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Why copper gaskets are annealed

The essential qualities of a copper gasket are softness and toughness (provision for cold 
working without fracture). In its place the copper gasket is subjected to considerable 
mechanical stress, resulting from elastic strains internally balanced. These elastic strains are  due to jamming of dislocations, which occurred during cold deformation (during service).
If  this cold worked gasket is heated to a sufficiently high temperature the total energy available  to the distorted regions will make possible the movement of atoms into positions of  equilibrium so that the elastic strains diminish and the locked-up energy associated with  them ‘escapes’. Tensile strength and hardness will fall to approximately their original values  and capacity for cold work will have returned.

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