What microstructure can result from faster cooling in the heat-affected zone of steel?

• A. In the heat-affected zone, faster cooling can form martensite or brittle structures.
• B. Faster cooling always reduces hardness.
• C. Faster cooling has no effect on microstructure.
• D. Faster cooling only affects surface color.

Answer: (A)

Rapid cooling in the heat-affected zone pushes the steel's transformation path toward a diffusionless change of austenite into martensite. When cooling is fast, carbon atoms don’t have time to diffuse and form the equilibrium phases like pearlite or bainite, so the structure becomes martensite—a hard, very brittle phase. That brittleness and increased hardness are the hallmark outcomes of rapid quenching in the HAZ, which is why this option correctly describes what can form. The other statements miss the mark by implying no microstructural change, or by claiming hardness always drops or surface color is the only effect, which isn’t true.