Which of the following processes uses heat to alter the properties and microstructure of metals?

Heat treatment is a critical process in metallurgy where heat is applied to metals to alter their properties and microstructure. This method involves heating the metal to a specific temperature and then cooling it at a controlled rate. The purpose of heat treatment can include increasing strength, improving ductility, enhancing wear resistance, and relieving stresses within the material.

During heat treatment, various techniques such as quenching, tempering, or normalizing can be employed, each leading to different microstructural changes and resulting properties in the metal. This process is vital in manufacturing and engineering applications, allowing materials to be tailored for specific performance requirements.

In contrast, cold working involves deforming the metal at or near room temperature, which increases its strength but does not use heat to alter its microstructure. Welding involves joining two or more pieces of metal through melting, which also requires heat but is not primarily intended to change the overall properties of the base metal itself. Annealing, while it does involve heat, is considered a specific type of heat treatment aimed at softening the metal and relieving internal stresses, but it is not an encompassing term for all processes that utilize heat in this manner.