Brazed copper joints will withstand ....°C?

[Captain Nemesis]

Ive got an old copper hot water cylinder which Im thinking of upcycling into a firepit, either by slicing in in half vertically and lying it on its side (like your traditional oil drum BBQ) or more likely as I think that might be too big, eat wood, and be hard to get close to, just use one of the ends like this:

My question is about fabricating legs/support/frame out of copper tube and/or sections of the cylinder. Obviously soldering is doomed to fail - would brazed joints made with a MAPP torch hold?

Ive also got an oil-filled Oxford with the 12V single-contact carbon rod brazing output. Never used it on anything other than steel - does anybody know how it fares with copper?

 

[TechnicAl]

bolt some steel legs on

 

[Captain Nemesis]

MMm. Could do I suppose, but if I want any swept bends then copper is easier to work with than steel, and I would like an all copper item. But if brazing aint gonna work it aint gonna work, and no amount of wishing it would will change that.

Thanks.

 

[123hotchef]

a round copper loop with copper legs so the base is free standing and pit rests in it could be square with compession Ts in each corner

 

[Spark plug]

Should look nice, but once the copper gets hot and becomes annealed it will be very soft and deform very easily.

if I was going to try this I would make a steel ring for the top from round bar and roll the copper around it to give it some strength.

 

[Kram]

Depends what brazing rods you use. SiB tig rods have a high melting point close to copper, might work with the carbon arc? Too hot for mapp.

 

[The_Yellow_Ardvark]

Brazed on steel legs will be ok.

 

[optima21]

seem like bbq paint is supposed to be fine up to 650C, if you get the copper glowing that roughly that temperature. if its a brazed joint (rather than silver solder) the melting point of the alloy may be 800c to 900c, so based on that as long as its not glowing you'll be fine.

if it does start glowing, its time to step back.

 

[123hotchef]

seem like bbq paint is supposed to be fine up to 650C, if you get the copper glowing that roughly that temperature. if its a brazed joint (rather than silver solder) the melting point of the alloy may be 800c to 900c, so based on that as long as its not glowing you'll be fine.

if it does start glowing, its time to step back.

really i have had copper washers glowing annealing them with a wee blow torch then poof they dissapear in a puff of smoke cant imagine that little blow torch is capable of
900C

 

[The_Yellow_Ardvark]

really i have had copper washers glowing annealing them with a wee blow torch then poof they dissapear in a puff of smoke cant imagine that little blow torch is capable of
900C

They have a higher tin content.

 

[Turbo]

Once that copper gets hot it will have the rigidity of plasticine so you'll need to add a ring of steel or similar to support it & weld your legs to that.

 

[dyno-tron]

I used to do a bit of brazing (not very successfully) with the carbon rods attached to an arc welder. Main issue is that the arc is massively hot so you have to be very careful to keep the brazing rod away from the arc otherwise it vaporises the zinc.

 

[Captain Nemesis]

Replying to the points raised overall, rather than specific posts.

I had wondered about the heat from the fire softening the copper. The copper firepits you can buy do have steel legs/frames, but often they dont look as if theyd do much good if the bowl got soft. I could easily knock up some kind of support out of dexion just to make it safe to try a burn and see what happens to the bowl.

MAPP gas will give a flame approaching 2000°C

Brazing with my welder doesn't use a carbon arc torch, and does not (should not) create an arc.

 

[123hotchef]

Replying to the points raised overall, rather than specific posts.

I had wondered about the heat from the fire softening the copper.

MAPP gas will give a flame approaching 2000°C

Brazing with my welder doesn't use a carbon arc torch, and does not (should not) create an arc.

thats why my washer dissapeared in a piff puff poff

 

[zeppelin builder]

it could be tigged no problem
but as mentioned thin spun copper dish end around 22swg typicaly will be as soft as wet cardboard when hot and will be operating red hot most of the time so will continuously be producing scale and deteriorate rapidly!
some iron skeletal basket on the outside ,would be a way to stop it deforming