Which of the following processes is used to enhance the hardness of a metal's surface?

Case hardening is a process specifically designed to enhance the hardness of a metal's surface while maintaining a tougher, softer interior. This is achieved by diffusing carbon or nitrogen into the outer layer of the metal, resulting in a hard, wear-resistant surface. Case hardening is particularly beneficial for components that are subject to high levels of wear and require durability, such as gears and bearings.

In contrast, annealing is primarily a heat treatment process used to relieve internal stresses and improve ductility, rather than to increase hardness. Quenching does increase hardness but is primarily a method for hardening the entire volume of the material, which may lead to brittleness without further treatment. Tempering, on the other hand, is used after quenching to reduce brittleness by slightly reheating the material, which modifies the hardness rather than enhancing it. Therefore, case hardening stands out as the process that directly targets hardness enhancement at the surface of the metal.