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

The process of case hardening is specifically designed to create a hard, wear-resistant layer on the surface of a metal while maintaining a tough and strong core beneath. This is accomplished by infusing elements like carbon or nitrogen into the surface layer of the metal, thereby increasing its hardness. The core remains relatively unaffected by the treatment, retaining its toughness and ductility, which is critical for applications where materials need to withstand stress without breaking.

In contrast, quenching involves rapidly cooling a heated metal, typically leading to increased hardness but can also create brittleness in the material without a soft underlying layer. Tempering, while it improves toughness after quenching, does not specifically create a wear-resistant surface. Annealing is a heat treatment that softens metal, which is opposite to the desired outcome of achieving a hard surface. Therefore, case hardening stands out as the most suitable choice for achieving both a hard exterior and a strong, resilient core.