Which statement about carbon in steel is correct?

• A. Increases hardenability and strength
• B. Increases ductility
• C. Decreases strength
• D. Decreases hardness

Answer: (A)

Carbon content tunes the balance of steel’s properties. Adding carbon strengthens steel because it contributes to solid-solution strengthening and enables the formation of harder microstructures, such as martensite, after heat treatment. Higher carbon content makes the steel harder and, after quenching, increases the depth and degree of hardness achievable—this is what we mean by greater hardenability. At the same time, increasing carbon raises hardness and strength but reduces ductility, so there’s a trade-off with brittleness and weldability. That combination—more hardenability and higher strength with less ductility—is why this statement is correct.