Fig. 1. Typical coils of copper tubing

Question:
We have some bent coils of 1/8-inch (3.2 mm) diameter copper tubing. The curved portions are "work hardened" by the bending process. If this material were left in a shed where the ambient temperature reaches approximately 100ºF (38ºC), would the tubing anneal over period of time?

Answer:
As you probably already know, annealing is a process that softens and improves the ductility (and/or toughness) of copper and copper alloys. The process involves heating, holding (soaking) and cooling. Annealing is primarily a function of metal temperature and time at temperature.

Cold-worked metal is annealed by heating to a temperature that produces recrystallization (rearrangement of the "stressed" crystal structure). If maximum softening is desired, it is heated well above the recrystallization temperature to cause grain growth.

Generally, annealing is categorized as 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 onset of rapid grain growth.

Copper tubing (Fig. 1) is typically "annealed" in the temperature range of 700-1200ºF (370-650ºC). Method of heating, furnace design, furnace atmosphere and shape of workpiece are important because they affect uniformity of results, finish and cost.

Hopefully, this information will provide you with an explanation as to why the material will not anneal at 100ºF (38ºC) irrespective of how long the material is stored.