What is the purpose of case hardening in metals?

The purpose of case hardening in metals is to create a hardened exterior shell. This process enhances the surface hardness of a metal while maintaining a softer, tougher interior. The technique is particularly valuable in applications where wear resistance is critical, such as in gears, shafts, and bearings. By hardening just the surface, the metal retains the necessary ductility and toughness internally to withstand stresses, making it suitable for dynamic and demanding applications.

Case hardening processes, such as carburizing or nitriding, involve diffusing elements like carbon or nitrogen into the outer layer of the metal at high temperatures. This results in a surface that can resist wear and fatigue much better than untreated metal. Therefore, the choice that highlights the creation of a hardened exterior shell aligns perfectly with the objectives of the case hardening process in metallurgy.