Re-hardening annealed coper tube

[1Participant @1]

Restoring silver soldered copper tube to original state

[1Participant @1]

I have silver soldered some test pieces of 5/32 copper tube and found that after they had cooled down they were annealed and softened to the point of being tricky to handle. In fact by merely picking one piece up with pliers to drop it into a quench I had crushed it (did I just hear a cry of "clumsy so-and-so"  . Before I get round to soldering the actual parts I am making I would appreciate some guidance.

My vague memory is that copper age hardens. Is this true and if so over what timescale and is there a method of accelerating the process? If age hardening is just what is happening to my arteries is there a way of putting a bit of rigidity back into the tubes?

Jim

Edited By Im Indoors. on 07/06/2013 14:05:34

[Anonymous]

I think that age, or precipitation, hardening relies on an impurity in the metal coming out of solution over time and disrupting movement of the metal lattice. Therefore, by definition, a pure metal cannot age harden. Some copper alloys can be age hardened, but not, as far as I'm aware, pure copper.

Pure copper will, of course, work harden, but then you'd probably need to anneal it again due to the distortion caused by working it…………..

Andrew

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Jim ,

Quick answer is that there is no easy way of re-hardening copper tubes after annealing .

Copper will age harden eventually but with the nearly pure copper commonly used in model work this can take several months .

Copper can be hardened rapidly by work hardening it – ie stressing the metal and releasing again several times . Not usually any practical way of doing this with most model engineering components though it does seem to happen of its own accord in copper boilers ..

In practical terms use slightly thicker tube to get a bit more strength in the annealed condition .

For interest only :

There are ways of rehardening annealed copper which are used just occassionally in specialist industries . All these rely on work hardening .

(1) Using hydraulics – pipes are cyclically pressurised and de-pressurised in a controlled way .

(2) Using ultrasonics to generate thousands of small stress / release cycles ..

(3) Using shot blasting and machine peening/planishing .

Michael Williams .

[nigel jones 5Participant @nigeljones5]

Best to let it cool on its own – ive never lost a copper piece by crushing it but ive lost a fair few brass pieces this way. I leave mine out in the garden to harden – no scientific proof of this working but the warm/cool/wet/dry changes seem to speed up the hardening process which can be desirable before pressurising a new boiler.

[David JuppParticipant @davidjupp51506]

From my original training in metallurgy, and some quick checking of a few technical web sites, I'm not convinced that there will be any significant age hardening in copper as used in typical ME projects.

If age hardening did occur, it could be speeded up by heating to the correct temperature (to speed up diffusion) – heating too much would drive the precipitate forming elements back into solution and hence soften the metal.

There is age hardening in materails like Beryllium Copper, and Copper is used as an age hardening alloy addition in Aluminium and some other metals.

As commented by others, very significant work hardening does take place in copper.

Edited By David Jupp on 15/06/2013 11:29:18

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

I leave mine out in the garden to harden – no scientific proof of this working but the warm/cool/wet/dry changes seem to speed up the hardening process which can be desirable before pressurising a new boiler.

Very well observed Fizzy .

(1) The surface of the copper begins to acquire a coating of oxide and usually various compounds of the Verdigris family . Some of these compounds are harder than soft copper so the copper begins to become hardened on the outer surface .

(2) These surface compounds with help of atmospheric water , possible atmospheric acidity and low level thermal cycles migrate slowly from the surface layers into the body of the copper . These traces of compounds can then initiate precipitation hardening . Doesn't usually go very deep but has an effect something like anodising – a harder outside layer with a useful thickness is formed .

(3) Another type of hardening which can occur is relaxation hardening . The grains in newly annealed copper are relatively big and are all tumbled about in the metal structure . Low level thermal cycles cause these grains to very slowly move about on a microscopic scale and lock together in a stronger arrangement .

MikeW

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