Metallurgy of Copper

While tidying the workshop recently, I surfaced a coil of 10 mm OD copper tube.

I bought this coil about fifteen years ago thinking it might come in handy (it hasn't!! ).

Here's the thing: I'm sure that when I bought this coil of copper tube it was soft tempered. It certainly isn't soft now!

Does copper harden with age? It hasn't been worked, merely picked up from one place and put down in another.

Can it be annealed without needing such a high temperature that it will form surface scale inside and out? I'm thinking here of the domestic oven.

Best regards,

Swarf, Mostly!

Red heat, so no, I would think.

Maybe this is your imagination - or forgetfulness? They made it much thicker back then.

Edited By not done it yet on 01/06/2022 16:32:48

.

Try searching for ‘hardening’ here: **LINK**

https://www.copper.org/search.html?q=&submit=Search

It’s my go-to resource for all things Copper

MichaelG.

If the problem you need to avoid is scale inside the tube, you might make up a bung for each end, and fill the tube with eg nitrogen, or carbon dioxide. Remembering to leave one end not perfectly sealed, to allow for expansion. Then use a blowlamp (etc). Or even try adding eg tissue paper or newspaper in the tube, and this will burn and use up the oxygen quickly.

Scale on the outside can be rubbed off with a brass wire brush, or a kitchen scouring pad (also used to matt a gloss surface).

And yes, old tube does seem to harden on its own - but my experience is with vibrating old engines so not quite 'storage'.

Hope this helps

Cheers, tim

Age hardening of copper is normal. in oder machinery all connecting copper tubing was made with a coil in it.

I seem to remember that in service or o/haul copper tubing was annealed to soften it again.

Clive that's hardening by movement/vibration.

It still causes embrittlement cured by annealing, i have seen fuel tubes on static geneartors crack at the nipples and as you say caused by vibration. Cured by fitting a flexible connection.

Over the last two weeks I've been replacing the vented hot water cylinder with a larger, unvented, one. yes, I'm a slow worker!

I've been re-using a lot of the old copper pipe that I installed in 1988, along with a length that I've had in the workshop for quite a while. The old pipe is very definitely harder than new. I'm toying with the idea of annealing those ends which are to have compression fittings on them, prior to tightening the nuts and olives on to them,

It does seem the age hardening is actually work -hardening by years of small but repeated alternating strains from vibration or heating and cooling

I wouldn't, it makes it very easy for the ends to go out of round during install. if the tube is too soft wouldn't it allow the copper to move away from the olive so you get a poor seal?

Heat/anneal the olives and not the pipe? A damned site easier and simpler. KISS Principle in operation.🙂

The olives are supposed to cut into the pipe!

I've ended up just using some Molyslip, my big spanners and giving the nuts a bit of welly. half a dozen compression joints, plus a number of new and re-joined end-feed and Yorkshire fittings are hopefully undergoing a successful leak test at the moment.

I cannot agree that the olive needs to cut into the pipe. Once the copper pipe starts you yield under the force from the olive, you will never achieve a leak-free joint, however much you tighten it.

I certainly agree that the olive needs some lubrication. I alway use a smear of Boss White or similar. I know plumbers who swear by silicone sealant but that is not my preference.

Cut the olive off a pipe, you will see that it has cut into the pipe. Failing that, try to remove the olive without utting it off, You cannot.

There is a reason that Ermeto and Swagelok fittings used with stainless pipe require the pipe to be saw cut and not roller cut. It's upsetting to have 2000 psi steam turning a gauge or transducer pipe into a snake because some twit didn't ensure that the olive could bite into the tube. It's even more upsetting having to explain to the Station Manager why you tripped his turbine...

In hydraulics an olive is called a cutting ring - is that a clue to what it does ? Noel

I think the olive causes a uniform compression of the pipe under the circumference of the olive, the term “bite into the pipe” is probably a little misleading and describes a brutal action. Because the copper is malleable then it is easily compressed by the olive, because the pipe can only distort inwards which ensures that it will seal against the olive once the olive has been tightened by the coupling nut. Dave W

I am surprised that olives are permitted at pressures even approaching 2000psi.

Automotive brake pipes have formed metal-metal joints. I would not be using an olive. Diesel injection systemd, likewise, do not use olives. Your gas cylinders seal perfectly well with metal-metal contact.

The requirement is for the two surfaces to be in contact such that there are no leaks between the two and that longitudinal movement is not going to happen under any circumstance.

I plumbed in a 5000psi air system in my old firm using tungham tube. This looks like brass, but stronger. The joints were made mostly using a setting tool in a vise to set the olives which had a small sharp edged step in the bore which cut into the tungham. This allowed better repeatability of joint strength than forming in situ. I seem to remember that the fail pressure for that size, about 9/16" od was about 20000psi. The pipe fittings and olives were made of steel.

Edited By old mart on 03/06/2022 13:50:10

We used tungham pipe as well as stainless steel on aircraft hydraulic systems, as far as I can remember our pipes all used flared couplings, don’t remember making any hydraulic pipes with olives, our system pressures were mainly 4,000 psi. Dave W

I've actually tested Ermeto fittings on 3/8" steel pipe up to 10,000 psi, which is well over what was found in power stations, but the highest we could go with our gear. To be fair, they are designed for that pressure and almost fool proof for certain values of fool. One of their major benefits was that they had a positive stop to the compression, unlike central heating fittings, which can be over tightened.

Curiously, on one job in South Korea, the customer wanted to use ball valves for instrumentation isolation. We were extremely dubious about their suitability. Out came the Dead weight tester and I couldn't get one to leak or split even at 3,000 psi, which was 50% more than they could see on that unit.

I think part of the reason for automotive brake pipes to have flared pipe joints is all to do with cheap. Using something such as the Ermeto or Swagelok fittings would cost a company like Nissan about £30 million per year just for tittings for brake systems in their UK production.

Old Mart's fittings do sound very much like the Ermeto design.

A good reason for using flared fittings on aircraft would be the ever present desire to keep weight to a minimum.

I apologise for dragging the thread way off topic. I'll shut up now!

Edited By Mark Rand on 03/06/2022 14:33:04