Which statement about heat treatment is true?

• A. To maximize corrosion resistance
• B. To produce different levels of hardness and tensile strength by controlling grain size
• C. To reduce density
• D. To avoid any change in mechanical properties

Answer: (B)

Heat treatment changes a metal’s microstructure by heating and cooling in carefully chosen ways, which alters grain size and the distribution of phases. This directly affects hardness and tensile strength because dislocation movement is impeded by grain boundaries and by transformed phases. By selecting the right heat-treatment process and controlling grain size, you can tailor how hard the material is and how strong it feels in tension. Smaller grains raise hardness and strength, while larger grains lower them but can improve ductility, and various treatments like annealing, normalizing, quenching, and tempering achieve these effects.

Other statements don’t fit because heat treatment isn’t primarily used to maximize corrosion resistance, and density is essentially fixed for a given material (it doesn’t change meaningfully with heat treatment). Also, heat treatment inherently changes mechanical properties, so avoiding any change is not accurate.