What is a common outcome of the heat treatment process?

Heat treatment is a controlled process used to modify the physical and sometimes chemical properties of a material, particularly metals. One of the most significant outcomes of heat treatment is the alteration of mechanical properties. This process can result in changes such as increased hardness, improved tensile strength, and altered ductility depending on the specific heat treatment techniques applied, such as annealing, quenching, or tempering.

For instance, quenching can harden a steel by transforming the structure from austenite to martensite, which significantly increases its hardness and strength. On the other hand, annealing can relieve internal stresses and improve ductility by allowing the microstructure to stabilize.

The other options do not accurately represent common outcomes of heat treatment. While certain heat treatments may increase costs and can lead to changes in thermal expansion for specific materials, these are not as universally applicable as the concrete alteration of mechanical properties. Similarly, heat treatment typically does not affect the weight of the material meaningfully. Thus, the transformation of mechanical properties is the most relevant and widely recognized outcome of the heat treatment process.