Which of the following actions increases the hardness of steel?

• A. Sintering
• B. Stretching
• C. Hammering
• D. Adding carbon

Answer: (D)

Hardness in steel rises as carbon content increases because carbon atoms disrupt the motion of dislocations in the iron lattice and promote the formation of harder microstructures, like cementite-containing phases, which resist deformation. This change in composition directly tightens the steel’s internal structure, making it harder and often stronger, though it reduces ductility if too much carbon is present.

Sintering is a manufacturing process that fuses powder particles with heat; it changes density and grain structure but doesn’t inherently make steel harder in the same direct way carbon does. Stretching and hammering involve deforming the metal; those actions can increase hardness locally through work hardening, but they’re not the straightforward method of increasing bulk hardness via composition and microstructure. Therefore, adding carbon is the most direct way to increase hardness among these options.