Which element is associated with increasing hardness and toughness in steel?

• A. Nickel
• B. Zinc
• C. Copper
• D. Chromium

Answer: (D)

Hardness and toughness in steel are often improved by alloying elements that strengthen the iron matrix and form hard carbides. Chromium stands out because it both strengthens the lattice and forms hard chromium carbides inside the steel. Those carbides impede dislocation motion, increasing hardness and wear resistance, while the overall alloying also helps resist crack growth, enhancing toughness when the steel is tempered. Chromium also improves hardenability, allowing deeper sections to reach higher hardness without becoming too brittle, which is a key part of getting a tougher, more wear-resistant steel.

Nickel tends to boost toughness and ductility, especially at low temperatures, rather than primarily increasing hardness. Copper improves corrosion resistance with less effect on hardness, and zinc is mainly used for galvanizing rather than enhancing mechanical strength. So chromium is the element most closely associated with raising both hardness and toughness in steel.