Repairing Cast Iron - SailNet Community

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  • 2 Post By GaryHLucas
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Old 06-21-2012
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Repairing Cast Iron

There are two threads going here currently about broken cast iron manifolds. I have suggested brazing them, and don't currently have the equipment to do it. So I thought I'd post how to do it.

I repair cast iron using common bronze brazing rods and the very common Brazo flux. I have tried special cast iron brazing fluxes but haven't personally gotten good results.

The first step for a broken casting is to clean the parts well with a wire wheel to remove all surface rust. Then V out the two sides of the joint so that you have about a 30 degree v groove between the parts and they only touch on the inside edge. As much as possible you'd like the brazing to penetrate all the way through.

Don't be in a hurry, you need lots of patience here. Carefully clamp all the pieces back together. Theere aren't any large foreces involved, but once you start they must not move. Heat up the whole casting to at least 500 F. I often do this over a plumbers propane lead melting burner, especially if the part is large. Touching it with some lead solder should melt the solder instantly. Once you have the whole part hot you can then start heating the area around the joint using an oxyacetylene or oxy-propane torch. Unless the part is very small and you use MAPP gas you will not be able to use a torch without oxygen. You MUST use dark brazing goggles, it will be very bright, and you need to see well. Wear your reading glasses!

Heat the area around the whole joint until the metal glows a dull red everywhere, and quite bright where the flame is. Make sure you use a neutral flame, cut back the acetelyene until the blue streamers just go away. You don't want to add more carbon to the brazing joint. This heating is very important, almost everyone hurries this, the metal is too cold and the brazing metal doesn't flow into the joint well.

Once it is good an hot around the repair area, heat the brazing rod with the torch and dip it in the flux. Then heat the start of the repair. Cut the flame back, you aren't in a hurry, you want very good control of the temperature. Do not melt the rod with the torch. It must melt when it comes in contact with the hot metal. Keep heating, and touching with the rod until it starts to flow into the joint. You'll need to keep refluxing the rod as you do this.

Once it starts to flow you need to keep the temperature of the metal just bleow the melting point of the rod. At this temperature the rod will melt in the heat of the flame, flow and solidify on the metal. The puddle should be bright and shiny the whole time. You can stir it with the rod and the rod will slowly melt into it as you move. If you get popping you have too much heat, cut the flame back or get a smaller tip. Note that to keep the temperature just right you will need to keep moving the torch away from the puddle and back again.

Controlling the temperature is the whole secret here. Note that small pieces heat much quicker than large pieces. This means the flame needs to spend most of it's time on the large piece. Small pieces tend to overheat. When the metal is too hot the brazing material just runs off and won't stick. If it too cold it also won't stick. Get the temperature right and it is a thing of beauty how the metal flows.

You'll find that if you have the temperature just right you can build up missing areas using just the bronze. For parts actually missing just get some mild steel and use a grinder to fit it into the casting. Then just braze it in the same way.

Don't be in a hurry. Let the whole thing cool slowly until you can pick it up with bare hands. The flux will be a glassy coating on the bronze that chips off with a hammer. If you have areas built up a little too much a grinder or hander file will easily take care of it.

I wish I had pictures of some of the parts I have fixed this way, You'd be amazed at how well this can work! I've done quite a few exhaust manifolds, one broken in at least 6 pieces. I've done fork lift wheels with two of the bolt lugs broken completely off, and a third one misssing altogether. I recently repaired my cast iron keel where the bolt holes were completely gone and two tabs were broken off and missing. Last year I repaired several pieces of a bandsaw gear shift mechanism that were broken.

Gary H. Lucas
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Old 06-22-2012
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Re: Repairing Cast Iron

As a retired shop teacher I appreciate your lesson plan. Further, I've had success fixing simple manifold cracks with arc. and cast rod. Short bursts and quick peening minimizes distortion.Dodge slant 6 manifolds were my specialty. I keep oxy/acet on board the Thane.Mostly used to fix others problems up da coast.

Last edited by Capt Len; 06-22-2012 at 12:37 PM.
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Old 06-22-2012
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Re: Repairing Cast Iron

Quote:
Originally Posted by Capt Len View Post
As a retired shop teacher I appreciate your lesson plan. Further, I've had success fixing simple manifold cracks with arc. and cast rod. Short bursts and quick peening minimizes distortion.Dodge slant 6 manifolds were my specialty. I keep oxy/acet on board the Thane.Mostly used to fix others problems up da coast.
Capt,
Many of the cast iron repairs I have done are quite highly stressed. The fork truck wheel I mentioned, a handle on a pipe bender, the foot pedal on a bandsaw, etc. The tensile strength of bronze is very close to that of cast iron. So when heavily loaded they both stretch at about the same rate. So the weld zone doesn't see the high stresses you get when the materials are very different. Welding with nickel welding rods does work. You have to peen constantly though because of the great shrinkage stresses that must be relieved. So in the end it won't really be as strong.

I have used basically all of the metal joining methods, soldering, brazing, gas welding, stick welding, Tig welding and Mig welding, and each has its place.

Gary H. Lucas
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Old 06-23-2012
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Re: Repairing Cast Iron

Ahoy Gary,

How well does your process work when the cast iron get heavily crystalized from the heat/salt and exhaust gases? I tried to get my exhaust manifold welded and nobody wanted to touch it...I ended up getting schedule 40 mild steel pipe and making a new one.but thought I could have gotetn it fixed with the right know how. Thanks for your info!
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Old 06-23-2012
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Re: Repairing Cast Iron

Aeventy,
My 25 year old keel sat in salt water the whole time and some areas lost a 1/4" or more of metal. I chipped off the heavy scale with a hammer, then had it sandblasted. I wire wheeled the area where I needed to braze and it came out fine. You DO need metal, rust can't be welded or brazed! For small parts and things like marine manifolds I'd probably try pickling them in acid. You could probably get galvanizer to do it for you, and skip the galvanizing step. They dip everything in hot caustic which strips off all oils and dirt. Then they dip the parts in hot acid, which removes the rust. I get everything I design at work galvanized, so they re-galvanized my old really rusty anchor for free. I chipped off all the heavy scale rust with a hammer, wire wheeled it and they did the rest.

By the way Draino drain cleaner which is just caustic would probably clean all the oil and carbon out of a manifold pretty easily. If you rinsed it good and then used a sulphuric acid drain cleaner on it you'd probably get it quite nice and clean. When you heat it red during brazing it'll dry out completely.

Gary H. Lucas
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