Which heat-treatment operation changes the surface composition to increase hardness by introducing carbide or nitride elements?

• A. Temper
• B. Normalizing
• C. Case hardening
• D. Annealing

Answer: (C)

The concept being tested is hardening the surface by adding alloying elements that form hard compounds. Case hardening does this by diffusing elements such as carbon or nitrogen into the surface, where they form hard carbides or nitrides. This creates a very hard, wear-resistant outer layer while the interior remains softer and tougher, which is ideal for components like gears and shafts. The other treatments don’t change the surface composition in this way: tempering lowers hardness after quenching to improve toughness, normalizing refines grain structure without adding new elements to the surface, and annealing softens the metal and relieves stresses rather than creating a hard surface layer.