Which heat-treating process produces a hard, wear-resistant surface over a tough core?

Case hardening is a heat-treating process specifically designed to enhance the surface hardness of materials while maintaining a tougher core. This duality is essential in applications where parts need to withstand wear and abrasion due to surface contact, yet also require the ability to absorb shock and resist fracturing, which is provided by having a tougher internal structure.

During case hardening, the outer layer of the metal is enriched with carbon or other hardening elements, typically through methods like carburizing or nitriding. The process involves heating the material to allow carbon or nitrogen to diffuse into the surface and then cooling it rapidly, creating a hard, wear-resistant layer. This hardened surface significantly improves the part's durability without compromising its overall toughness, ensuring it can handle various mechanical stresses in service.

In contrast, quenching is a method that cools metal rapidly after it has been heated to harden it, but it usually does not introduce additional hardening materials, which is why it may not deliver the same surface toughness benefit as case hardening. Annealing is primarily used to soften metals and relieve internal stresses, while alodizing is a surface treatment that primarily improves corrosion resistance rather than hardness. Thus, case hardening stands out for creating a hard surface with a resilient