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    Default Need Advice on Silver Brazing

    I need to Silver braze some mild steel blocks (like a sleeve) to a 5/8" diameter mild steel shaft; the blocks are not much bigger than the 5/8 shaft and do not have much mass; radial clearance is .001 to .002". The blocks are 1.5" long and I have added a 1/4" hole through two sides to aid in the braze flow and provide an area for adding the braze material in the middle of the block. First attempt failed because I overheated the flux (turned black) - I was using a #4 tip on a oxy/acet welding torch with a slight carburizing flame. I thought about using the rosebud tip, but thought that would be overkill. To me, the big problem seems to be heating up the 5/8 bar without overheating the flux. I did start with clean, well fluxed metal on both parts.

    I have machined off the first block (did get some flow of the braze material) and now have about .003 to .004" radial gap, and sanded off the burnt flux. The flux I have is Harris Stay-Silv and I am using Silvaloy A45 brazing alloy. I need maximum surface area adhesion between the bar and blocks so TIG welding or TIG brazing won't work for strength reasons.

    I did check several pages of posts under Silver brazing and didn't see any other similar posts.

    Thank you in advance for any tips or suggestions you can provide.

    -Jim
    Last edited by sabre1fv; 08-25-2019 at 02:11 PM. Reason: clarification

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    Sounds like you need a softer larger flame, …...you'll have to steadily heat (as in not fast!!) all the material right through more or less at the same time, ……..silver brazing - indeed all brazing is not a fast process like welding.

    FWIW as long as your rose bud isn't a monster it could well be the tool for the job - another old fave of mine was a cutting nozzle.

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    Stick the whole assembly in an oven to pre-heat it. If your oven gets hot enough, you could silver solder it in there. For large quantities, you may want to put together a kiln with fixtures to hold the parts in place. It will eliminate the contamination and heat control issues you will have with a torch. Induction heaters work really good too

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    Another item you might want to consider, Harris makes two types of flux, a white paste and a black paste. I find the black easier to deal with on steel. It doesn't seem to burn as easily.

    Tom

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    Get the propane bottle from your BBQ and swap it with the acetylene, the regulator will go right on the propane bottle. This will give you a "softer" flame that is also cooler so it will heat the part slower, without burning it as easily.

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    Coming from an applications engineer at a silver solder rod supplier, if this is not what you are using then disregard the rest. You HAVE to heat everything up to the right temp with the hottest part being the farthest from where you introduce the solder. When everything is just right you should introduce the solder and it should fill the entire joint in one shot. If it doesn't you CAN NOT heat it back up to add to it. A coworker at the time did production silver soldering of custom road bike frames and said the same procedure was strongly enforced in that shop.

    Supposedly propane is too dirty to silver solder with.

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    Propane with oxygen is clean. Propane (or acetylene) in a plumbers torch that just uses air from its venturi is "dirty".
    Here is a link to compare acetylene to propane for cutting. Info is good for heating as well.
    Andys Place, Propane vs Acetylene Cutting
    This has some interesting data from Carbide Processors, they like hydrogen:
    Brazing Gases for Torch Brazing
    Last edited by Rob F.; 08-25-2019 at 05:21 PM. Reason: add links

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    Steel itself has a pretty low thermal conductivity. So you cannot depend on the entire assembly being at flow temperature, if one part
    is at flow temperature. Basically you need to slowly bring the assembly up to temp by playing a soft flame over the entire thing, until
    it is all dull red.

    If you dig in with a smaller, fast flame you can blow the flux off and expose bare metal to the heat and it will oxidize. You are allowed
    to add more flux on as you go, Best if you can get full coverage with the flux when it goes transparent. Once the flux goes tranparent
    you are getting close to the solder flowing.

    As mentioned the silver solder will flow *towards* the hottest area. If you put on solder and it only beads up that is too cold.

    Use a larger tip than you think you need, and flow it with less gas to produce a softer flame that will tend to not blow the flux off.
    Also keep the flame in motion, do not let it dwell in one spot for long. Resist the temptation (happens to us all) to dig in with the
    flame as you begin to get the solder to flow.

    I would use oxyacetlyene for a job like this.

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    If you havnt done much silver brazing, practice on some scrap of similar dimensions first, maybe starting smaller and working up.

    There isnít anything that says you must only use one torch, two torches will help get even heat all around the piece.

    Use the black flux, itís higher temp and intended for steel

    MAPP gas is good for brazing, as is a multi flame tip like the preheat on a cutting tip (put your hand under the cutting jet lever!)

    Iíd probably use a cutting attachment preheat flame adjusted slightly carburizing with acetylene and O2, and a MAPP air torch, aka ďbernzomaticĒ yellow can, together.

    If itís not getting up to red heat in 20 sec or less you donít have enough heat, and you will trash your flux before it get there.

    Use lots of flux, add more as itís getting hot.

    Clean clean, clean, itís got to be clean.

    If it doesnít go and the flux starts pulling away, stop! Clean everything and go again.

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    My advice for brazing.. PRACTICE..

    If I haven't done it in a while, I need to practice a bit, and
    the last parts I do always look a lot better than the first parts.

    Like has already been said, warm it ALL up slow, and then work on the
    area furthest from where you will be applying the silver.. Just holding
    the heat on one spot is not going to do you any favors (rarely).

    I second using the cutting torch. Its like a little mini
    rosebud.. Generally running very low pressures.. If you have
    flaky regulators, that really can really screw with you also.

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    Could you rotate the part to heat evenly? I have clamped a big old monster 3/4 electric drill in a vise and lowered the speed way way slow with one of the cheapo router speed controls. I have also done it in a big old worn out lathe.Contrary to what others have said, I have successfully joined some large 2 inch diameter tubes that were sleeved together.I was not able to heat the entire joint at one time,because the circumfrence was over 6 inches.I was able to chase the heat around and apply solder 3 or 4 times.Of course it would have been better to have a bigger or multiple torches.The most important part is ,try to keep the actual FLAME away from the place that you are introducing the solder. If you burn the flux there ,you are screwed. Edwin Dirnbeck

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    Despite whats said ...propane/LP gas does cause problems...there is a heavy oily residue in the gas..and the stink.......which plays havoc with braze or silver......I ve found to use propane,which I do,the surface must be protected from the direct flame,then the filler metal will flash all over.......any kind of brazing is "heat,heat ,heat....enough heat" but not a concentrated welding flame.....IMHO,do it in a furnace,and the process will work like magic.......incidentally,writing (lead) pencil is a widely used dodge to stop the silver going everywhere.

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    Thank you for all the advice so far. This is a one-off so hopefully I won't be doing it again...... yeah right ;-). I am limited to my oxy/acet welding equipment so some of the suggestions I don't think I'll be using, but will refer back to them if my attempts continue to fail. In reading about the black flux that does look like a possibility and will purchase some tomorrow at Madco in Mt View, Calif - great shop been there it seems like forever (yes it is a plug but they are worth it). I will mock up some parts and practice as was wisely advised. Using the cutting torch head for heating is a great idea and will try that as well as my rosebud tip. The 'finesse' suggestions are also gratefully accepted since I rarely do any oxy/acet brazing.

    Thank you to everyone who took their time to help me with this problem - one of the great things about this website; I always learn something every time I visit here.

    -Jim

    PS: To john.k - great suggestion on the lead pencil trick. Years ago I took a welding class to learn MIG welding, and a comment the instructor shared to achieve the same result was to scribe a small line where you wanted the boundary for silver braze to stop flowing; never had a need to try it however. The pencil seems better to me than scribing a line.

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    a MAPP torch is only 40 bucks or so, and really handy for all kinds of things like sweating copper water tubing, freeing up rusted nuts, etc., so you might rethink the "I am limited to"...

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    In fact loss of heat due to convection or draught is one major cause for difficulty......its very easy to make a small furnace with a single gas cooker type burner inside a steel shield .....no insulation needed ,just keeping cold winds off is enough,and the extra heat makes the whole operation childsplay.

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    I silver brazed thousands of silver contacts on copper blocks. Not the same as steel, but the basic rules are the same. I started with acetylene and changed to propane. We quickly learned to heat the block, not the contact. There are countless formulations of silver solder. We started with so called easy flow and changed to medium because easy was too runny, but the easiest flow is probably what you want. For a hand torch I used a Smith rosebud with the holes drilled out to keep the flow velocity down with propane.

    We also needed to assemble some small parts so we put them in a tin can with a lid in a pottery kiln and put a 1/4" copper tube in a hole to supply a very small flow of argon. Even with the inert atmosphere we had to flux the parts but they came out clean with perfect silver solder fillets around them. If you have access to a furnace, that is the way to go.

    Bill

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    An aside - back in the late 70's I came up with a cost saving plan for bushing and refurbing cast and machined bronze wheels (about 6'' dia) 00's upon 00's of them from a paper sack works, FWIW the cost saving over new parts less the scrap value of the old ones, was <> 40% so they nigh bit my hand off (and Sami was on a real sweet little earner )

    To biz - basically ??? 9ya know what I mean ) the job involved boring out and silver brazing in a bronze bush that I'd made, then a 2nd & final machining to size and true running.

    I built a rough & ready simple ''double barbecue'' out of the two semi circular halves of a 200litre oil drum and some reinforcing mesh, and fired same with charcoal (which I could get for nix from the client )

    And the routine was to assemble fluxed bush and wheel and place on grid, by the time the barby was full the first had preheated enough for no more than a 15 - 20 second ''blast'' with the OA rosebud and in with the silver solder, the pre heat not only made the job easy but a reliable consistent process.

    Yes, a bit shade tree, but I don't recall a bad joint, ……...and it put food on the table - which is the name of the game - that some folk tend to forget ???




    P.S. And of course have a bit of barby type feed with me mates afterwards

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    Limi, you folks really do have a different language. Nix is what we use to get rid of lice and other nasty insects... Other definition in yer part of the woods?

    L7

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    Quote Originally Posted by lucky7 View Post
    Limi, you folks really do have a different language. Nix is what we use to get rid of lice and other nasty insects... Other definition in yer part of the woods?

    L7
    Nix = nothing, as with the charcoal above, ...........as in I got it for free

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    Quote Originally Posted by cyanidekid View Post
    If you havnt done much silver brazing, practice on some scrap of similar dimensions first, maybe starting smaller and working up.

    There isnít anything that says you must only use one torch, two torches will help get even heat all around the piece.

    Use the black flux, itís higher temp and intended for steel

    MAPP gas is good for brazing, as is a multi flame tip like the preheat on a cutting tip (put your hand under the cutting jet lever!)

    Iíd probably use a cutting attachment preheat flame adjusted slightly carburizing with acetylene and O2, and a MAPP air torch, aka ďbernzomaticĒ yellow can, together.

    If itís not getting up to red heat in 20 sec or less you donít have enough heat, and you will trash your flux before it get there.

    Use lots of flux, add more as itís getting hot.

    Clean clean, clean, itís got to be clean.

    If it doesnít go and the flux starts pulling away, stop! Clean everything and go again.
    Really good post. I have to especially agree with the "get to heat in 20 seconds"! The flux is only active for a short time, you must be finished before it becomes ineffective. I don't know when time really starts, probably around 500F. Get in, get out, quit playing about.

    For cleaning, you can't beat spray on oven cleaner for cleaning off grease and oils. If water beads up when dropped on the part, the part is to dirty for proper brazing. After you have inspected the brazed part put some oil back on it to prevent rust. Rust is the downside of a properly cleaned part.

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