Which statement about quenching in steel is accurate?

• A. It forms austenite at room temperature.
• B. It forms pearlite with slow cooling.
• C. It leads to grain growth and reduced strength.
• D. It forms martensite by rapid cooling, increasing hardness.

Answer: (D)

The effect of cooling rate on steel microstructure and hardness during quenching is being tested. When steel is heated into the austenite range and then cooled rapidly, diffusion is suppressed and austenite transforms into martensite, a hard, brittle phase with a distorted lattice that is supersaturated with carbon. This diffusionless transformation markedly increases hardness because dislocations are more effectively blocked by the carbon in the lattice. Slow cooling, by contrast, allows carbon to diffuse and form pearlite or bainite, which are softer than martensite. Quenching thus specifically yields martensite through rapid cooling, boosting hardness.