Which heat treatment increases the hardness of certain metals?

• A. Annealing
• B. Cold working
• C. Quenching
• D. Tempering

Answer: (C)

Hardness changes when a metal’s internal structure is altered by heating and cooling. Quenching increases hardness by heating the metal into a high-temperature phase region and then cooling it rapidly. In steel, this rapid cooling traps a hard, brittle microstructure called martensite, which resists deformation much more than the softer forms formed during slow cooling. This is why quenching is commonly used to boost hardness. However, the rapid cooling also makes the metal more brittle, so tempering—reheating to a lower temperature and holding—is often used afterward to restore some toughness while preserving much of the hardness. Annealing softens the metal by slow cooling to relieve stresses and allow grain growth, reducing hardness. Cold working can increase hardness through deformation at room temperature, but it is not a heat treatment.