What is the primary function of carburizing in case hardening?

The primary function of carburizing in case hardening is to diffuse carbon into the metal's surface. This process is critical because by introducing carbon, the hardness and wear resistance of the outer layer of the metal are significantly enhanced. Carburizing typically involves heating the steel in an environment rich in carbon, such as in a carbonaceous gas or solid carbon materials.

As the carbon atoms penetrate the surface during the heating process, they interact with the iron in the metal, leading to the formation of a hardened layer once the metal is subsequently quenched. This hardening process increases the material's performance under wear and stress, making carburized components ideal for high-strength applications such as gears and other mechanical parts that experience significant surface fatigue.

The other options do not accurately describe the primary function of carburizing. Cooling the metal quickly is related to quenching, while introducing nitrogen into the metal pertains to nitriding, not carburizing. Reducing the metal's hardness is contrary to the objective of case hardening, which aims to increase hardness for better performance.