heat treating copper (wire)

[Tmoose]

Is it possible to make fairly pure copper brittle or even hard by heating and cooling?

Seems like crimped connections are the modern standard in at least some MIL specs. Suggesting a reliability advantage at some level with crimping.

In any of the online crimp vs solder wire connecion debates someone (a crimp only advocate) will eventually mention something like this -
"The heat required to get a good solder joint will destroy the annealing in the copper and make the metal brittle"

I have experienced wire behaving badly at the solder/no solder transition but wrote it off to the sudden extreme restraint causing a big stress concentration during bending.

Vintage motorcycle lore is that to soften/anneal a copper gasket for re-use it should be heated and (pick one) quenched in water or allowed to air cool.

 

[EdStainless]

Pure Cu can only be hardened by cold work.
Any heating will relax stresses and reduce the strength and hardness.
Depending on how pure the Cu is and how much cold work it has seen it is possible to get full annealing at some very low temperatures.
There are published stress relaxation curves for Cu.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[redpicker]

Electrical copper wire is typically pure copper.
Pure copper is not hardenable by heat treatment. It will work harden dramatically, you can increase its strength (hardness) by something like 50% through work hardening.

Heating will anneal work-hardened copper. Annealing is accomplished by heating the copper to around 400F-1000F. Longer holding times for lower temperatures. Cooling rate from annealing temperature is not important, although rapid cooling (as with a water quench) can lead to distortion due to uneven thermal contraction.

There is a small amount of truth to the statement

"The heat required to get a good solder joint will destroy the annealing in the copper and make the metal brittle"

but it is surrounded by so much misinformation that it can be hard to see.

Yes, the soldering temperatures (typically 400-500F) are high enough to anneal (soften) the copper wire. At the low end of the range, you have to have long hold times to do much annealing, however; the short amount of time the copper is exposed to the eleveated tempareture is not going to soften the material much.

This heat does not destroy the annealing, it causes annealing. Annealing is another word for softening. The annealed wire will be softer than the drawn wire. Softer wire is easier to break, which could make some people consider it more brittle (people who are confused about what brittle means).

On the face of the statement, it is blatently false, since the heat can only cause annealing (not destroy it) and makets the material more ductile (not more brittle). But, if you take a laymans understanding, the heat can cause changes in the material's internal structure that will make it easier to break, which is probably the point of the statement. Even with that, however, the length of time you would have to hold the material at soldering temperatures is in the order of hours, not seconds, so it really isn't much to worry aobut.

rp

 

[btrueblood]

Copper wire for wiring is generally pretty pure, and no, it won't harden from any heat treatment, only soften.

 

[swall]

Sometimes these anecdotes, like "...heating makes the metal brittle" have a grain of truth to them. Torch heating, i.e. silver brazing, aka silver soldering, can lead to grain boundary penetration/embrittlement of copper alloys, mainly when there has been at least one re-heat (repair) of the joint. I would not expect to see this with tin-lead solder at lower tempertures. Silver brazing of copper for refrigeration requires oxygen free copper, otherwise cracking can occur. Hydrogen related, if I recall.

 

[NickE]

Ahh.. The Crimp vs Solder debate... As a metallurgist and live sound engineer I have often been interested in this debate.

In live sound a common and critical connection is the MASS connector, W4 MASS, this connector contains 176 pins/sockets in a very high density connector. It also sees a significant amount of abuse, both flexing and pulling.

When building these connectors it is the recommended practice to crimp and not solder the wire to the ends, pin or socket. This is because the solder flows up the stranded wire through capillary action. Once the fine strands of the wire are bonded together by the solder the wire becomes very stiff, and brittle. This is not a result initially of the metallurgical state of the junction, but the mechanical difference between a solid "bar" and one that is comprised of many fine fibers that can move in relation to each other. This stiff and brittle section of the wire is also at one of the highest stress points in the cable, and has been known to cause failure.

I know in soldered connections that after some time the kinetics of the Pb/Sn/Cu system allow the interdiffusion of the solder and the copper. I've often thought that this along with electro migration of atoms across the solder joint causes failure of rather old (>40yrs) audio signal carrying connections on the PCB. That may be another reason for the use of crimps instead of soldered connections.

Or they're just cold solder joints that finally gave way.

IF the spec calls for crimp there are likely several reasons why, not all are obvious from initial examination.



Nick

 

[IRstuff]

I've seen that wicking effect. But I think that the issue is that the wicked region subtracts from the service loop that is required to mitigate bending fatigue from handling and thermal expansion/contraction.

TTFN
faq731-376

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[NickE]

exactly what I was going for IRstuff. Thanks, didn't realize there was a name for the wire btw the connector and strain relief.



Nick