Need some help with bronze brazing

Rockmonton · Apr 22, 2021

Dropped a cast iron piece a while back.

Made my first attempt ever at brazing

Heated for about 20 min with a tiger torch, then oxyacetylene to braze.

Doesn't look pretty, and there's some spots I definitely need to go back and try again.

I'm looking for some tips on getting it better. Should I melt apart and try again in total?

Limy Sami · Apr 22, 2021

Please bear in mind this is what I was taught 50 years ago (so may be out of date).............oh, and on another continent to boot

1, Did you flux the joint before preheating?

2, was the CI perfectly clean and well ''veed'' out, ..........preferably with a grinding wheel that's been used for nothing else.

3, IMO it's nigh impossible to use too much flux when bronze welding CI

4, Looks to me like you've porosity in the weld deposit, ......often caused by the zinc in the bronze burning, .......an oxydising (as in excess oxygen) flame suppresses zinc burnout.

HuFlungDung · Apr 22, 2021

It looks passable for a first attempt.

The joint prep is the most important step, and then you have to observe whether you are getting good wetting of the surface as you go. Can't tell too much by looking at what is on top. And the flux always makes it look messy, but grinding the deposit to make it pretty usually makes it look worse. Just buff the flux off with a wire wheel or chip it off with a chipping hammer.

Rockmonton · Apr 22, 2021

I was using flux coated rods, didn't add flux until hitting with the oxyacetylene. It definitely looks like I didn't get wetting and fill the entire vee in one part of the braze.

I used a fairly dirty grinding wheel to vee the CI out, there was maybe 1/16" left of the original break to help line things up. Everything was washed down with denatured alcohol beforehand. I was having a lot of trouble seeing the puddle with #5 glasses, seemed like the flame and flux really obscured things. I think the lowest my welding helmet goes is #9. Is there any workaround to this? Or is it just positioning and practice?

Mike C. · Apr 22, 2021

Agree with both the above, I'll also add that some of the old books say to only chip with a chisel or file for prep, grinding smears graphite. Get a can of powdered flux (you can occasionally catch it at Tractor Supply) to flux hell out of the joint. Restaurant type salt shaker works great for this. Myself, I'd have also fixtured that and drilled/tapped and countersunk some screws into the breaks... does two things, fixtures and holds the parts in correct alignment during brazing, and also acts as reinforcement just like rebar in concrete. Makes a VERY strong repair.

Rockmonton · Apr 22, 2021

When you say drill and tap, are you saying to make a bracket to hold the pieces together?

crossthread · Apr 22, 2021

I think what Mike is saying is drill down into the crack and tap the hole then put counter sunk screws in. No bracket. Of course you have to clamp the pieces well together to accomplish this. I would have used something a bit stronger to remove any oil from the casting which is always a problem with cast iron. Say acetone. I have held on to an old tombstone buzz box just for nickel welding cast iron. I have always had good results if you skip weld and preheat. Your problem looks like not enough heat, or not enough flux.

Dan from Oakland · Apr 22, 2021

What Crossthread said- your part was not hot enough and it looks like your v'd out joint could have been larger. Get some gas welding goggles so you can watch the braze flow out- you're not welding here- flux is your friend and only works with enough heat- if the rod balls up on your part, your base metal needs more heat. Flux coated rod can be problematic as you have no control over how much gets used- get a can of it, heat the rod and stick it in the flux to adhere it to the rod- rinse and repeat as needed. Don't make the mistake of using 1/16" braze rod for a casting repair- you just cook and oxidize the bronze Good luck.

neilho · Apr 23, 2021

What Sami said...

Flux, oxidizing flame. Good flux coverage is assured if the flux is premixed in a small container with a few drops of water, then painted on the joint with an acid brush. And if a thin layer is brazed on each side of the joint before the finish brazing, it's really easy to see where the bronze doesn't stick. Stir it around till it does stick.

Fixtured after the skim coat is on, then reheated and the bronze will flow right in. The base metal wetting is already done, so shaping the joint is the only task.

The best thing about that joint is that it's a clean break. Makes it easier to get a good bond, thermal expansion/contraction isn't a problem, can redo easily if not satisfied. Not always options if it's a crack repair.

HuFlungDung · Apr 23, 2021

If the casting is oil soaked, heat it red hot to burn the oil out. Dull red is sufficient. Depending on the part, you may need to heat the whole thing, but not for something like this, just heat each edge of the crack and let it cool off and do the prep.

I can't say grinding smears any amount of graphite in particular. There is always graphite present there anyways, and the flame will oxidize it in the presence of the flux. If you like you can grind most of it, then go over it with a carbide burr. This isn't going to make much, if any difference.

Gordon Heaton · Apr 23, 2021

neilho said:
What Sami said... Flux, oxidizing flame. . .

You guys got it backward from the way I learned it! You want a slightly carburizing flame, one with a bit of 'feather'. An oxidizing flame is one with too much oxygen. Its harsh both in sound and appearance.

Limy Sami · Apr 23, 2021

I sorta knew this would come up, but I was definitely taught oxydising flame for bronze welding, using both manganese or silicon (for copper) and nickel bronze (for steel) filler rods ..........and it served me well for over 30 years (long time since I did any welding, let alone bronze)

Maybe - as in JUST MAYBE

the two country's alloys are different.

cyanidekid · Apr 23, 2021

Limy Sami said:
I sorta knew this would come up, but I was definitely taught oxydising flame for bronze welding, using both manganese or silicon (for copper) and nickel bronze (for steel) filler rods ..........and it served me well for over 30 years (long time since I did any welding, let alone bronze)

Maybe - as in JUST MAYBE the two country's alloys are different.

has nothing to do with different alloys...the oxidizing flame is useful for burning off the graphite in cast iron. pre-heat with a SLIGHTLY O2 rich mix, with no flux, to burn off the graphite, then switch to a neutral or SLIGHTLY carburizing flame to reduce any oxides. pile on the flux, do the actual braze.

the only possible way an oxidizing flame REDUCES zinc boil-off is because it reduces the temperature. free oxygen is what you DONT want in the flame when actually doing any joining with the flame!

hobbyman · Apr 23, 2021

Mike C. said:
Agree with both the above, I'll also add that some of the old books say to only chip with a chisel or file for prep, grinding smears graphite.

Use a cutting tool such as a burr but avoid grinding for the final bit. Go along the Vee joint (90 degrees) and heat, add flux and and rod. Wet both sides of the joint. The alloy should wet both sides. after that you can start braze welding. The additional brazing alloy will easily bond now that you have wetted both sides of the joint. It is much the same as tinning both sides of a solder joint.

Big B · Apr 23, 2021

When I've had problems with brazing cast iron it has usually been from not enough preheat. Once I get it hot enough it brazes a lot like steel, the bronze flows right on. I have a lot of flux coated rod so that's what I use for flux.

Picture #3 near the bottom looks like it may not have bonded on one side and that was what I would get with not enough preheat. It does seem odd that a smaller cross section like that wouldn't be hot enough though.

neilho · Apr 23, 2021

For welding anything but cast iron, neutral or slightly rich flame, yes.

Linde's Oxyacetylene Handbook, 2nd edition, pg 248. "The flame should be adjusted to be slightly oxidizing." Next page: "..it appears that during welding the bronze flows in to occupy the space left by the oxidation of some of the graphite flakes."

And from my experience brazing exhaust manifolds (yeah, I know, crazy, but when yer broke and need an inspection sticker...) anything but an oxidizing flame just didn't work.

BTW, in the books the term brazing refers to a very thin joint formed by capillary action, while a joint involving bronze buildup is braze welding. Pretty much irrelevant in this case, since it's clear what the OP is trying to do...

Rockmonton · Apr 23, 2021

Just want to say that I'm enjoying how much learning is going on. Thanks!

cyanidekid · Apr 24, 2021

neilho said:
For welding anything but cast iron, neutral or slightly rich flame, yes.

Linde's Oxyacetylene Handbook, 2nd edition, pg 248. "The flame should be adjusted to be slightly oxidizing." Next page: "..it appears that during welding the bronze flows in to occupy the space left by the oxidation of some of the graphite flakes."

And from my experience brazing exhaust manifolds (yeah, I know, crazy, but when yer broke and need an inspection sticker...) anything but an oxidizing flame just didn't work.

BTW, in the books the term brazing refers to a very thin joint formed by capillary action, while a joint involving bronze buildup is braze welding. Pretty much irrelevant in this case, since it's clear what the OP is trying to do...

Btw, the term “braze welding” is an an egregious abuse of the English language. There is capillary brazing, in which the filler flows like water, and wicks along a closely spaced joint (as in jewelry “silver soldering or gold soldering”, or a “slushy” state braze, where the filler occupies a substantial gap (“V”) or fillet.

neilho · Apr 24, 2021

cyanidekid said:
Btw, the term “braze welding” is an an egregious abuse of the English language. There is capillary brazing, in which the filler flows like water, and wicks along a closely spaced joint (as in jewelry “silver soldering or gold soldering”, or a “slushy” state braze, where the filler occupies a substantial gap (“V”) or fillet.

Yeah, but I ain't gonna stop using it.

Agreed, but commonly used, now in the lexicon, enshrined in the literature, etc and makes a useful distinction between two different processes.

Limy Sami · Apr 24, 2021

Rockmonton said:
Just want to say that I'm enjoying how much learning is going on. Thanks!

Stick with it Rock, and you'll end up a confused gibbering idiot cowering in the corner - just like every other PMer

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