Building a brazing furnace

[RichardF]

I am looking for advice here in terms of building a moderately sized brazing furnace.

 

Background:   I have working on and off for years to make a copper cowboy hat.  This problem has proven to be very challenging.   I had hoped to weld the brim on, but shrinkage became a big problem, so I gave up on that approach.  In order to braze, I ended up building a three sided furnace with a propane torch to preheat, and using an oxy-actetylene to weld.  But brazing had shrinkage problems also  I decided to try what I thought would be a simpler project, which was to make a steel top hat by tin-lead soldering the pieces together. with an acetylene-air torch.  That turned out to be difficult as well, due to the pieces shifting when they were heated.  (See the photo of a top hat, approximately 14 inches by 12 inches by 5 inches.) I decided to make a kind of scale model, and see if I can solve the problem at a smaller scale.   I think now that I may have solved the problem by using an oven.  The oven produces fairly uniform heat, and I am able to clamp the pieces together.  See the photo.  I was able to use the kitchen oven for soldering.  But my goal is to braze a copper cowboy hat, and I want the color match of silicon bronze, not a silver line of tin at the join.

 

What I am thinking that I need to do is to build a medium sized brazing furnace, propane fueled.  Furnace brazing is a standard industrial process.   The inside of the furnace would need to be at least 16 inches by 14 inches by 8 inches tall, for a volume of about 1800 cubic inches, or around 1 cubic feet.   I would probably build it from the usual white soft insulating fire bricks.   So I have some questions.

 

What burners should I use?  How many?  I made my own burners for my blacksmithing forge, but was disappointed that I could not get it close to welding heat.  On the other hand, I do not need welding heat, maybe just build two or three Reil type burners.   Or just buy a few burners off of Amazon or wherever.  Who makes good burners?

 

What should I do for the roof of the furnace?   I could simply use a large plate/sheet of steel, and then put fire bricks on top of it.  This would have the advantage that I could easily take it apart, and perhaps reconfigure it at a future date. For a more permanent solution, I could weld wire mesh to a large plate/sheet, and then glue the fire bricks to it.  What could I use to bond the bricks to a sheet of metal?  Given my situation, a furnace that can be reconfigured for each project is probably the way to go.

 

The clamping arrangement shown in the photo uses 1 inch square tubing, 16 ga wall thickness.  This will probably be inadequate at brazing temperatures; Perhaps use 1” by 1/2 inch bar.

Richard Ferguson

[Frosty]

Welcome aboard Richard, glad to have you. If you put your general location in the header you'll have a much better chance of meeting up with members living within visiting distance.

Do we know each other? Your name sure rings a bell with me. Artmetal or theforge lists maybe?

A couple things about managing heat on sheet metal. First off an air fuel torch is way too soft a flame, it's designed to heat wide areas and is the opposite of what you need here. You are applying torch and filler on the inside of the joint between brim and hat / cylinder / can (?) on the top hat. This causes the flame to heat everything in a wide area around the joint. It can't do anything BUT WARP. Flip it over and weld the outside of the corner formed by the joint. This will let you just kiss the steel with a fine torch flame and minimizes the HAZ (Heat Affect Zone) and warping. Move FAST, tack it up and skip solder it.

Same for the copper. Use a low melt and only heat the outside corners of the joints. 

Also aim the flame so the tail impinges the can shape section. Cylinders are have structure and it will be contained within the brim so it can NOT warp as far nor easily. You should be holding the torch body over the brim with the flame just touching the joint with the tail flowing off into the open hat body. Placing something to shield the far side from the heat is a good idea.

Better still, buy an inexpensive or used TIG welder, practice on scrap then braze it up with the TIG torch. It'll go so fast the hat won't have a chance to warp. If the brim is slid down so the hat body extends 1/16" above it you won't need filler rod at all.

Frosty The Lucky.

[RichardF]

Frosty:  We do know each other from Art Metal.  I do not think we have met face to face.  I am in Colorado.   I mostly TIG weld, but I go kind of slowly.

[George N. M.]

Richard,

Welcome aboard from 7500' in SE Wyoming.  Glad to have you.

If I wanted to make a copper cowboy hat I would be thinking about solder to join the rim and the crown.  Or, at least, I'd do one with solder as a test before I tried brazing.

"By hammer and hand all arts do stand." 

[Frosty]

Too cool Richard, long time no read! What have you been up to?

If you have a tig that's the only way to go, you can cut a little of the parent stock for filler and match the color exactly. 

I wish I had a tig. <sniff>

Frosty The Lucky.

[Mike BR]

Have you tried peening the welds?  As I understand it, warping happens mostly because the weld puddle freezes at a relatively high temperature, then contracts as it cools, pulling the surrounding metal with it.  If you can peen the weld and spread it back out the the size it was when hot, you can often reverse the warp.

It looks like you might be able to support the brim joint from the outside of the hat (inside of the angle) against something like the edge of an anvil, or better, a swage block, and then work the weld from inside the hat with a hammer.

You're probably way ahead of me, but I've had good luck using electrical wire (stripped down Romex, for example) as TIG filler for copper.

[Frosty]

That is true but more in heavier section welds. Sheet metal is much harder to control as the HAZ can cause deformation of the parent stock at a greater distance that doesn't come back as it cools. 

Heavier stock can absorb the force of the weld's HAZ without significant movement but the stresses can cause problems down the road. Peening welds relieves that stress. The easiest way to deal with "pulling" caused as the puddle cools is to tilt the joint so it pulls itself to the correct angle. Or tack and weld on opposing sides, skip welding, etc. 

I just don't know how much good it'll do with sheet stock. Then again I'm so far from my school time and practice I could be completely off the mark.

Frosty The Lucky.

[RichardF]

Thanks to all for your comments.