Why is steel tempered after being hardened?

• A. To increase its hardness and ductility
• B. To increase its strength and decrease its internal stresses
• C. To relieve its internal stresses and reduce its brittleness
• D. To improve corrosion resistance

Answer: (C)

Tempering after hardening is done to relieve internal stresses and reduce brittleness. When steel is quenched, it forms martensite, a very hard but very brittle structure with lots of residual stress trapped in the lattice. Tempering allows the structure to relax: carbon atoms diffuse slowly, some martensite becomes a more stable tempered martensite, and tiny carbides precipitate. This lowers the internal stresses and makes the steel tougher, so it can absorb impacts without cracking while still staying hard enough for wear resistance. It isn’t about increasing corrosion resistance, and while hardness may remain adequate, tempering mainly aims to improve toughness and reduce brittleness rather than simply increasing hardness.