ANS:D - carried out to get extreme hardness in its core.

"Carried out to get extreme hardness in its core" refers to a specific type of heat treatment process known as through-hardening or full hardening. In this process, the goal is to achieve uniform hardness throughout the entire cross-section of the material, including its core, rather than just hardening the surface layer as in case hardening. Through-hardening involves heating the material to a temperature above its critical transformation temperature, followed by rapid quenching to achieve a fully hardened structure. The rapid quenching process, typically done by immersing the heated material in a quenching medium such as oil, water, or polymer, helps to "freeze" the microstructure of the material in a hardened state. The result of through-hardening is a material with uniform hardness and strength properties from the surface to the core. This extreme hardness throughout the core makes the material suitable for applications requiring high wear resistance, cutting tools, and components subjected to heavy loads and impact. Through-hardening is commonly used for steels and other ferrous alloys but can also be applied to non-ferrous metals and alloys. It is often followed by tempering, a process of reheating the material to a lower temperature to relieve internal stresses and improve toughness while retaining most of the hardness achieved during through-hardening. Overall, through-hardening is carried out to achieve extreme hardness in the core of the material, providing uniform mechanical properties and enhanced performance in demanding applications.