Storage of copper wire

Peter,
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.

Richard

Hello again Gan,

Since I am confident you are not breaking the laws of physics, you must look for a situation where you are either stretching the conductor or work hardening the conductor. This can be intermittent. Copper can also be work hardened by many reverse bends on small diameter sheaves.

I believe you may have drawn a wrong conclusion as to the reason for a drastic reduction in conductor elongation by short term storage. You need to carefully re-evaluate how you came to this conclusion.

Sincerely,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

Hello everyone,

Thank you all for your comments. I would like to give a few more information based on the questions posted by Peter and spectre07

To Peter:

We are following the rule of annealing. We anneal the wire at intermediate machines for all sizes. We also check the elongation of each spools that comes out of the intermediate machines. For the wire that I have mentioned, we found that the intermediate wire has good elongation. Only after the enameling process then the elongation drops.

When we encountered the problem, we switch to another bare copper wire which has a shorter storage time, and the problem was gone. We then put the problemetic wire onto another machine, the problem is still there.

We use electrical annealer pulley at the drawing machines and oven annealing at the enameling machines. At the drawing stage, everything is good (resistance and elongation of the bare wire). But after storing the wire > 6 weeks, the finished product(enamel wire) has lower elongation.

To spectre07:
The sample of the wire is taken after removing the few layers at the end of the production spool. This means that it is not the end if the full spool , but could represent the entire spool's condition.

So, i don't know whether you guys out there have anymore ideas? Thanks.

Hi Richard,

You brought something to mind that I had long forgotten.

I once saw a fascinating problem where the elongation of the conductor was significantly reduced because the timing on the electric annealer pulleys was incorrectly set, slightly. Thus the wire was being stretched but not broken during the annealing process itself. It did however give a superficial indication that the wire was incorrectly annealed but that of course was not the case at all. We also knew we had a problem with elongation throughout the entire length of wire on the reel.

Moreover I have seen a few cases where the annealer electric power was somewhat re-routed randomly because of a fairly high and intermittent resistance to ground say through one of the annealer contact pulley water seals on an annealer driven shaft.

As far as hot wire on a reel goes, I have seen this after the fact and the outer layers are very soft and spongy. Often the wire has to be actually cut off the reel with a chisel because paying it off by any means is virtually impossible.

I think the writer's problem has more to do with a faulty solid state voltage controller for the annealer power.


Best personal regards,

Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

Peter,
I've certainly never heard of such a thing. I know that some enamelled wire loses it electrical insulation properties over time, you know the old thermoset vs thermoplastic thing where it continues to cure and eventually becomes overcured.

Obviously this has no effect on the conductor.

Do you think that they could have such a high winding tension that the wire is near it yield so as it cools (and shrinks) the tension increases?


It would be nice to know where the sample is being taken. Typically you set up a machine, pull samples and if they pass, put it into production. After that each full spool is pulled but only the end is checked and if you do a complete test only about 300-400 feet are used. This being true you can easily see that when checking the end of a 20-40 pound spool of 30 awg not much gets checked. If his later samples are taken from inside a spool that would possibly lay some credibility to the tension question.

I have never heard of such a thing in copper wire.

Was this copper wire intermediate annealed before final annealing? Before each annealing, the cross section area should not be reduced by more than 90%. Are you following this rule?

Is it reliably annealed along its full length?

I do not see how the conductor metallurgy (grain size) could result in a reduction of elongation during short term storage. After all, annealing, which is grain growth, is a much, much longer time-temperature phenomenon and this would increase the elongation.

Sincerely,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com