What are you using to heat the parts, if the torch does not have the power to heat the base metal you will end up just melting blobs of solder onto the surface.

J

To solder (braze) successfully it helps to understand how it works.
Soldering refers to the process whereby pieces of metal are joined by melting another metal (the solder) so that it wets the joint and holds it securely when it freezes. That's basically it, but there are a few "gotcha's".

The important word is "wets". It's just like water. Put a nice clean pipe in water and freeze it; it's then difficult to get the pipe out, but use an oily pipe and it's much easier. The difference is that the water wetted the clean pipe, but couldn't wet the oily one. If the pipe wasn't oily but dirty, the effect would be the same. Solder works in exactly the same way. So all you have to do is make sure the joint is nice and clean and you'll get a good joint, right? Well, no, it's a little more complicated than that, but not a lot.

To melt the solder you have to heat it up. The trouble is that the very act of heating it also makes it dirty. The oxygen in the air is only too eager to oxidise everything it touches, and, as far as solder is concerned, metal oxide is dirt. Most metals oxidise rather slowly at room temperature, but heat them up and the effect is very rapid – so rapid as to make soldering impossible, unless you prevent the oxygen from reaching the hot joint.
There are only three ways to do this: solder in a vacuum, solder in an atmosphere devoid of oxygen, or coat the joint with an oxygen barrier that can withstand the heat. The first two options are rather impractical, but the third is fine; the barrier is called a flux. The job of the flux is to cover the joint with a barrier to stop the oxygen from oxidising it. Resin is a good flux for soft solders, and borax for hard solders or brazing. If the joint and the solder are nice and clean then these fluxes work rather well, but that's all they do; they are known as inactive fluxes. If the joint is a little dirty these simple fluxes do nothing except act as a barrier, but there are others that can do a small amount of cleaning too. Baker's Fluid is one such active flux for soft solder and EasyFlo is one for hard solder. The active fluxes are certainly better than the inactive ones, but they are not magic, you shouldn't rely on them to do the cleaning for you. Active fluxes usually contain some rather toxic and dangerous additives (potassium biflouride?) which do the cleaning bit.

So, now you have a nice clean joint and a flux, so it's plain sailing, yes? Well almost. Most problems are caused by heating the solder rather than the joint. All that happens is that the solder melts, goes into a ball, and refuses to flow into the joint because it freezes before it can wet it. The secret is to heat the joint, not the solder. When the joint gets hot enough it will melt the solder which will then flow nicely into the clean, fluxed joint. The final thing that can go wrong is to burn the flux. If you heat the flux for too long, longer than a minute or so, it will lose its properties and allow the oxygen to pass. This is normally the result of insufficient heat, so if it happens, remove the heat, clean the joint, and start over, perhaps with a better source of heat, or better insulation to prevent the heat from leaking away. If you can't get the joint up to temperature with adequate insulation then you need a greater quantity of heat – ie. a bigger flame. Your photos show that you aren't using enough heat to get the boiler hot enough to melt the solder.

So, the four main points for a good soldered joint are:

1. Make sure the joint and solder are both clean.
2. Use a good appropriate flux.
3. Heat the joint not the solder.
4. Complete the joint quickly.

I hope this helps.

Gary

And doesn't just liberally apply flux over the joint, try to get some IN the joint, ideally before assembly.

Neil

Well put Gary wooding

A good explanation, I have never built a boiler, and my silver soldering is rather basic. Your explanation will improve my work.

Regards
John

Edited By John McNamara on 16/11/2013 23:16:14

Hi Mark, I have been using silver solder since I was an apprentice some um….. years ago, when easyFlo came my way for free. It was not until last year that I started to have any problems as it was the first time I was s/soldering heavy gauge copper. Copper (unlike steel, s/steel or brass) EATS HEAT like nothing else – Its a good conductor of heat and so it is essential to heat either side of the joint first and then start to concentrate on the joint itself. Use plenty of hearth bricks to both retain and reflect the heat back onto the component. Just for an experiment, silver solder a fitting onto some 15mm copper pipe and see just how your SF55 flows. You will only need a dab of solder. If you have the right flux and temperature I am sure you will see a nice fillet being formed. Good luck.

Bob H

HI Mark, some really good advice here. The guys are right, your problems seem to be twofold, using the wrong flux and not enough heat. Tenacity5 (or as I prefer HT5!!) is fine for the high temperature alloys and indeed will work with the low temperature but as the residues are quite corrosive please ensure you clean as much off after brazing. Do not use wet and dry as this can leave residues that the flux cannot remove. The joint area doesn't have to be really, really bright and clean, as long as there is no oil or grease the flux will sort out the rest. Ensure you have plenty of flux all around the joint area. if you are mixing a paste as it seems it should have the consistency of a thick yogurt. The 3941 burner isn't producing enough heat especially for the sf24 (or, again as I prefer 424 – sorry guys for the plugs!!). Try a 2941. The rest is as per the excellent advice given above, particularly from Gary but also the links to CuP Alloys' hints and tips as posted by keith. Especially, as we always say, 'heat the joint, not the rod'!

Here Goes I'll stick my Totally Biased Opinion on good Brazing-Silver Soldering technique.

All advice offered so far is good practice.
The bit that I would like to add is that you use the flame and hot gas stream to protect the area of molten metal at all times by NOT flicking the flame sideways on and off the area to be Joined!!!
Move the Torch closer or further away from the joint keeping the flame on the area you are working on this will allow you to control the temprature of the molten puddle from liquid to solid.
The Enemy of molten Silver Solder is AIR.
Whatever source of heating is used it is the hot and almost inert gas that is the flame and hot gas stream that will protect the puddle.

Eric

Julian,

Curious – On boiler tubes, do you mean put the rings on the inside of the boiler and apply the heat from the firebox side or put the rings on the firebox side and somehow get the heat to flow between the tubes on the 'inside' of the boiler. Same question for the smokebox tubeplate.

Bob H

Go onto the CupAlloys site you will find everything you need

Bob