A few days ago I picked up a length of silver solder (he said 45% Ag, IIRC) from the welding store and some black flux appropriate for it.. well I'm wondering what I'm doing wrong, joints with brass and copper proceed just as brass brazing on steel (except for the lower temp and wider melting point) but on steel it works about as well as soft solder, breaking away from the steel on testing - and also the solder gets brittle. From what I hear I'm not supposed to flow it in from the rod like soldering and brazing, but that shouldn't account for this...??
Tim
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Tim, i have used the 45% silver solder in HVAC copper to steel, brass to copper, brass to steel and it works well. i use the silver solder flux that is white and a paste(add water if dries out) the solder should bond with the metal long as it is clean and the flux is working( my guess the flux you got maybe the problem?) good luck!
I think you probably heated it too fast or unevenly. The black flux is fine as it takes more heat before breaking down than the white paste version. As expensive as this solder is I would recommend cutting it up in tiny chips (pallions) like 1/32' long and setting it in place with a small pointed brush (wet with flux). If this is not possible then feed it from the wire but this is wasteful and sometimes too much solder causes problems. Anyway, heat the assembly sloooowly and evenly, watch the flux as it starts to melt and get shiny, flow point should be right NOW. Maybe you need a bigger torch tip. It is vital to have the whole joint at flow temp at the same time! This is really what makes soldering different from brazing. If you continue to have problems try the 56% Au from Harris. This is more ductile and has a lower flow point but same principals apply.
Johnson Matthey site. Sounds like you are overheating the steel joins and perhaps using the wrong flux. Silver soldering (brazing) is not like brazing with brass alloys as much as it is like soft soldering. You don't heat the steel to red heat before you start soldering. Instead you apply flux, heat the joint a bit, touch the Silver filler to the joint and if it starts to melt get to soldering. Never heat the Silver filler with the flame, let the parts to be joined melt the Silver filler. The closer a fit you have between parts to be joined the stronger the join will be. Also note that Silver filler (like soft solder) will flow TOWARD the hottest part of the joint and this fact can be used to advantage by using the torch somewhat like a brush to pull the Silver to where it is needed.
I used to use "40T" Brazing rod in a previous production job for stainless steel blade assemblies. When I started working there they where using a Black flux (can't remember Product names) It was some nasty stuff, A LOT of fumes and it left the assembly looking like crap. Then they switched over to a white flux, it seemed to do a much better job at protecting the stainless from burning and had considerably less fumes.
If it's hard solder, borax paste is the flux. Clean the joint, mix powdered borax with water, bring to a dull red heat, and touch the solder to it. When the work is hot enough to melt the silver solder it's just right. Over heating will boil the brazing material, and ruin the joint.
Thanks guys for the replies.. more: It's Harris(?) Stay-Silv black paste flux. I usually use a lot of it because the way I've been applying it, it's hard to get a small amount on the work ;) The mini-MSDS that came with it indicates fluorides and boron based stuff, FYI. I guess I'm used to a stronger flux, when brazing I can go through mild rust, scale obviously, and dirt and grime on the work is N/A. I think what I've been doing wrong is overheating it (used to heat=flow), heating the rod with the work (brazing with a propane torch, it's pretty much required), and probably too thick joints. But I don't understand how an otherwise ductile material can't be used, wastefully though it is, to make a nice fillet. And that doesn't explain why it isn't wetting the steel either ;)
Plain borax flux ( or Boraxo powder from the laundry section of the supermarket ) boils and swells a lot before the water is expelled and it becomes Borax Glass. Borax flux becomes a clear liquid like coating on the item being heated at around 1100-1200 deg. F. Which is a good indicator that the joint is nearly hot enough to apply the Silver solder and have IT melt. Jewelry makers use this behavior of borax fluxes to their advantage, because Sterling Silver melts only a few degrees hotter that the so-called "hard" Silver solder alloy (1450 deg.F.) does.
The transition time, when the water is boiling out of the flux, is the time that joints tend to get burned by inexperienced torch operators. Key is to keep the flame soft and light, and always moving, especially during the 'white and fluffy' stage.
Otherwise the flux burns off of local hot spots and that's where the oxidation happens. This is true even after the flux 'flows' like glass, but to a lesser degree. I was taught to go with a larger tip, but flow it with a small amount of gas, to prevent some of this.
Jim
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Tim Williams wrote: And that doesn't explain why it isn't wetting the steel either ;) ^^^^^^^^^^^^ This is just my guess, for what it is worth. When you rely on a close fit-up, and capillary action to draw the solder into the joint, you know, when the solder "wets" the metal, that you have succeeded. If you lay in a heavy fillet of solder, you can't really tell its wetting--you are just trusting that with a mass of solder, it must be wetting. But it probably is not. The weight of the solder, resting in the joint area and cooling, is giving you a sense that the job is done, but without wetting, as you know, you don't have adhesion.
Some of the solders will wick thru the entire length of the joint - even a closely fit one. For example when I was silver soldering some brass bushings onto stainless tubes recently, I could tell the joint was 'done' because there was a ring of solder that appeared at the other end of the bushing. The fit up in this case was about a thou or slightly less.
My favorite story about this effect was told to me by the man who taught me (well, he let me watch him, anyway) to silver solder, he was named Bud (Arthur) Embry, and he's dead now and drinking martinis with Teenut I suppose.
Anywho, Bud's story was that he was brazing anti-aircraft gun barrel liners into the barrels during WW2, and they would press-fit the liners in and then set the barrels on supports and hit the whole thing with several rosebuds at once, a couple of guys working on them. Once at temperature, they would wipe the one end with the solder and would travel the entire length of the barrel (many feet) and appear at the other end, as a ring of brass colored solder between liner and the barrel.
I guess Bud was personally responisible for a number of enemy planes shot down. In a sense.
Jim
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I may have overstated things, but you really want to start with surfaces that are not oxidized and not rely on the flux removing any oxides. As you say you may burn up a bit of a joint anyway and have a spot to redo. If you start with any oxides on the surface, you are almost certain to end up with one or more bits to redo ( Or at least I am ). And as you say it is easier to remove the residue if it has less oxides.
Vincent stayed in the city and worked all night. I suppose that is why he was off when you saw him.
Tim, The reason you can't get a nice built up fillet with these types of solders is that they are more liquid when molten than bronze rods and tend to flow out rather than build up. BUT, there is no reason for a built up fillet. The silver solder joint is extremely strong when done correctly. Solder also has a much better capillary action than bronzes so it penetrates the whole joint. Once you get a nice clean joint look at it closely with a lens and you will see that there actually is a tiny fillet at the edges of the joint. If the reason you want a big fillet is so you can blend in the joint like a brazed bike frame, well then, go back to bronze. Just what are you building down there?
Tim, The reason you can't get a nice built up fillet with these types of solders is that they are more liquid when molten than bronze rods and tend to flow out rather than build up. BUT, there is no reason for a built up fillet. The silver solder joint is extremely strong when done correctly. Solder also has a much better capillary action than bronzes so it penetrates the whole joint. Once you get a nice clean joint look at it closely with a lens and you will see that there actually is a tiny fillet at the edges of the joint. If the reason you want a big fillet is so you can blend in the joint like a brazed bike frame, well then, go back to bronze. Just what are you building down there?
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