[Shop-talk] Annealing Copper bars

[Tom Coradeschi]

It’s been decades since I’ve dealt with copper busbar, but even the half hard stuff you have (which presumably has been bent, so it’s work hardened slightly) is still pretty darn soft in absolute terms - esp when compared to ferrous materials.

Annealing copper to the temperatures folks have noted will soften it. You can cool it slowly or quickly, it really won’t make any difference (unless you’re impatient, in which case cooling it quickly is always better).

Back to the OP’s note, I would suggest that what you really want in such a situation is the hardest material you can get your hands on. I’m sure that sounds counterintuitive, but follow my train of thought for a bit. What’s I presume you’re looking to do - quite rightly - is to minimize the electrical resistance across the bolted connection. 

The interesting thing about electrical connections like those, is that a large surface area is only of value if it’s properly mechanically preloaded. Even machined surfaces, which appear flat (macroscopically), are microscopically rough and probably have an oxide layer on them to boot. Holm’s work on defining a-spots as the mechanism for electron flow through contacts has been expanded on by others in the field. Most of the math goes over my head these days, but generally speaking, surface prep and preload are key.

Assuming that you’re using steel fasteners and washers, you’ve got a mechanical connection which is about twice as stiff as the conductors, so the number of fasteners is actually going to dominate the situation. In addition, if you back up the fasteners with steel (either plates or washers) to distribute the compressive load, you will be doing a lot to help yourself.  

We used to, routinely, run megampere (pulsed) currents through bolted copper and aluminum busbar connections. Surface prep was important (particularly for aluminum, which forms an oxide layer in nanoseconds on exposure to oxygen). In that regard, Electric Joint Compound (EJC) is critical for aluminum, and quite helpful for copper connections. One lab was all aluminum busbar, and had literally hundreds of bolted connections (it was a fairly sophisticated set of pulse forming modules). Much of this busbar had been through bending processes and we did nothing to it in terms of material properties. What we did do was pay careful attention to the cleaning, EJC application and bolt preload on assembly. We also measured the resistance across each connection and documented it - spot checking it was a semi-annual maintenance check.

Not sure any of the above really adds any true value, but it was fun to pull up some memories of my time as a Lab Rat, early in my career, so...

—
Tom Coradeschi
tjcora at icloud.com


> On 08 Aug 2020, at 10:21 AM, Pat Horne <patintexas at icloud.com> wrote:
> 
> I thought quenching made the metal less soft.
> 
> Annealing requires slow cooling. 
> 
> Peace,
> Pat
> 
> Pat Horne 
> We support Habitat for Humanity
> 
> 
> On Aug 8, 2020, at 9:18 AM, Bob Spidell <bspidell at comcast.net> wrote:
> 
>  Question for the List: Is it proper to quench--in water, presumably, or oil maybe--copper to achieve softness after it's been heated 'cherry red?'
> 
> Bob
> 
> On 8/8/2020 6:43 AM, old dirtbeard wrote:
>> I sort of like your idea about the self-cleaning oven cycle. It would be very even heat, the right temperature, should not hurt the oven. 
>> 
>> Maybe just wait to do it while the wife is away for an hour or so...  :-)
>> 
>> best,
>> 
>> doug
>> 
>> On Sat, Aug 8, 2020 at 5:37 AM Pat Horne <patintexas at icloud.com> wrote:
>> Pay a local shop with an oxy/acetylene rig to heat them? Muffler shop, body shop, A/C contractor? A/C contractor sometimes use air/acetylene. Will that get hot enough?
>> 
>> Peace,
>> Pat
>> 
>> Pat Horne 
>> We support Habitat for Humanity
>> 
>> 
>> On Aug 7, 2020, at 11:25 PM, Jack Brooks <JIBrooks at live.com> wrote:
>> 
>> 
>> I have fabricated a number of 1/8 and 3/16th inch thick busbars for an Van RV build electrical system which I am assembling in my shop.  The busbars are already bent to shape, but I’d like to anneal them, because the copper was purchased in the  “Half-Hard” state and making them “Dead-Soft” will insure that they lay completely flat to maximize the conductivity when I final assemble the system.
>> 
>>  
>> I usually anneal copper by getting it hot enough to glow with a propane torch and then allow it to cool.  With the mass of these busbars, I can’t get them up to a temperature to where they glow with propane.  An Oxy/Acetylene torch would work, but I don’t have one.  MAPP gas is hotter, but I don’t think it’s a lot hotter.
>> 
>>  
>> I am considering running them through the cleaning cycle in the self-cleaning over, as a self-cleaning oven will typically runs up to around 900°F.  Copper needs 700-1,200°F to anneal, so it should be fine.
>> 
>>  
>> I’d prefer almost any other solution.  Thoughts?  The last time I used our oven for a shop project, it was to cure the paint on the jugs (cylinders) of my ’74 Norton motorcycle.  Mrs. Jack was not happy.  Copper bars should be fine as they won’t stink up the house.
>> 
>>  
>> Thanks in advance,
>> 
>>  
>> Jack
>> 
>> 
> 
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