The hardness of steel can be increased by all of these methods except

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

Answer: (B)

Hardness measures how resistant a material is to localized plastic deformation, which is most effectively increased by changing the microstructure or density of the steel. Adding carbon creates harder carbide phases and overall a harder structure; hammering (cold work) increases dislocation density and strengthens the metal via work hardening; sintering densifies powdered material, producing a tougher, more compact structure that often correlates with higher hardness when followed by appropriate heat treatment. Stretching, by itself, mainly changes shape and dimensions and does not reliably produce the microstructural changes that raise hardness. Unless paired with additional steps that induce work hardening or phase transformations, stretching does not increase hardness, making it the exception.