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  1. #1
    Join Date
    Dec 2007
    Location
    Adelaide
    Posts
    96

    Default Silver solder advice

    Although I'm not a plumber I have done a fair amount of solver soldering with copper pipes, from a small repairs to a complete house re-plumbs in copper and have never had a problem until now.

    The project involved solver-soldering a small internally threaded mild steel lug (7mm diam and 10mm long) onto a small length of 20mm diam mild steel tube. Knowing how important cleanliness is I ensured both mating parts were bright clean - applied a small coating of silver solder flux (same stuff I use for copper) and proceeded to heat it (oxy/acetylene) to the point where the silver started to melt - the result was a black joint with almost no silver solder flowing/adhereing to the joint, it looked like a classic example of bird sh*t welding. I tried ordinary 5% silver solder some of the more expensive stuff which I think may be 15% and also some 45% I had on hand - same result with each. I eventually had to throw the pieces out and ended up machining it from solid to get the project finished.

    I have heard that there is a flux called BLACK FLUX - has anyone heard of this - is it available locally ? is this what I need or do I need to rethink my technique ?

  2. #2
    Join Date
    Jun 2010
    Location
    Wimmera
    Posts
    78

    Default

    Being steel you were trying to weld, I would have used bronze.

    Hope this helps,

    John.

  3. #3
    Join Date
    Dec 2005
    Location
    Mid North Coast NSW
    Posts
    8

    Default

    Agree with above post. I used to use Tobin Bronze rods for steel.

  4. #4
    Join Date
    Dec 2007
    Location
    Adelaide
    Posts
    96

    Default

    I know bronze welding is possible but did not want to do this, as I did not want a fillet of bronze around the lug, the steel tube sits in a machined half round groove, the welded lug sits in a hole which locates the tube, in addition bronze welding is done at a higher temperature than silver soldering and I did not want to warp the tube - as it was I did warp the tube so it ended up looking crap for 2 reasons hence making a new one.

  5. #5
    Join Date
    Dec 2013
    Location
    Sydney
    Posts
    101

    Default

    What flux were you using? This sounds to me like the joint was heated too much, or too unevenly, and some areas got too hot and the flux burnt.

    As long at the part is not too big, I prefer to use an LPG flame or an LPG/air torch for silver soldering. It's all too easy to heat parts of a joint up too much with an oxy torch. If I do need to use oxy I'll run a large tip, keep the gas turned down, and keep my distance from the work and let it warm up slowly and evenly. Its always tempting to get in too close and heat it up faster, but this is just going to risk overheating flux in the area directly under the flame. It's always best to go slowly and not force the solder to melt into the joint. If I'm going to be feeding in solder from a wire, I'll cut a bit of the wire off and rest it somewhere on the joint to indicate when it's time to start feeding in more wire.

    Also, for most silver soldering jobs I will make up some sort of insulating oven around the job. I use firebricks, CFC sheets, blocks of steel, old TIG fingers, whatever is to hand. Doing this ensures that the heat gets to the joint from all directions and things warm up nice and evenly. If the job is tricky I'll often stand it on a block of aluminium to help even out the heat.

    If you can post some pictures of your particular situation I may be able to offer some more specific advice re positioning and heating.

    I have dozens of different fluxes, black, white, clear, some from local sources and some imported, collected for various bicycle frame soldering tasks and other jobs. I'll see if I can dig up some of the places where I got them from. I never use that plumbers silver solder. I only use the "proper" ~45% varieties. For this solder, and on mild steel, you should be fine with the standard Comweld Silver Brazing No. 2 flux which is available all over the place (just don't heat it up too hot). More exotic fluxes are needed for soldering things like stainless steel that need more aggressive cleaning, or for brass brazing and higher temp solders that need a flux with a higher activation temperature to cope with the higher melting point of the filler material.

    Graham.

  6. #6
    Join Date
    Dec 2007
    Location
    Adelaide
    Posts
    96

    Default

    Thanks for the input Graham, the part was 200mm long and 20mm diam with a 2mm or so wall thickness, My father in law (he's now in his 90's) owned at one stage a bike shop and he has described how he made the frames using silver solder and a kero blow torch to hard solder the lugs to the steel frame tube, the tube and frame lug were nestled in a bed of coke and then heated, these days it's all done with TIG which I don't have, I have never had a problem with copper so I thought how hard can it be.

    I have fire bricks but did not use them - perhaps I should have, I just sat the tube on top of the slightly open vice.

    I don't have the failed part as it went into the bin. I believe you are right that I over heated the flux and I need to refine my technique, the common old Comweld No.2 is the one that I have - doing some google research it looks like the stuff called Black Flux will take more heat. The googling led to a lot of dead ends, what the folk in the USA call silver solder is actually our soft soldering and what we call silver solder they call hard solder. More internet searching leads me to believe that Black Flux is either not readily available in Australia or those that do have it don't advertise on the internet.

    I might do a few test pieces just to practice so when this comes again I'll feel more confident, I have an intended project coming up that needs a part to be fabricated by silver soldering.
    Attached Images Attached Images

  7. #7
    Join Date
    May 2011
    Location
    Murray Bridge S Aust.
    Age
    66
    Posts
    3,371

    Default

    The silver solder rod that you used, was it the same as you used for the copper???
    It requires the same flux but a different rod, a piece about 500 long and 1.6 daimeter is about $22.00, as it contains 45% silver, hence the expense. It's the same as I use for repairing my wood bandsaw blades.
    Hope this helps,
    Kryn
    To grow old is mandatory, growing up is optional.

  8. #8
    Join Date
    Dec 2013
    Location
    Sydney
    Posts
    101

    Default

    OK, if I'm reading your sketch correctly you're soldering a small diameter short boss to a larger tube? In which case it's a classic situation of a small and large part, and almost certainly you overheated the small part. Bicycle frames, at least the old fashioned soldered ones, have many parts like that - in fact they are called braze-ons.

    You may also have overheated the solder because plumbers "silver solder" melts at a lower temperature and the flux may not have done it's cleaning job before the parts got to solder melting temp. So you heated further to get the flux to work, which was too hot for the solder.

    If I was soldering this job I'd never let the flame touch the small boss. All the heat should be put into the larger tube. When it starts getting close to glowing dull red I'd keep tickling the joint with the solder until it flows around the joint. The boss will pick up enough heat from conduction, and particularly when the solder melts and provides a heat path to the boss. The flux will also tell you when you're getting close to temperature, it will suddenly change consistency and you'll see it etching the metal as it gets to its activation temperature which will be close to where the solder melts.

    Like Kryn says, don't bother with the cheap plumbers solder, go for the proper 45% silver stuff. The flux you have will be fine for this job. Just make sure you heat it gently, and only apply heat to the large tube.

    Graham.

  9. #9
    Join Date
    Jul 2006
    Location
    Athelstone, SA 5076
    Posts
    3,416

    Default

    5% and 15% is not recommended for steel.
    why plumbers use 5% and down to 2% on copper has got me beat.....unless of course they need a repair job later on in life.

    15% flows so mush easier it actually makes the time spent on the braze quicker...and no need to use flux unless it really dirty...but emery tape sorts that out

  10. #10
    Join Date
    Dec 2007
    Location
    Adelaide
    Posts
    96

    Default

    I have a pack of the 45% stuff that I picked up from BOC a few years back, 5 sticks about 1.5 mm or so in diam. the docket and 4 1/2 sticks still in the bag - $45. I did try this as a last resort but by then the parts were looking well used and I had noticed that the tube was a bit banana shaped so I had probably mentally given up and was already planning to machine from solid. I altered the design by boring out a length of bar stock and leaving a short section of the bore with a smaller diam. creating a thicker wall at that point that I could drill and tap a thread into for a locating peg.

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