of cementite at low temperatures.
• C. It is an austenitic transformation product found in some steels and cast irons forming ferrite with a fine dispersion of cementite between pearlite and martensite temperatures.
• D. It is a mixture of martensite and cementite.

Answer: (C)

Bainite is a diffusion-controlled transformation product of austenite that forms at temperatures lower than where pearlite forms but higher than where martensite forms. In bainite, the austenite decomposes to a ferrite matrix with a fine dispersion of cementite, so you get a ferrite plus very fine carbide structure. This occurs in steels (and some cast irons) in a temperature window between the pearlite and martensite temperatures, giving a combination of strength and toughness that’s different from both pearlite and martensite.

This description matches the idea of bainite as an austenitic transformation product that yields ferrite with a fine carbide distribution, between the temperatures where pearlite and martensite form. The other statements aren’t correct because bainite is not simply cementite-rich like pearlite, it isn’t just diffusion-limited growth of cementite at very low temperatures, and it is not a mix of martensite and cementite.