Annealing Aluminum Conductors

When annealing a reel compact or compressed aluminum conductor in a furnace what is going on? The conductor goes in hard, then does it go soft at a certain temp. then start to harden again or does it slowly soften as it soaks in the heat??

Just looking for the basic understanding of the process

Hi Dennis,

Generally speaking, EC grade 1350 aluminum such as that normally used in ACSR is not annealed after drawing.

I wonder if you are talking about aluminum alloys instead of EC grade aluminum. Please clarify this for us. Thank you.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641-3212

8000 series

The 8000 Series of Aluminum Alloys
The 80000 series of aluminum alloys are the aluminum - lithium alloys. Moreover we believe the aluminum building wire in use in the United States today and the only one approved by the NEC, is the 8000 Series of aluminum alloys. Even though the cable is approved or sizes 12 AWG and larger, we understand no manufacturer produces sizes smaller than #8 (for underground service entrance cable or SEU cable) and #6 AWG for single conductor.

Work Hardening
Adjoining crystals called grains (Not individual atoms.) within alloy structures can “slip” against each other along “slip planes” as the material is loaded or cold worked. An alloy in which slippage of the crystals has not yet occurred is found to be soft. As that soft metal is cold worked (hammered, rolled or stretched (drawn), all at room temperature) then adjoining crystals will move along a slip plane. Each slip plane has a limit to the amount of movement available and so further working will result in successive slippage along other planes. As more work is applied to the metal, the resistance to movement increases greatly and the material is said to “work harden”. Over-working will eventually cause the metal to fracture.

Annealing
a)The original workability of the alloy can be restored by heating the material to a point where a new crystal structure forms, complete with a new set of unworked slip planes. The critical temperature at which this occurs in an alloy is called the recrystallization point and the process is called annealing.

b) Another critical factor in achieving correct annealing is the soak time. This is the minimum time over which the alloy must be held at the designated temperature for full recrystallization to occur. The required soak time is dependent on alloy type and material thickness. Thus as you asked, the alloy takes some period of time to regain its original properties.

Annealing is usually carried out in an air furnace and can be used to remove the effect of cold working non-heat treatable alloys or to remove the effects of heat treatment and cold working from heat treatable alloys. Following the annealing operation, aluminum alloys are designated as being at condition “0”,

c) The air circulation inside the furnace is also critical to ensure that all points within the furnace are at the same temperature so that all the aluminum alloy receives the correct annealing temperature and soak time.

If the annealing temperature is too low or if the soak time is too short to achieve complete recrystallization the most serious issue will be a partial anneal and thus the annealing process will have failed. This means that the alloy will still have some degree of work hardening and thus exhibit a higher than fully annealed strength.

d) Finally, a controlled cooling cycle time is also critically important for heat treatable aluminum alloys once the the soak time is complete. This is because a slow cooling rate allows the small percentage of soluble elements that have found their way into solution the opportunity to once more precipitate out at the grain boundaries. This gives the best possible anneal. Cooling rates for the heat treatable aluminum alloys are typically in the range of 40 to 50 degrees F per hour.

I do hope that this improves your understanding of annealing aluminum alloys.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641- 3212

Thank you!