Annealing
Softening or annealing of cold worked metal is accomplished by heating to a temperature that causes recrystallization and, if maximum softening is desired, by heating well above the recrystallization temperature to cause grain growth. Method of heating, furnace design, furnace atmosphere, and shape of work piece are important, because they affect uniformity of results, finish, and cost of annealing.
For copper and brass mill alloys, grain size is the standard means of evaluating a recrystallizing anneal. Because many interreacting variables influence the annealing process, it is difficult to predict a specific combination of time and temperature that will always produce a given grain size in a given metal.
Several copper alloys have been developed in which the grain size is stabilized by the presence of a finely distributed second phase. Examples include copper-iron alloys such as C19200, C19400 and C19500, and aluminum-containing brasses and bronzes such as C61500, C63800, C68800 and C69000. These alloys will maintain an extremely fine grain size at temperatures well beyond their recrystallization temperature, up to the temperature where the second phase finally dissolves or coarsens, which allows grain growth to proceed.
Generally, two annealed tempers are available: light anneal, which is performed at a temperature slightly above the recrystallization temperature, and soft anneal, which is performed several hundred degrees higher, at a temperature just below the point at which rapid grain growth begins.
When annealing copper that contains oxygen, the hydrogen in the atmosphere must be kept to a minimum to avoid embrittlement. For temperatures lower than about 480oC, hydrogen preferably should not exceed 1%.
