Pick out the correct statement.

ANS:D - Tempering of a material improves ductility & toughness but reducess hardness & brittlenes.

Tempering is a heat treatment process often applied to hardened steel to improve its mechanical properties. When steel is initially hardened, it becomes very hard but also quite brittle. Tempering involves reheating the hardened steel to a temperature below its critical point (lower than the temperature at which it was hardened) and then cooling it, usually in still air or slightly agitated air. During tempering, several changes occur in the steel's microstructure:

1. Reduction in Hardness: Tempering reduces the hardness of the steel compared to its fully hardened state. This is desirable because extremely hard steel can be too brittle for many applications. By tempering, some of the hardness is sacrificed in favor of improved toughness and ductility.
2. Increase in Ductility and Toughness: Tempering redistributes the carbon within the steel, reducing its overall strength but increasing its ductility and toughness. This means that the steel becomes less prone to fracture and more capable of absorbing energy before breaking. This property is crucial in applications where impact resistance is important, such as in tools and structural components.
3. Reduction in Brittleness: While tempering reduces the hardness of the steel, it also reduces its brittleness. This is achieved by relieving some of the internal stresses that were introduced during the hardening process. As a result, tempered steel is less likely to crack or shatter under sudden or heavy loads, making it safer and more reliable in many applications.

Overall, tempering is a balancing act between retaining some of the hardness from the hardening process while improving the steel's toughness and ductility. The specific tempering temperature and time duration are carefully chosen based on the desired mechanical properties for the intended application of the steel.