Annealing Copper without oxidation - what is needed?

[irpheus]

Hi all,

I have an annealing chamber question that I hope one of you may be able to help with ...

As it is I will soon be in need of dead-soft annealing copper wires and to this end have built a small annealing chamber from Stainless Steel (SS) surrounded by a ceramic insulation.

Before starting the annealing process I pour Argon into the annealing chamber, place a SS plate on top of the chamber, and then start heating the argon from inside the chamber with a scrapped kitchen stove heater plate. This works fine in terms of being able to reach sufficient temperatures (~725 degr. C) ... BUT ... the copper plate is heavily oxidized when cooled down.

I reckon the reason why this oxidation happens is that the argon when heated "flows turbulently" inside the annealing chamber, and then, since the chamber is not gas tight on the transition between the bottom chamber box & the top SS plate, the argon slowly pours out into the surrounding air and - when cooling takes place - atmospheric air enters the chamber and here causes the oxidation (my guess ).

However, I now have on hand a vacuum pump capable of reaching a vacuum of 0.002 mmHg ... so I wonder if this would be sufficient to ensure a non-oxidizing annealing - in case the annealing chamber can be kept gas-tight? And, if not, what could be a "kitchen table" non-oxidizing annealing chamber solution? The annealing chamber needs not be that large - 20*20*10 cm suffices - but it should be able to reach temperatures that will also allow annealing of other metals as well (silver & gold mainly).

I would appreciate your feedback

Cheers,

Jesper

 

[WKTaylor]

irpheus...

Thermal treatment of metal sounds simple... but never is. The set-up/processes You describe may/may-not be capable of doing what You are asking.

Something as simple as 'validated' thermal measurement and control requires relative sophistication... especially within the 3D cavity of the oven: soooo.. is the temperature relatively even through-out and is the temperature measurement calibrated?

Likewise for atmospheric controls...

BTW a vacuum chamber at 0.002 mmHg will generate substantial 'crushing forces' on the chamber, which is a very serious added design element.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[WKTaylor]

BTW... 'copper wires' is a pretty non-definitive description of the copper.

Copper alloy wire...
or brass/bronze copper alloy wire...
or electrical grade copper,
etc...

WHAT SPEC 'copper wire'...

BTW... MIL-HDBK-698 Copper and Copper Alloys has a short section on copper heat treatment.

Also, there are ASM handbooks dedicated to all-aspects of coper, thus...
ASM Specialty Handbook: Copper and Copper Alloys ISBN: 978-0-87170-726-0

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Compositepro]

You need to use a continuous purge of argon during the anneal and cooling to prevent O2 from entering your chamber. The flow does not need to be large. If you do go with high vacuum then any leakage at all will result in O2 in your chamber. Bear in mind that hot gas is low density, so you must not have any unsealed joints at the top and bottom. That would result in air entering at the bottom and flowing out the top, which requires much greater purge flow to stop, compared to just a pinhole type leak. You could let the exhaust flow from your chamber bubble through a pan of water to ensure that you always have positive pressure in your chamber so air cannot leak in. Make sure that under no circumstances can water be sucked back into you chamber.

 

[irpheus]

Hi both - & many thanks for your feedbacks! ... What you write is indeed food for thought ... From reading your feedbacks it does, however, seem that vacuum annealing is not the simplest way to go - at least not if I want to do this in a "kitchen table" way.

Will ponder ... please feel welcome to comment further if you come to think of something

Cheers & thanks,

Jesper

 

[dgallup]

Can you not just purchase annealed copper wire?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[WKTaylor]

dg' has a good point...

There are standards for annealed 'pure copper electrical-grade wire'... unless You want annealed copper-alloy wire... which would beg the question 'for what'?

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[3DDave]

I would expect that sealing the box fairly well except for a hole on the top would work, preferably a tube about as tall as the box depth. Then, as the argon heats and expands it will generally leave by the easiest path. The density should keep cooler air from entering and will need a makeup flow until expansion from heating exceeds the loss; as part of cool-down, increase the bleed of argon into that tube; the denser, cooler argon will displace any lower density hot oxygen or nitrogen that might leak in while also making up for the volume of argon that exited due to the original temperature increase. I'd invest in a flow meter as well to allow a reproducible condition if that's important.

 

[irpheus]

Hi - good morning ...

@dgallup & WKTaylor:

... Yes, it is a good point because, yes, normally I would be able to do so. But I would like to anneal other things as well and the wire sizes/shapes I would like to anneal are not readily available already annealed.

@3DDave: Thank you also for your comments ... Considering your feedbacks I think I will stay with the annealing chamber I already have made and then see if I can make an Argon inlet/"output" arrangement that will make sure that the chamber is always filled with argon.

So ... more pondering ;-)

Have a good day,

Jesper

 

[berkshire]

One of the easiest ways to monitor your argon flow, is to use a pellet type flowmeter of the type used for Tig welding machines.
B.E.

You are judged not by what you know, but by what you can do.

 

[irpheus]

Hi B.E. .. thanks for the suggestion ... Would it be something like this?

https://www.ebay.com/itm/CO2-Mig-Ti...787&pg=2047675&_trksid=p2047675.c100005.m1851

Cheers,

Jesper

 

[berkshire]

irpheus ,
Yes that is the animal, with that you can regulate the flow of argon into your oven. Remember the object of the exercise is to keep the oxygen out, so sealing as much as you can and keeping a slight positive pressure in the oven is the order of the day.
B.E.

You are judged not by what you know, but by what you can do.

 

[irpheus]

Thanks B.E. ... will look into it ...

Cheers,

Jesper

 

[irpheus]

Hi again ... Hmmmm - as you seem to be quite knowledgeable about annealing & vacuum I wonder if you may also know of a sealant that will withstand up to 800 degr. C - and maybe, just maybe will also work in a vacuum chamber context? I've found 3M 1137 but it seems quite costly so if you might know of an easily accessible and not too costly alternative I would appreciate a hint about this ... Preferably available in Europe

Cheers, Jesper

 

[Compositepro]

Vacuum seals on furnaces are placed outside of the insulation so that they are not exposed to high heat. If the seal must work at high temperature, copper gaskets are usually employed. These are only for one time use and must be replaced every time the joint is opened.

 

[berkshire]

irpheus,
I have no knowledge of vacuum annealing ovens, only Hydrogen and Argon/helium inert gas muffle furnace/annealing ovens. For the most part the ones I have been involved with do not use a sealing material, they just use a close mechanical seal, and rely on the flow of gas to keep oxygen and nitrogen out of the furnace atmosphere.
B.E.

You are judged not by what you know, but by what you can do.

 

[EdStainless]

I have done some small part annealing in an open furnace by putting the parts into a bag made from stainless steel foil and then purging the bag with Ar.
Or if you really need bright surfaces you can use Ar+10% hydrogen as a purge gas.
This way you can make a bag with folded edges with the part inside and let it purge at a low flow for a hour before you put it into the furnace.
You can even take it out of the heat for it to cool with the bag still being purged.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[irpheus]

Hi All & thanks again for your very helpful feedbacks

@Compositepro:
My goodness! When I read this sentence of yours I had one of those moments where I was thinking why I hadn't thought of this before .. ?? The annealing chamber I've made places the ceramic insulation outside of the stainless steel - yet the ceramic insulation itself isn't warmer than that I can actually touch it with my hand even at the maximum internal temperature ... I guess this also means that basically it would be possible to make a vacuum annealing chamber with just plain steel on the outside and ceramic insulation on the inside since heating of the steel would be less than what causes it to oxidize in normal atmosphere. Very helpful post - many thanks

@berkshire:
Thanks for clarifying .. I will just consider how I can do this in the most feasible way. Since I will be doing some annealings it may make some sense to me to reduce the amount of argon used.

@EdStainless: Thank you also for your feedback. Sounds as a quite accessible method although I can't quite see from your description how air is kept out of the stainless foil bag when the air around the bag is cooled ... Might I have missed something here?

Thanks again to all of you for helping out!

Cheers,

Jesper

 

[mdm55]

Regarding ceramic insulation inside a vacuum chamber, one should consider that low density ceramic insulation typically has a large surface area that can adsorb moisture. For furnaces that are kept under vacuum when not in use, this is not a problem after initial outgassing. For furnaces that are exposed to atmosphere for any length of time between runs there will be significant outgassing upon reheating to 750°C. In vacuum the dominant heat transfer mechanism is radiation and nested sheets of a heat resistant alloy such as inconel separated by alumina beads provide a means of reducing heat loss without the outgassing problems associated with ceramic insulation. As has previously been mentioned, Ar with a small percentage of hydrogen would be an effective approach for annealing copper.

 

[EdStainless]

You make a bag of SS foil by folding all of the edges over.
Before you fully close it you put your part inside.
You can also tuck in a small diameter SS tube (1/8") tube connected to your gas source.
The other option to using gas is to put something more reactive inside the pouch, we used small piece of Ti foil as it will react first leaving the Cu clean.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube