Work Hardening from: http://www.tpub.com/content/doe/h1017v2/css/h1017v2_88.htm

• Work hardneing is the appearent hardening of metal due to plastic deformation, changing the shape or size of an object due to applied force.
• makes metal more resistant to change in form
• makes metal more brittle
• requires more stress to deform as metal hardens
• smaller grain size makes for harder metal
• larger grain size makes for more malleable metal