Which microstructure forms at intermediate cooling rates during steel cooling?

• A. Martensite
• B. Bainite
• C. Pearlite
• D. Austenite

Answer: (B)

When steel is cooled from austenite, the speed of cooling dictates what phase can form. At slow cooling, carbon diffuses enough to arrange into alternating layers of ferrite and cementite, giving pearlite. At very high cooling rates, diffusion is suppressed and a diffusionless transformation produces martensite. The intermediate cooling rates strike a balance: there’s enough diffusion and time for transformation to occur, but not so much warmth or time that pearlite forms, nor so little diffusion that martensite happens. This middle-ground transformation leads to bainite, which is a mixture of ferrite and cementite with a distinctive needle-like or feathery morphology depending on the exact temperature window. This region sits between the pearlite and martensite regions on the transformation diagram, so bainite forms at intermediate cooling rates.