Oxy/Acetylene brazing for cast iron, what type of rods/flux?
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    Default Oxy/Acetylene brazing for cast iron, what type of rods/flux?

    I have an old machine that needs various cast iron repairs. All my research leads to the best fix being brazing with silicon bronze. I have a good oxy/acetylene torch setup, no TIG.

    I've welded and done some brazing, but am an amateur. I've done enough and watched enough videos I have no concern about technique or actually doing it.

    I'd just like to know what would the recommended brazing rod be? Also what type of flux?
    Thanks!

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    All I have ever done [admittedly not a lot] I just use the white coated rod. Also bare rod with the caned white or pinkish flux. Sil-Bronze is usually used with TIG IME. Larger repairs with a lot of filler build up I probably use the bare rod as once all surface is wetted with brass you really don't need much more flux.

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    tig imparts a lot of stress into material. The bronze and brass with torch has been used for decades to repair cast machines with less failure rate than ni-rod or tig superficial brazing. The wider/softer heat of the torch is less stressful for the parent material and allows slow cooling of joint. ni-rod and tig are just easy, not better.
    If you have a crack, you heat the material thru- brass flows better (silver best) than bronze; bronze tends to be stronger with fillets and is more forgiving on flame control. Super clean material is best, wire brush till blinding shiney is better than grinding or sanding alone.
    The coating rods are fine, unless you braze relatively often it is better/easier to get rods + some as you need them.

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    Quote Originally Posted by memphisjed View Post
    Super clean material is best, wire brush till blinding shiney is better than grinding or sanding alone.
    I'll disagree with you there. A rough ground surface is best for adhesion of the bronze, more surface area. Bronze is basically a very high temperature glue. Yes, there is some mixing of the material for a few thousands depth, but if its deeper than that you are running way to hot and have burned out the zinc.

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    His point about cleaning is correct - use alcohol or other "burn safe"* solvents to clean, clean, and clean again to remove as much oil or other contaminants as possible before heating up the part.

    *Burn safe as some chlorinated cleaners are very dangerous if still present when heated

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    I've done a bunch of cast iron repairs and the best I've found is welding it with oxy fuel. They make square filler rods and cans of flux that you have to dip the rods in. Its an older method but it works out pretty good. It matches the color of the original casting better than any other method, and it seems to take to all sorts of crap castings far better than nickel based fillers or brazes. From what I've read its machinable after, but I've never had to try. It also puts a LOT of heat into the part which is a good thing for cast iron. Still need to preheat and slow cool tho.
    Cast Iron Filler Metals for Gas or TIG welding
    First site that had what I was talking about.

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    Quote Originally Posted by PlasmaOnTheBrain View Post
    I've done a bunch of cast iron repairs and the best I've found is welding it with oxy fuel. They make square filler rods and cans of flux that you have to dip the rods in. Its an older method but it works out pretty good. It matches the color of the original casting better than any other method, and it seems to take to all sorts of crap castings far better than nickel based fillers or brazes. From what I've read its machinable after, but I've never had to try. It also puts a LOT of heat into the part which is a good thing for cast iron. Still need to preheat and slow cool tho.
    Cast Iron Filler Metals for Gas or TIG welding
    First site that had what I was talking about.
    +1
    i have found it somewhat hard to machine afterwards.

    i have always been able to get the rods and flux at the local welding supply

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    I'm not sure of the grade as we haven't needed to buy it in awhile, but I use a bare brass rod and canned flux. Punch a hole in the lid, warm the tip of the rod, stick it in, and the flux will stick to it. I don't use a whole lot of flux though. You need a little, but heat will help the most getting the rod to stick. I've had super good results using a rosebud tip to braze. The heat is higher but more dispersed so the whole area heats up, cooks out contaminates, and helps the rod flow adhere and level. A good brazing job is the same as a good welding job: you want the bead to be fairly level with little to no clean-up grinding. Piling brass on top won't strengthen the bond near as much as having the brass flow deeper into the joint.

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    You can also get an agressive flux called "scaling powder" if the iron wont wet.....use only for initial wetting ,because it causes bubbles in the braze.....Oil soaked castings always cause problems unless the oil is burned in a very hot preheat.

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    Quote Originally Posted by john.k View Post
    You can also get an agressive flux called "scaling powder" if the iron wont wet.....use only for initial wetting ,because it causes bubbles in the braze.....Oil soaked castings always cause problems unless the oil is burned in a very hot preheat.
    John, do you have a link to this "scaling powder"? I did a quick search and didn't find anything.

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    sear the surfaces to be brazed with an oxidizing flame and stop when there are no more sparks then braze in well vented area


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