Even if you have a decent amount of welding experience under your belt, you may not feel much motivation to learn how to weld cast iron. Not only are all cast iron alloys brittle, but some of them can’t even be welded. If you’re talking about the types that can be welded, there’s still a higher risk of cracks developing as the metal cools.
That being said, you shouldn’t necessarily let any of that discourage you. With the right information and techniques, you could learn how to weld cast iron like a pro.
Why do so few people try to learn how to weld cast iron?
As a carbon-iron alloy, cast iron is exceptionally hard; unfortunately, the trade-off is that ductility is quite low. With 2-4% carbon content, cast iron is prone to cracking when exposed to sudden temperature variations – such as the kind caused by a welding torch, for example.
Then there’s the issue of carbon migration, which is caused by extreme heat. Flakes of carbon migrate into the weld metal, excess carbon leads to weak points in the weld…and you can probably picture how that usually ends. This is what often happens when you MIG weld cast iron (or TIG weld cast iron, which is a similar process). For more consistent results, it’s best to stick weld cast iron.
It’s important to find out which alloy you’re working with
You’ll get slightly different results from each cast iron alloy. Here are the main types you’ll come across when learning how to weld cast iron:
- White cast iron is pretty much un-weldable; it’s just that brittle.
- Grey cast iron is slightly less brittle than white cast iron, but it’s still tough to weld.
- Ductile or nodular cast iron is by far the easiest to weld, thanks to microstructural differences. It has much better fatigue resistance than either white or grey cast iron, which is exactly what you need if you’re going to weld this metal.
If you don’t know which type of cast iron you have, there are a couple of ways to find out. The most accurate method would be a spectrochemical analysis, which is foolproof. At-home welders may not want to splash out on this kind of test, but fortunately they may not have to.
You may also be able to identify the type of cast iron you have by looking at its fracture points. Both white cast iron and ductile cast iron will appear white along a fracture, while grey cast iron will (of course) be grey along a fracture. This still won’t help you to differentiate between white and ductile cast iron, though, so you may have to look into that spectrochemical analysis after all.
Make sure the casting is cleaned thoroughly
No matter what alloy you’re using, it has to be thoroughly cleaned of grease, oil, paint, or any other surface contaminants. To confirm that the surface is completely clean, simply deposit a weld pass onto the cast iron; if there are any contaminants left, the weld pass will have a porous appearance. Let it cool, grind it off, clean the metal, and repeat until the weld pass looks normal.
Determine which welding technique you’ll use
Assuming this isn’t your first rodeo where welding is concerned, you may already have a preferred welding technique. If you’re learning how to weld cast iron, though, you may have to abide by which welding technique the cast iron prefers. Here’s what to expect with some of the most common welding techniques.
- If you’re going to MIG weld cast iron, you’d better prepare yourself for some frustration – this technique is really only appropriate if you can establish the procedure for a reoccurring application. There will probably be a lot of trial and error involved, and there’s a good chance that you’ll end up with a weak weld due to carbon migration.
- If you’re going to TIG weld cast iron, you could get a similar result as with MIG welding. Since it’s an open-arc process, there isn’t much of a chance to prevent carbon migration. You’ll also have to use nickel wire if you’re TIG welding cast iron, which is pretty pricey.
- If you’re going to stick weld cast iron, you’re on the right track – it’s one of the most simple and effective techniques for anyone who’s learning how to weld cast iron. You do have to use the correct welding rods, though; ferro-nickel and pure nickel rods are preferred. With this technique, ferro-nickel will give you the strongest weld.
Decide on a pre-heat method and temperature
This step will depend on the welding technique you’re using; the main thing is to minimize the temperature gradient between the surfaces that are being welded and the rest of the metal. The higher the temperature produced by your welding method, the more you’ll need to pre-heat the metal.
What if you can’t pre-heat the metal? In that case, try cold welding or any other lower-temperature welding technique, and use rods or wires with a low melting point.
Pre-heat and cool the metal slowly
If you’re learning how to weld cast iron, keep in mind that the idea is to avoid sudden temperature changes. First you pre-heat the cast iron, then you complete the welding, then you let the metal gradually cool down.
Practical tips to prevent cracking when learning how to weld cast iron
Developing cracks is a pretty big risk if you’re welding cast iron, but there are a few strategies that could help prevent cracks in your newly welded masterpiece.
Perhaps the most important advice would be to manage the pre-heating and cooling very carefully. Pre-heat in an oven if possible; this heats the entire piece, which is what you want. When it’s time to cool it down, the best approach is to insulate it with wool blankets or hot sand, or even put it back in the oven at gradually decreasing temperature settings.
Another technique that minimizes the risk of cracking is called “peening”. The welder uses a ball-peen hammer to gently strike the weld while it’s still malleable; this reduces tensile stress as the cast iron cools.
Are you going to learn how to weld cast iron?
Welding cast iron may require more preparation and skill than some other metals do, but it’s a great way to advance your welding abilities. Whether you’re re-attaching the handle of a beloved cast iron skillet, repairing a piece of machinery, or taking care of whatever else life throws at you, you’ll be glad you figured out how to weld cast iron.
