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The high carbon content (2-4%) of cast iron makes it a different beast than working with regular steel.
The high carbon content makes cast iron extremely durable, and it can offer decades of unwavering use. However, it is also more brittle, and when exposed to rapidly changing temperatures, or extreme stress, it will fail.
There are two major types of cast iron.
- Gray Cast Iron – This is the most common cast iron. It has less tensile strength, but since the carbon is precipitated out as graphite during the forging process, it is slightly more ductile and is weldable.
- White Cast Iron – White cast iron has the carbon in the form of iron carbide. This makes it too hard and brittle and is considered unweldable. It is used in the liners of cement mixtures, in extrusion nozzles for manufacturing and in slurry pumps.
When cast iron cracks, the owners just give up and throw it away.
However, our readers who own welders are wanting to try welding weld cast iron.
The answer is that both stick welding and TIG welding lend themselves to cast iron repair. It is do-able with MIG but isn’t typically the first choice. You can also use Oxy-acetylene to braze the metal back together.
When we are talking about something like an engine block being able to weld cast iron can mean saving hundreds or thousands of dollars.
Welding Cast iron requires more care but is quite doable.
It shouldn’t be undertaken as your first project. You should also choose some cheaper pieces of cast iron to play with, first, before repairing a high-dollar item.
The Steps To Welding Cast Iron
1. Clean to Shiny Metal
Cast iron often has heavy pits and rust deposits. Or, when it breaks off, it does so with a lot of rough edges and granulation.
In order to have any hope of welding it, you will want to get some flap discs for your angle grinder and shine it to a smooth, bright shine. (This will work with a solid grinding disc if you prefer).
Take some time to grind out the crack as well and make sure that you have a shiny surface for good adherence to all parts of the crack. If it is a hairline crack on the surface, and you try welding it, you’ll only cover the damage without repairing the deeper crack.
Open those small cracks up with a grinding wheel so you can get full penetration. While you are doing that, bevel the edges of the crack so that it is wider near the top.
2. Identify The Crack
Now that you are getting through the rusted area clean off the metal with some kerosene and a rag. This will help highlight the crack and show any hairline fractures that you might have missed.
Continue grinding and cleaning until all of the damaged areas are exposed.
3. Drill The Crack
If you don’t drill out both ends of a crack, it is likely that it will re-occur from residual stresses.
When repairing a crack, you want to drill a small, 1/8-inch hole at each end. This increases the crack tip radius and distributes the cracking pressure across a larger radius.
By doing so, you disrupt the crack and stop it from re-establishing itself as easily.
You’ll want to drill the thickness of the crack. A more superficial crack will need shallower holes drilled at each end.
4. Start With Pre-Heating
Cast iron responds poorly to dramatic temperature swings. If you try welding without pre-heat, it is likely that you will hear a “tinkling” noise, which comes from fractures in the metal due to the extreme temperature swings.
It helps to pre-heat it with a torch and to get the piece up to 500 degrees or so. This keeps it from having to jump straight from room temperature up to welding temperature.
If you heat cast iron too quickly, it will crack. The same thing goes for cooling it off too fast.
The other thing to consider is that once you get started welding, you’ll want to keep at it until the job is done (taking frequent mini-breaks as you’ll see below). If you break long enough for the metal to cool, you’ll need to use the propane torch and reheat the piece of metal.
If you heat the metal just to the point that it starts to glow, you are going to have it heated to about 900 degrees. Work slowly and try to heat the piece as evenly as possible with the torch.
This also helps clean off the surface of oils and other residues.
5. Don’t Overheat the Metal
Use the lowest current possible to still get a good weld. It is possible to weld cast iron with as little as 50 amps.
It is easy to overheat cast iron with a welding arc. The arc is much hotter than the 1,450 degrees Fahrenheit melting point of cast iron.
Because of this, it is recommended to weld using short, 1-inch segments.
A good way to do this is to weld one end of the crack, and then move to the other end and weld the other side. By working from end to end (or “skip welding”), you can get the crack filled more quickly without overheating it.
Since you have to restrike the arc for each 1-inch segment, keep a file handy. You can briefly brush over the edge of your last weld bead and get a fresh surface for getting the next arc started.
Immediately following each 1-inch section, hit that weld with a ball-peen hammer to relieve any stress in the weld.
This helps keep the weld from forming in a manner that will cause it to crack later.
7. Slow Cooling
Air cooling is too fast of a cooling rate for cast iron. You’ll want to use the torch to slowly reheat the metal and slow the cooling rate.
Alternatively, you can bury it in the sand or wrap it in a fireproof blanket.
The best cooling rate for cast iron is about 50 degrees per hour. From a 1,000 degree heat, it should take nearly a day for the metal to cool to touch.
Welding Techniques and Filler Metals
For welding processes that require shielding gas, a 98% argon/ 2% carbon dioxide mixture works quite reliably.
MIG Welding Cast Iron
While it is possible to MIG weld cast iron, it isn’t the preferred method. If you are going to try that, a nickel-based wire can work well. These wires are pricier than some of the other items but are soft enough to apply at a lower amperage while being durable enough for machining later.
Another idea that works well for partial cracks for filling in the affected area is to go with a brazing wire. This softer wire works well for filling in gaps in cast iron without overheating the surrounding cast iron.
Stick Welding Cast Iron
Stick welding (shielded metal arc welding or SMAW) is one of the more popular ways to repair cast iron. It uses a consumable electrode rod covered in flux that also acts as the filler material.
A nickel-iron rod is great for joining dirty cast iron, and iron submerged in oil. Mixed metal rods are popular as well. A pure nickel welding rod can work great for cast iron and looks like nickel when you are laying it down. This should work with AC/DC currents.
For repairing cracks in engine blocks and manifolds and for keeping the cracks from reforming, the
Muggy 77 welding rod is the one most people choose. It requires you to prepare the metal well, but then uses a combination of three metals to provide a more resilient bond and offers better stretch capabilities for scenarios where the metal heat and cools regularly (such as an exhaust manifold). It has limited porosity and is easy for novices to master.
TIG Welding Cast Iron
As with the other types, TIG welding works well if you choose consumables with high nickel content. Blue Demon makes a good 99% nickel rod that works well for Tig. Using a high pulse on your TIG setup works well, if you have it, and the foot pedal can help you keep from overheating the metal as you work.