What does case hardening achieve in metal processing?

Case hardening is a process used in metal processing primarily to enhance the surface hardness of steel while maintaining a tougher and more ductile interior. The correct answer highlights that this process specifically produces martensite in the outer layer of the metal.

Martensite is a very hard form of steel that is achieved by rapidly cooling (quenching) austenitized steel. In case hardening, the outer layers of the metal part are often subjected to a heat treatment process that alters the microstructure of the surface, converting it to martensite, which significantly increases hardness and wear resistance. The core of the metal, which does not undergo the same rapid cooling, retains a softer, more ductile structure, which is advantageous for mechanical performance and resistance to fracture.

This selective hardening allows components such as gears and bearings to withstand wear while still having enough toughness in the interior to avoid catastrophic failure. The other options do not accurately represent the outcomes of case hardening: it does not make the entire structure brittle, it does not completely transform the metal into austenite, nor does it soften the interior of the metal component. These features combine to make case hardening a valuable technique in manufacturing and metalworking.