What does case hardening achieve in metalworking?

Case hardening is a heat treatment process that specifically increases the hardness of the surface layer of a metal while preserving a softer core. This differential hardening is beneficial for components that require a hard exterior to resist wear and abrasion but need to retain ductility and toughness in their core to withstand impacts and stresses.

By hardening the surface, case hardening ensures that parts can endure high friction and contact without deforming, while the softer inner structure can absorb shocks and resist cracking. This combination often leads to enhanced performance in mechanical applications, providing both durability and resilience.

The other options do not accurately reflect what case hardening achieves. Hardening the entire metal piece would negate the benefits of having a tough core. Enhancing ductility contradicts the purpose of hardening, as ductility often decreases with increased hardness. Finally, while case hardening can have some effects on thermal properties, improving thermal conductivity is not a primary goal of the process. Therefore, the focus of case hardening on achieving a hard surface with a softer core is the key takeaway that supports the correct answer.