Quenching, dipping heated metals in oil or water, can change the hardness of the metal.

• A. True
• B. False
• C. Not sure
• D. Sometimes

Answer: (A)

Quenching changes hardness because rapid cooling after heating alters the metal’s internal structure. In steel, heating into the austenite region and then quenching quickly in oil or water prevents carbon diffusion and transforms the structure into martensite, a very hard (but brittle) phase. This phase change is what makes the metal harder than it was before quenching. The degree of hardening depends on the alloy and how fast it cools; some metals may not harden much with quenching, or may require different quenchants or subsequent tempering to achieve the desired hardness.