Which of the following occurs when a mechanical force is repeatedly applied to most metals at room temperature, such as rolling, hammering, or bending?

• A. The metals become artificially aged
• B. The metals become stress corrosion cracked
• C. The metals become cold worked, strain or work hardened
• D. The metals become annealed

Answer: (D)

Repeated mechanical deformation at room temperature causes cold working, also known as strain hardening or work hardening. When you roll, hammer, or bend a metal, its crystal lattice develops many new dislocations. These dislocations multiply and become entangled, which makes the movement of later dislocations harder. The metal therefore becomes harder and stronger, but less ductile and more prone to internal stresses. Annealing would soften and relieve those stresses, which is the opposite of what happens with cold working. The other options describe different phenomena (artificial aging is a heat-treated precipitation phenomenon, and stress corrosion cracking is a failure mechanism due to environment), so they don’t fit this situation.