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Thread: Brazing bronze with a mapp torch

  1. #1
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    Default Brazing bronze with a mapp torch

    It appears that to pull this off, I'll have to use a nickel-silver rod. A plain old garden variety mapp torch won't melt a bronze brazing rod and I'm not willing to fork out the $$ for an oxyacetylene getup. OK so here's the question... Would it be bad to braze a bronze part with ni-ag rods if the part is going to spend most of it's time under water?

  2. #2
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    Default Re: Brazing bronze with a mapp torch

    It should be OK. Nickel silver is very close to phosphor bronze in the galvanic series. The corrosion rate in seawater is on the low side for copper alloys. Corrosion rates in seawater are listed here: http://www.dtic.mil/dtic/tr/fulltext/u2/743872.pdf which is one of very few such studies.

    Only the voltage scale on the chart is important, not the vertical scale. All the vertical scale does is put the metals in order, from high to low voltage, but as long as the voltage difference is below 0.2V, the metals should be compatible. The braze alloy is lower in Ni than the wrought alloy, but the corrosion rates are very close. The braze alloy is close to Ni brass in the dtic paper, while nickel silver (CDA 752) is the wrought or cast alloy.


    Nickel silver, aka German silver is a copper zinc nickel alloy. It is essentially a brass, but unlike most brass the 10-20% nickel component prevents dezincification, unless the part is work hardened or bent, which is not an issue in a braze joint. https://www.copper.org/applications/rodbar/pdf/7013.pdf

    Braze alloy https://www.brazing.com/Pdf/Copper%2...%20Silver).pdf
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  3. #3
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    Default Re: Brazing bronze with a mapp torch

    Interesting side note which was new to me in spite of being associated with shipyard welding for many years. . .
    The hydrogen content of MAPP/Propane does NOT seem to be a problem for solder/brazing judging by it's wide use in bicycle and refrigeration work.

    The instructors that taught me welding were themselves retired from the now closed down Charleston SC naval shipyard and they all remembered when oxy-fuel gas was much more widely used not only for steel and cast iron, but even for aluminum . . . imaging that. We have a local MC tank builder that gas welds his custom MC tanks.
    My impression is that alternative fuel gasses are most widely used for large scale oxy-fuel cutting. You often see propane/mapp for cutting in scrap yards.

    ( https://www.polytechforum.com/weldin...lene-2493-.htm )
    There is a lot more hydrogen available in the MAPP flame then in the acetylene flame, so welds done with MAPP (or any other LP derivative) suffer from hydrogen embrittlement.
    Here is the section on fuel gases out of the notes from Ernie's gas welding class:
    Fuel gases:
    Acetylene: made from dripping water on calcium carbide, acetylene used to be user- generated for everything from toy "carbide cannons" to household gas installations which had an acetylene generator in an outbuilding, generally a blockhouse about 100 feet from the main home with 3 walls made of concrete blocks and the 4th wall (the one that faced away from the house) made of wood so that if the acetylene exploded, the wood wall would fail and the blast energy would be directed away from the home. Acetylene generators were also used in industry and are still around as quaint period pieces, but insurance practices now dictate that acetylene production all be done in large plants. The welding gas industry would love to see acetylene go away entirely because of the astronomical insurance costs associated with handling it, but it is necessary for welding as no other gas has its attributes of heat without excessive amounts of elemental hydrogen being introduced into the weld metal.
    very expensive fuel only fuel gas appropriate for welding steel
    Propane/Chemtane/MAPP/Propylene/Flameall: (LP gasses) not suitable for welding, best choice for heating or cutting use same regulator as acetylene propane is a much cheaper gas than acetylene
    Grant Erwin Kirkland, Washington Seattle Metalheads welding class facilitator (Ernie Leimkuhler, teacher)
    Last edited by George Ray; 07-31-2018 at 05:57 AM.
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  4. #4
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    Default Re: Brazing bronze with a mapp torch

    The problem with using a hand held mapp gas torch for brazing is primarily BTU's, there is often not enough, or maybe just enough, which means it will take about a half hour to heat up your part...

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    Default Re: Brazing bronze with a mapp torch

    I hear that a mapp torch can be heated up considerably if you buy the slightly-higher priced version which allows the attachment of a separate oxygen cylinder. However, I ALSO read that these little oxy cylinders are good for about ten minutes of use. So I could go through two or more oxygen cylinders, getting the hollowrod hot enough to melt the brazing rod.

    The answer might be to buy all the parts, drill and countersink the straps, bend the straps around the hollowrod, and then take the stuff to someone who knows what they're doing to do some bronze welding.

  6. #6
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    Default Re: Brazing bronze with a mapp torch

    Most, if not all 'MAPP' gas sold in the US is now propene (Propylene) and doesn't burn as hot as the old MAPP gas. The label on the cylinder says MAP/Pro. Wikipedia and some bike building sites said this and the safety data sheets confirm it.

    There is also a different torch type marketed as a turbo or turbo extreme torch that is supposed to burn hotter than a standard torch. ESAB makes a turbotorch stk and tx. I don't know enough about these, but it sounds like they are worth looking into. ESAB is a reputable welding supply house.

    https://www.esabna.com/us/en/product...nt=50&pageno=2

    The O2 cylinders from the big box stores hold one ounce, a whole cubic foot of oxygen, so you will go through a lot of expensive gas if you ise the map-pro/oxygen torches. They do work though.

    Hydrogen embrittlement is NOT a problem for bronze. High strength steels (uts > 100,000 psi (689 MPa)) do have a problem with hydrogen, along with titanium and a few others of no interest here. http://www.sandia.gov/matlsTechRef/
    Last edited by MN Dave; 08-02-2018 at 04:12 PM.
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  7. #7
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    Default Re: Brazing bronze with a mapp torch

    Quote Originally Posted by Canoeyawl View Post
    The problem with using a hand held mapp gas torch for brazing is primarily BTU's, there is often not enough, or maybe just enough, which means it will take about a half hour to heat up your part...
    One way to combat this is to preheat your work before lighting the torch, I do a lot of cast iron repair and preheating and controlled cooling are mandatory.
    I picked up one of those electric oven/roasters that you see being used at thanksgiving and Christmas,I found it at a secondhand store for ~$15.00 and it will heat up to ~450*.
    Now that I have it I have found other uses for it, the most common use is it makes spray painting metal parts turn out much nicer I use the high temp engine paint and preheat the parts to 250* before painting and then after drying bake at 350* for ~1 hour and the finished product is outstanding in both the finish and durability of the paint.

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