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Nonferrous topics such as copper and aluminum, annealing, etc. go here.

TOPIC: Storage of copper wire

Re: Storage of copper wire 2 years 7 months ago #1640

I am on a different computer and could not remember my password.

After reading the additional information I have a couple of thoughts/questions:
1) Are you trying to enamel annealed or soft wire? If so you are work hardening it. From your original post I was under the impression that you were telling us that after enamelling the wire, you put it in storage and after abotu 6 months it lost elongation.

2) Most enamelling systems have pre-annealers. I say pre-annealer because the annealing process is continued in the enamelling process. Do you have an annealer or are you depending upon the oven?

3) If the pre-annealer temperature is to high sometimes it will cause your finished wire to have less elongation than with a lower temperature. I know, doesn't make sense.

4) The best wire for enamelling has somewhere between 25% reduction and 80% reduction. This is typical with the inline wire drawing process. Soft wire or nearly soft wire is fed into the inline drawing machine, drawn to size and goes directly into the enamelling system. Typically it will pass through the pre-annealer which performs more of a cleaning operation than pre-annealing. Annealing takes place as the wire passes through the oven. What is unique to this process is that you have improved ductility, good elongation, and higher tensile strength. I’m not a metallurgist but there were some papers written on this in the 60’s and I think I quoted it in one I wrote. It has to do whit the alignment of the crystals and their being all alike. One theory is that the slower wire drawing speed allows the crystals to slide better and not fracture as much (I think that is the wording).
5) If you are checking your bare wire and it is OK when you draw it but not OK when you enamel it I think your problem lies in you payoff tensions, pre-annealer temperature setting, perhaps the diameter of the sheaves on the enamelling oven, even the bearing in the sheaves could be causing drag, resulting in a slight draw down of the diameter of the conductor.
6) Material properties may change over eons but not weeks or months in normal storage. You might also check the purity of the copper. In the USA/Canada the purity is really good and other places often do not have the same high degree of purity and that of course affect annealing while enamelling.

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Re: Storage of copper wire 2 years 7 months ago #1641

I think the cause is the oxidization of the bare wire. Try to keep your bare wire isolated from oxygen ,such as a room filled with nitrogen.
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Re: Storage of copper wire 2 years 7 months ago #1642

As a result of Mr. Tang's statement and knowing that oxidation on just drawn, bright clean copper is only about 3 Angstroms thick, I have contacted copper metallurgists through Copper Development Association to learn the following:

(1) What would be the anticipated thickness of copper oxide after 6 weeks of storage.

(2) If they ever heard of the situation as gan described it.

(3) What they think of Mr. Tang's suggestion that it is the result of oxidation.

I should get a reply early next week.


(a) I need to know from you what the diameter range of the copper conductors you have seen this phenomenon on. (No longer required, see page 2)

(b) Please remember that filling a storage room with nitrogen is life threatening to any employee going into that room. Nitrogen is clear, colorless, without odor and makes up 78% (volume) of our atmosphere. That makes pure nitrogen in an enclosed space extremely dangerous.

As an experiment, it would be just as easy to heat seal a few spools of copper in heavy polyethylene bags filled with nitrogen.

Peter J. Stewart-Hay
Stewart-Hay Associates
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Re: Storage of copper wire 2 years 7 months ago #1643

Hi Richard,

Your item #3 above makes sense to me. What it means to me is that the wire was soft enough that it was permanently deformed (elongated) slightly so when you did the elongation test, some of the elongation value had already been lost prior to the test.

gan, I think we have found your problem. Go to Page 2 for the details and the experiment you need to do to prove it.

Best personal regards,

Peter J. Stewart-Hay
Stewart-Hay Associates

PS to Richard - You sure caught me off guard in the thread "Process Research". I had no idea that was your comment when I answered it. After reading your response, I went back there and checked the IP number and then I knew it was you.
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Re: Storage of copper wire 2 years 7 months ago #1644

Here is the reply I received from a metallurgist at the Copper Development Association this morning.

"The initial reaction step involves the adsorption of a mono-layer of oxygen on a clean metallic surface. Electrons then pass from the outermost layer of the metal atoms. Film growth takes place through the diffusion of metal atoms outward through the film to the surface, where they react with the oxygen.

From the kinetic standpoint, the oxide film as a result of progressive oxygen adsorption, thickens very slowly to a low stable value.

I would suspect the loss in ductility is a result of hydrogen embrittlement. Oxygen free copper will oxidize the same as an oxygen bearing copper but will be immune to hydrogen embrittlement. Try annealing some oxygen free copper with the oxygen bearing copper and see if the elongations are affected. If the oxygen bearing copper is affected and the oxygen free copper isn't affected, you may conclude that the formation of the Cu2O has nothing to do with the low elongation."

gan, there is your experiment please let us know how you make out.

Mr. Tang, you may want to perform this experiment in your plant as well.

Peter J. Stewart-Hay
Stewart-Hay Associates
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Re: Storage of copper wire 2 years 7 months ago #1645

Hydrogen Embrittlement

Electrolytic tough pitch copper contains oxygen in the form of cuprous oxide (copper oxide or CU2O), a eutectic mixture. (A mixture of two or more elements which has a lower melting point than either of its constituents.) For more on the oxides of copper, see thread:

If electrolytic tough pitch copper is heated under reducing conditions with hydrogen as the reducing atmosphere, the following chemical reaction will take place:

CU2O + H2 = 2CU + H2O (Gaseous)

The steam (water vapor) is dissolved in the copper and the pressure at the grain boundaries forces the copper apart causing separations. Brittle delayed failure of normally ductile materials when hydrogen is present within is called internal hydrogen embrittlement. (This delayed phenomenon is certainly a new one for me.)

Oxygen free copper is not susceptible to hydrogen embrittlement in a hydrogen atmosphere unless it has been first heated in an oxidizing atmosphere.

Gan, are you or your copper rod supplier batch annealing with hydrogen as the reducing atmosphere?

Peter J. Stewart-Hay
Stewart-Hay Associates
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