Which statement correctly describes the formation of pearlite, bainite, and martensite in steel?

• A. Pearlite forms by very rapid quenching; bainite forms by slow cooling; martensite forms by diffusion at moderate temperatures.
• B. Pearlite forms by diffusionless transformation; bainite forms at very rapid cooling; martensite forms by diffusion during aging.
• C. Pearlite forms by slow cooling as lamellar ferrite and cementite; bainite forms at intermediate cooling; martensite forms by very rapid quenching.
• D. Pearlite is a single phase ferrite; bainite is cementite only; martensite is a cubic phase.

Answer: (C)

In steel, the final microstructure after cooling depends on how fast you cool it. Pearlite forms when cooling is slow enough to allow carbon to diffuse and two phases to stack into alternating layers: ferrite (low-carbon iron) and cementite (Fe3C) arranged as lamellae. This diffusion-controlled transformation happens gradually as the austenite cools, producing the recognizable layered structure.

Bainite appears at an intermediate cooling rate. It still involves diffusion, but the rate and temperature range lead to a non-lamellar mixture of ferrite and cementite, often with an acicular (needle-like) or feathered appearance. The result is a finer, more complex mixture than pearlite, formed before the conditions favor the full, lamellar pearlite arrangement or the complete suppression of diffusion.

Martensite forms when cooling is very rapid. The atoms don’t have time to diffuse, so the austenite transforms via a diffusionless, shear-like process into a supersaturated, high-carbon ferrite with a distorted lattice (body-centered tetragonal). This yields a very hard but brittle phase.

So the statement that pearl ite results from slow cooling with lamellar ferrite and cementite, bainite from intermediate cooling, and martensite from very rapid quenching best captures the distinct mechanisms and temperature regimes that produce these three microstructures.