During heat treatment, what happens to iron's crystal structure?

• A. It changes electrical conductivity with temperature.
• B. The crystal structure transforms between forms during heating and cooling - this is what allows steel to be heat treated.
• C. Its carbon content controls its microstructure.
• D. It becomes more brittle when heated.

Answer: (B)

Iron’s crystal structure changes with temperature, and heat treatment relies on those phase changes to tailor properties. At different temperatures, iron shifts between crystal forms, such as a softer body-centered arrangement at room temperature and a denser, more open face-centered arrangement when heated (austenite). In steel, the carbon present interacts with these transformations, so heating and then cooling drive how carbon is distributed and which phases form, which directly affects hardness, strength, and toughness. This is why the essence of heat treatment is that the crystal structure transforms between forms during heating and cooling, enabling control over the material’s properties. The other ideas miss the central mechanism: electrical conductivity isn’t the lever here, carbon content influences microstructure but doesn’t describe the crystal-structure changes themselves, and simply heating doesn’t inherently make steel more brittle.