Cast iron is a great material to weld. It’s strong and can withstand a lot of heat. It contains a higher than usual proportion of carbon. This makes it harder and more brittle than other types of iron, but also gives it better resistance to wear and tear.
Because of this, cast iron is often used for machinery components, tools, and other objects that need to withstand heavy use. You guessed it right, this makes it a great candidate for welding as well. However, cast iron is a tough material to weld.
With the right technique, you can weld cast iron with a mig welder. Although MMA welding is considered a better option. Even so, if MIG welding is the one you’ve decided on, then scrolling down will show you how you can weld cast iron with a mig welder.
How to weld cast iron with a MIG welder
A MIG welder can be used to weld cast iron. But for that to be successful, you need to take some precautions to avoid unwanted outcomes. This includes preparing the cast iron for the process, i.e., preheating it and cleaning it.
Step 1: Identify The Cast Iron Grade
First, identify the cast iron grade if possible. This will help you to understand what you are dealing with. Most of the available cast iron items are of gray cast iron, but they also can be malleable cast iron, ductile cast iron, or white cast iron.
Step 2: Clean The Metal
Now clean the metal. This might require you to use various solvents depending on the type of use or wear and tear the thing has gone through. Cleaning will allow a more consistent and stronger weld because impurities and dirt tend to meddle with its quality.
Remember that cast iron is a porous material. So if it has been in contact with oil, chances are it might have absorbed it. In such cases, acetone will not be enough to clean it. Using a break oil will be able to do the job. Make sure, however, that it is chlorine-free.
Step 3: Prepare The Casting
At this point in the process, you need to prepare the casting. It can be done with a chisel and a hammer, and then use a grinder. Afterwards, clean off the residue with a file.
Step 4: Heating The Iron
Then comes one of the most crucial steps, heating. Now, remember that cast iron does not handle fast heat or fast cooling well. That is why we are going to slowly heat the entire part. If it is small, this can be done in an oven. Otherwise, a flame torch can do the job.
Step 5: Set The MIG Welder
Before you start welding, set the MIG welder to a lower amperage and lower heat setting, as higher heat and amperage might break the iron. Make sure, however, that it is not too low, or else the resulting bond will become weak. The best condition for this would be the short-circuit mode.
Step 6: Precautions While Welding
While welding, use small beads to weld so that uneven temperatures can be avoided, and make sure to peen each bead immediately afterwards.
After you are done welding, try managing the heat so that it escapes slowly. This can be done by covering the piece with an insulator like sand. It will trap the heat while it cools down slowly.
Welding cast iron with MIG welder without preheating
Oftentimes, pre-heating is really not an option. It is especially true when dealing with large structures of cast iron that cannot be uniformly heated.
What to do in those cases?
Like welding done with preheating, you will have to weld very short beads and peen each bead immediately afterwards while it’s still red-hot. You will also need to use the lowest voltage and wire speed setting.
However, the main step here will be to stop welding after peening and wait till the bead cools down enough so that you can touch it with your fingers easily.
Despite these precautions though, welding without preheating will always be a risky business and the resulting weld will always be weaker than if it was done with preheating. There will always be a risk of breaking down.
Why is cast iron a difficult material to weld?
There are several problems when welding with cast iron. Cast iron is more brittle than you think it is, and especially so when it comes to welding. What MIG welding does is it heats the metal fast- something cast iron cannot handle. It is most likely to result in cracks when high heat is applied to it fast or if a heated cast iron is cooled down fast.
Why does this happen?
Firstly, it contains a high amount of carbon, which can be as high as 4%. This carbon happens to move during welding, resulting in cracks.
Secondly, cast iron has many types and grades and in most cases, you cannot really tell them apart. Thus, if you neither know the alloy of iron nor do you know the grade, you are most likely to be dealing with unpredictable behavior from the material which would eventually end in an unsuccessful welding session.
Which wire is best for MIG welding cast iron?
The MIG wire that is most suitable for cast iron is ERNiFe-CI (or ERNi55). This wire has higher tensile strength and ductility than the ERNi-CI (or ERNi99). Almost all ductile irons, dirty castings, and cast iron to steel can be welded with this process.
The welded metal has excellent machinability too. ERNiFe-CI has two more important characteristics that make them the best contenders for the job – they shrink less and resist the absorption of carbon. This makes cracking after the welding much rarer.
Can you weld cast iron with mild steel wire?
Yes, you can weld cast iron with mild steel wire but the result will be far from what you want. In such welding, the metal shrinks and usually absorbs a lot of carbon, making it brittle and hard. The weld may not hold when it is encountered with force or pressure.
Can you use flux-cored wire for cast iron?
Yes, you can weld cast iron using a Ni55FC flux-cored wire.
However, it is for professionals and hence there are a few downsides to this particular wire. Firstly, it is expensive. Secondly, smaller spools of this wire are usually not available. Thirdly, it does not weld thin castings. The final disadvantage is that it needs a shielding gas as it is not self-shielded. You should check the instructions for proper serrated rollers and polarity.
Bottom Line
Cast Iron is a widely used material especially in engineering, machinery, equipment, tools, and hardware. As a result, cast iron comes to the welder’s table very often. With the right tools, settings, process, and a great deal of care, the apparently impossible task of welding cast iron can be done smoothly. It is a technique you can easily master if you can work with patience, so that cast iron will cease to dread you.
