Which metal heat-treating process creates a hard, wear-resistant surface?

The process that creates a hard, wear-resistant surface on metal is case hardening. This technique involves altering the surface layer of a material—usually steel—by infusing it with carbon or nitrogen at high temperatures. As a result, the outer surface develops increased hardness and wear resistance, while maintaining a tougher, more ductile core. This combination is ideal for components that experience high surface wear, such as gears and other machinery parts.

Normalizing, annealing, and tempering are all heat-treating processes that modify the properties of metals, but they primarily focus on improving internal characteristics such as toughness, ductility, or relieving internal stresses rather than enhancing surface hardness. For instance, normalizing refines the grain structure for improved toughness, annealing softens materials to improve machinability, and tempering reduces brittleness in hardened steel. However, none of these processes specifically create the hard surface characteristic of case hardening.