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Why does rolling or hammering a metal make it harder?
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crystal
A crystal form of elemental sulfur. Sulfur has an atomic weight of about 32.06 atomic mass units (amu), which means that its molar mass is 32.06 grams.
Rolling or hammering a metal makes it harder through a process known as work hardening. While metals are naturally plastic due to the movement of dislocations—defects in the atomic arrangement—physical deformation introduces more of these defects into the crystalline matrix.
As you hammer or roll the metal, these dislocations multiply and begin to crisscross each other. Because of the complexity of the crystal structure, these intersecting dislocation lines act as obstacles that restrict further slippage of the crystals. Essentially, the more you deform the metal, the more these internal barriers impede the movement of atoms, thereby increasing the material's hardness and strength.12