Why is a piece of steel tempered after it has been hardened?

• A. Tempering removes some of its brittleness.
• B. Tempering increases hardness.
• C. Tempering coats the surface.
• D. Tempering adds carbon.

Answer: (A)

Tempering after hardening is done to reduce brittleness and increase toughness. When steel is quenched, it becomes martensite—very hard but extremely brittle because the crystal structure is distorted and locked in with internal stresses. Heating to a tempering temperature lets some carbon diffuse and the structure rearrange into a more ductile form, relieving those stresses. The result is a material that can absorb impact and resist cracking better, which is crucial for aviation components that face vibration and sudden loads. Tempering does not add carbon and does not coat the surface, and while it lowers hardness somewhat, the gain in toughness is the important benefit.