Which of the following is used to enhance the ductility of metals during processing?

Heat treatment is a process that modifies the physical and sometimes chemical properties of materials, primarily metals, to enhance their mechanical performance. In the context of ductility, heat treatment can significantly improve the capacity of metals to deform plastically without fracturing, which is crucial for many applications.

One common heat treatment method is annealing, where metal is heated to a specific temperature and then allowed to cool slowly. This process allows for the reorganization of the metal's internal structure, relieving internal stresses and promoting the movement of dislocations within the crystal lattice. As a result, the metal's ductility is increased as its ability to absorb energy and deform without failure improves.

In contrast, other options such as cold rolling and surface hardening typically reduce ductility. Cold rolling works by deforming the metal at room temperature, which can increase strength through strain hardening but often at the cost of reduced ductility. Surface hardening techniques are designed to increase hardness and wear resistance on the material's surface but may negatively affect ductility in the core. Quenching, while it can increase strength through the formation of martensite in certain steels, often leads to reduced ductility due to the internal stresses induced by rapid cooling.

Therefore, heat treatment is the appropriate