What is meant by tempering steel?

• A. The steel is heated to above its melting point.
• B. The steel is hardened; then some hardness is removed to relieve internal stresses.
• C. The steel is cooled rapidly in air to increase brittleness.
• D. The steel is coated with phosphate.

Answer: (B)

Tempering is a heat treatment done after hardening to reduce brittleness by relieving internal stresses, producing a tougher steel while keeping most of its hardness. After quenching, steel becomes very hard but brittle due to internal stresses. Reheating to a controlled, lower temperature lets those stresses relax and allows some diffusion and microstructural changes (tempered martensite and carbide formation), which decreases brittleness and improves toughness without greatly sacrificing hardness. The other options describe processes that are not tempering: heating above the melting point would melt the steel; rapid cooling in air increases brittleness (quenching); and coating with phosphate serves for lubrication or corrosion resistance, not tempering.