Cast iron metalworking is a historic trade known for its difficulties and frustrations in which welders from around the globe today will be familiar with. Cast iron is becoming less common however, occasionally you will come across a job requiring cast iron welding.
In this article, we provide the basic steps to follow to successfully weld cast iron and improve your metal-bending skill set.
Can Cast Iron Be Welded?
As cast iron contains a much higher carbon content than other metals when it is welded it most frustratingly likes to crack. This cracking occurs as the carbon travels to the areas being welded or to the heat-affected zone of the material which in turn increases brittleness or hardness. Overly brittle or hardened material cracks easily under pressure and as the weld cools, the material continues to move and reform itself many times which increases the probability of cracks occurring. Thermal stresses also play a part in the difficulty of successfully welding cast iron as it is particularly sensitive to rapid heating and internal stress.
It is important to identify the alloy correctly before welding the cast iron. There are different types of cast iron and each variety reacts differently to welding therefore, ask the producer what you are receiving.
Can You MIG Weld Cast Iron?
Yes, however, the MIG welding process is not recommended when compared to other welding methods. The most obvious reason for this is that MIG welds on cast iron will not be as strong as an MMA stick or a TIG welded joint, which is the general point when trying to weld cast iron. MIG welds do not penetrate steel as efficiently as MMA or TIG and weld penetration is more important than normal when working cast iron. Penetration will also aid with the removal of deep impurities which are present in the material.
How to MIG Weld Cast Iron?
If required to MIG weld cast iron, we recommend the following:
An 80/20 ratio of argon gas/carbon dioxide is the best choice for most situations. (Tip: even though cast iron is naturally prone to rust, you should avoid this method if you’re overly worried about rust eventually developing on the surface of the material as these gas combinations can increase the likelihood of rust developing.)
Use Nickel Wire
Use nickel wire with a wire thickness to match the requirements of the joint to be welded.
Tacking cast iron together can be problematic, especially with MIG. Easier tacking can be accomplished with the help of a brazing wire; however, this is likely to result in a weak and impure weld which isn’t great for projects that are likely to experience stress or impact.
Depending on the job you can also ensure the weld’s overall mechanical strength by utilizing studs.
Preheating the material before any cast iron welding is important (view the pre-heating section within this article for more information).
What Is the Best Method to Weld Cast Iron?
Manual Metal Arc Welding (MMA) is the most effective method of welding cast parts together. The next best option would be TIG welding although excess heat at the localized areas can be a problem. But an experienced welder can work his way around and when done correctly TIG welding works really well for cast iron.
1. TIG Welding
TIG welding is a welding method that is the cleanest and arguably the strongest when performed correctly. These are important factors when working with cast iron. Use the following information to help with performing a successful TIG cast iron weld.
- Thoriated tungsten electrodes are recommended for cast iron (use a 1% or 2% thorium rod). A clean and sharpened tungsten electrode is a must when TIG welding cast iron to allow good flow and control over the weld and prevent contamination
- The nickel-based welding rods work best for cast iron. This is because they match the cast iron’s thermal expansion temperature and properties. Select a wire thickness to match the joint you are about to weld.
- We recommend a shielding gas mix for TIG welding cast iron-made up of either pure argon or 75% argon / 25% CO2. Without shielding gas, the weld will be brittle and again, will likely crack.
- Preheating the material to be welded.
- Try to do a couple of test runs with your welder on some scrap material from the cast iron you are about to weld. We do this to understand better what temperature to run your welder at, how fast we need to weld and what gas flow rate and wire thickness work best.
- Welding cold cast iron is possible but challenging. If you cannot preheat the metal, it is very important when attempting to TIG weld cold cast iron to not allow the metal to become overly hot. Do this by welding in stitches and allowing the material to cool each time.
2. ARC welding: Best Welding Method for Cast Iron
The ARC welding process (MMA) is arguably the most effective and easy way to weld cast iron if the correct processes are followed.
- The most important step is the selection of the correct welding rods. Select Pure Nickel and Ferro-Nickel welding rods specifically manufactured for welding the cast iron. The nickel within these rods usefully helps to limit the migration of carbon into the heat-affected zone and the weld metal. While the overall strength is compromised with the use of any nickel electrode, it is preferred as the much bigger problem is the occurrence of weld cracking which is minimized with nickel.
- Iron rods can also be utilized however due to their high carbon concentration are less advisable as they are only useful for small weld runs and where color matching of material is important.
How to Weld Cast Iron?
Here is your step-by-step guide on welding cast iron without cracking.
Preparing the metal pre-welding is a must-do process. It will remove impurities within the surface of the iron such as paint, grease, and other obstructive materials. Use an angle grinder with a 120-grit sanding disc to remove the surface of the to be welded area.
Preheating the material before welding is important with cast iron. Due to its poor ductility cast iron is prone to cracking when subjected to localized heating. During welding, the area near the welding reaches a very high temperature while the rest of the workpiece is relatively cold. This creates thermal stress which will result in cracks. The solution here is to preheat the entire workpiece.
Use propane or oxy-acetylene torch to evenly heat the cast iron piece to approximately 400°C or 750°F. Move the flame across the workpiece to heat the work uniformly. Remember that the melting point of gray cast iron is 2200°F (1204°C). If you reach this temperature it will start melting and you will damage the workpiece.
3. Welding Rods and Wire
Select nickel-based rod and wire for all welding processes as this will limit the carbon migration that leads to cracking. The thickness of the wire or rod is determined by the material to be welded in the thickness, type, and shape of the joint.
After you have performed the welding, the weld will start cooling rapidly and this stress can cause cracking. Peening (or dressing) is a technique to apply compressive stress to the weld while it is hot which can help prevent cracking.
Use a ball-peen hammer to strike the weld with moderate force while it is still hot. This process will stretch the weld bead and reduces stress.
4. Post Heating
The final step in the process is post-weld heating. After welding, the workpiece is very hot and the atmosphere around it is cool. This may again result in rapid cooling and cracking. The solution is to allow for slow cooling.
You can place the workpiece inside a pre-heated oven and allow it cool gradually.