Which heat treatment hardens steel by rapid cooling?

• A. Normalizing
• B. Annealing
• C. Tempering
• D. Quenching

Answer: (D)

Rapid cooling from the austenitizing temperature locks in a hard microstructure. When steel is heated enough to form austenite and then cooled very quickly in a medium like water or oil, carbon atoms don’t have time to diffuse into softer phases. Instead, martensite forms, a distorted, supersaturated structure that is exceptionally hard, which is why this heat treatment hardens steel.

Other processes don’t achieve that rapid, diffusion-free transformation. Normalizing cools more slowly in air after heating, refining the grain but not producing the same extreme hardness. Annealing involves slow cooling to soften and improve ductility. Tempering reheats quenched steel to a lower temperature to relieve brittleness and reduce residual stresses, sacrificing some hardness in the process.