How to fix cracks in cast iron

Cast iron is one of those metals that you don't see too often except when it comes to certain engine components. Cast iron has a higher carbon content than mild steel and it has excellent heat properties. That is why the exhaust manifold on your car is always made of cast iron. Dutch ovens are usually made of cast iron because of their remarkable ability to heat evenly.

The problem with cast iron is that the higher carbon content makes it a brittle metal. If you were to drop a piece and it landed just right it would probably break or crack.

The proper way to fix a crack in cast iron is to first find how far the crack has travelled. You can do this by adding a little heat to it from a torch and it will ususally show the crack a little better. The next step is to keep the crack from getting any bigger. The way to do this is by drilling a hole about a half inch above and below the ends of the crack. It doesn't have to be a big hole. 3/16 to a 1/4 inch should do it. Remember, you will have to fill it with weld after you're done.

Once you have drilled your holes, the next step is to take a right angle grinder or a die grinder with a carbide bit and make a groove in the crack. The objective here is to grind out the crack because you're going to fill it back up with weld. Now you don't want to go all the way through the metal. If the thickness is a 1/4'' then you want to leave yourself at least an 1/8''.

Now lets talk about the proper filler to use. Of course you need to use the same filler rod to match the material. The nickel rods are the best to use. Cast iron rods are expensive, usually around $20 a pound. From my experience the best rods to use are the more expensive ones. The cheaper rods usually don't weld as well and leave a lot of porosity or pin holes.

This is where it gets a little tricky. I find that if you heat the casting a little with a torch and get the temperature up a few hundred degrees it typically welds better. You can not weld the entire crack in one pass. Its best to back step when welding cast iron. Back stepping is basically starting in the middle and laying about an inch of weld and then stopping. Then the next weld you lay will be behind that one and so on. You want to jump around a bit and just weld no more than an inch at a time. The other thing is to peen the weld after you stop. What I mean by this is to hit it lightly with a peening hammer. This relieves the stress that has built up after welding. It is a slow process but if you do this you will have a quality weld that will stand the test of time.

If possible, it's best to wrap the casting in a blanket to let it cool slowly. Too much of a drastic temperature drop could cause it to crack again. I have also brazed cast iron with a silicon bronze rod with good results. In a pinch I have actually used a 6010 rod, but this is only a temporary fix. Any kind of a blow to the piece will cause it to break. I have only used this to attach some broken scrolls on mailboxes. This is not the proper way to do it but it will get you by for the time being.
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