Welding cast iron is one of the most challenging jobs a welder can do. One mistake, and you could make things a whole lot worse. You could end up doing more damage, or worse still, produce a weld that looks solid but, is in fact, weak.
Be it ductile iron, gray, or any other type; the trick is to follow the correct procedure and choose the right filler material when you weld cast iron. Here is our guide to welding cast iron.
- Understand the type of cast iron: Cast iron comes in different types like gray, white, ductile, and malleable. Knowing the type helps determine its weldability and the right approach to take.
- Choose the right welding rod: Selecting the appropriate welding rod, such as nickel or steel, is crucial for ensuring a strong and durable weld.
- Preheat and post-heat the cast iron: Preheating before welding and allowing the iron to cool slowly after welding helps prevent stress cracks and ensures a stronger weld.
- Select the proper welding technique: MIG, TIG, or Arc welding can be used for welding cast iron, but each method has its merits and downsides. Choose the one that best suits your project and skillset.
What Type of Cast Iron Is Not Weldable?
There are several types of cast iron, but some are more challenging to weld than others. More often than not, you will achieve about a 50 percent success rate.
Gray is the most common type of iron, and it is widely used. It gets its name from the color of the fracture it forms, thanks to high levels of graphite. This type of iron is weldable, but with great difficulty.
White iron is hard and brittle. It is similar to gray iron in that carbon is present, but unlike gray iron, graphite does not form during solidification. White iron has uses in applications where abrasion resistance is imperative over brittleness. You find white iron in slurry pumps, cement mixers, flanges and pipe fittings.
White iron is almost impossible to weld.
Ductile iron is far less brittle than cast iron, so it bends without breaking. Widely used in the automotive industry and other heavy industrial settings, it is the ideal material for making wheels, pump housings, gearboxes, piping, and many other items.
Ductile iron is difficult to weld, but it can be achieved under the right conditions and with patience and skill. It is not something that should be undertaken by an inexperienced welder.
Malleable iron is not weldable because the heating process changes the material’s properties, making it useless. It starts out as white iron, but through heat treatment, it becomes malleable.
How To Weld Cast Iron
Once you’ve determined the type of iron you have, the next step is to decide whether you will preheat or cold weld.
1. Preheat or Cold Weld
There are two schools of thought on how to weld iron. Some say preheating is the best option, while others swear that cold welding is the right approach. Cast iron is brittle and cannot be bent or molded when hot.
Heating cast iron has a deforming effect on the metal, raising the potential that one part of the metal may heat or cool faster than another, causing cracking and stress fractures. At 1,400 degrees Fahrenheit, cast iron changes characteristics completely, potentially weakening the integrity of the metal.
By cold welding, the metal is placed under less strain, reducing the risk of cracking and warping, potentially creating a stronger weld. However, cold welding is not without its risks. Internal stresses may still occur and manifest themselves later, plus cold welding produces a weaker weld.
For this reason, experienced welders prefer the preheating method because they understand how the metal reacts and can control the situation to create a stronger weld.
2. Choose the Type of Welding Rod
The type and material of the welding rod dictate the quality of the weld.
Nickel Rod 99 Percent
These are considered premium welding rods. Nickel-based electrodes are expensive, but they are the best at producing welds with low to medium phosphate content and that is machinable.
Nickel Rod 55 Percent
These are a cheaper option than 99 percent electrodes, and they have a lower coefficient expansion, which produces less stress on the metal and fewer fusion cracks. This makes them better at repairing thicker sections of metal. These rods are also machinable.
Steel rods are the cheapest of the three options, making them the most economical. They are best suited to simple repair work and fillers. These electrodes are not machinable and require grinding to get the smoothest finish.
Steel rods are more user-friendly and can tolerate castings that are not entirely clean. It makes them the perfect choice when performing on-site repairs where the conditions may not be ideal.
3. Prepare the Cast Iron
Cast iron is temperamental, so make sure you pre and post-heat for the best results. The carbon within the metal causes the material to shrink and expand. For this reason, as much care should be taken with the pre and post-applications of the weld as to the process of welding itself.
Here’s what you need to be mindful of:
- Pre and post-heating preparation.
- Heat management when welding.
- Rod selection.
The process begins as follows.
Beveling the Fracture
Beveling is the process of cutting V-shapes using a cutting torch or a specialist beveling machine. The bevel makes a stronger bond between the metal surfaces and ensures that it won’t shear off at a later date.
Beveled but joints are far stronger than flat but joints.
Preheat the Cast Iron
Preheating can be done by applying a blowtorch to the surface. You can even preheat the metal by placing it in the charcoals of a lit barbecue. The idea behind preheating the cast iron is to slowly increase the metal’s temperature, ensuring a slower transition from room temperature to welding temperature. Think of it like transitioning the metal so it has less of a shock.
The ideal preheat temperature is between 250 and 650 degrees Celsius (482 to 1202 degrees Fahrenheit). Anything over 780 degrees Celsius will melt the metal.
Heating the Material
Both ends of the plates are heated steadily and slowly so as not to distort the properties of the cast iron. The plates are then joined together when the sufficient temperature for the metal gets reached. If an arc weld is used, slag will form, which needs removing with a hammer.
When MIG welding, no slag forms, thanks to the protective gas insert, ensuring the integrity and purity of the weld.
Post Heating the Cast Iron
Just as you transition the metal to cope better with the rise in temperature, you want it to acclimatize to the cooling process to prevent stress cracks and a weakening of the material.
Once done, you could use the same heated charcoal to post heat the metal, allowing the temperature to decline gradually. A bucket of sand will do the same thing, or placing the piece atop a wood fire oven will allow the metal to cool slowly.
4. Choose the Welding Technique
There are three ways to weld cast iron. Each has its merits and downsides.
MIG Welding Cast Iron
- Choose specialty nickel wire for the best results. It is the most expensive, especially if you choose the 99 percent purity, but you can buy 55 percent purity if you want to save some money. If the budget is too tight, choose stainless steel rods, but expect to have to rework the finishes as steel is not machinable and provides the roughest finish.
- 80 percent argon and 20 percent carbon dioxide gas gives the best results in most situations. The downside is that the weld will rust eventually. That said, cast iron is naturally prone to rust.
- Brazing wire will bond the metals, but it creates a weak joint that will not stand up well to impact.
- Studding is a possibility to add strength to the joint, but that depends on the job and the neatness of finish you require.
TIG Welding Cast Iron
TIG welding is only possible if you use nickel wire. The upside is that you get a strong and clean weld, while the downside is the cost of the nickel wire. You also might be able to skip the pre and post heat process if you use the correct gas, wire and heat settings.
The gas mix should be 75 percent argon and 25 percent carbon dioxide. This mix stops the weld from becoming brittle.
- Avoid rapid cooling as this places the iron under stress, increasing the chances of fractures. Use the pre and post-heat process for the best results.
- Avoid weld heat by using short weld beads. This stops too much heat from affecting the cast iron. Avoid one continuous weld line as this could cause cracking, especially if the weld and the cast iron are two different temperatures. Once you’ve created the weld bead, hammer it smooth. It increases the strength of the joint and alleviates shrinkage.
Arc Welding Cast Iron
Arc, or stick welding, is a favorite among home welders and DIY’ers. It’s affordable, and there are many different electrodes available to suit various tasks. It gives you flexibility and versatility. Most general repair tasks need a bronze rod, acetylene torch and some flux. This is the cheapest option, although you should be aware that brazing rarely bonds as well as other forms of welding.
- Clean each joint before welding. This ensures the hardest bond. Dirt in the joint creates weak spots that might come back to haunt you later.
- To get the neatest weld, lower the current to reduce splatter. It doesn’t affect the quality of the joint, but aesthetically, it produces a cleaner finish.
Getting the smoothest finish is the key to a neat and professional job. Using higher quality nickel electrodes does this without the need for grinding.
Post Heating and Cooling
The longer a weld project takes to cool, the better the joint is likely to be. Rapid heating and cooling are the enemies of a strong weld. Iron is brittle, and when heated, it distorts. This causes stress on the material, which weakens the integrity of your structure.
By easing the iron into a cool state after welding, you ensure a natural easing of the metal and a better result. Think of it like letting a juicy steak relax after frying to tenderize it. Metal does the same.
Peening is a way of preventing the shrinking force of the weld as it cools. It helps to remove tensile stresses created when the metal reduces in temperature. Using a small hammer to spread the weld when it is still malleable to work out any air pockets or stress fractures, makes the overall finish a lot stronger.
This method takes great care, because over peening could result in cracking as the weld cools too much, and weakens the joint.
Additional Tips for Welding Cast Iron
There are always hints and tips that make life easier, so here are a few to get you started.
- Keep the welding rod vertical when welding cast iron. It differs from most stick welding where you weld at an angle.
- Take it slow and steady. Cast iron does not weld quickly.
- Limit the amount you weld in one go. Use beads of weld rather than a continuous line.
- When repairing a crack, run a small amount of beading across each end to stop it spreading.
- Keep a steady hand and maintain a constant speed while you are working.
Cast Iron Conclusion
Be under no illusions that some metals don’t like to be welded, and cast iron is one of them. That said, it isn’t impossible. The watchwords are slow and steady because you cannot rush cast iron thanks to its brittle qualities.
Get the preparation right, and the rest will follow. Also, make sure you allow the iron time to cool after working with it. If you do these things, you should have a successful outcome.