How to Weld Cast Iron – 2020 Guide – All Facts You Should Know


One of the most common types of misconceptions about welding by those who are new to it or outside the welding world entirely is that one welder or type of welding is the same as another. In fact, nothing could be further from the truth. While it’s all welding, the different styles are highly distinct from one another and produce different results

Take cast iron welding, for example. The process is a lot more labor-intensive than other types of welding, and the type of metal they work with is quite astonishing. Cast iron can be quite strong when you work with it properly. That said, once again, not every type of cast iron is the same, so you need to know what you’re working with and how you’re working with these materials in the first place.

So let’s take a look at what cast iron welding is all about and how you can master this rewarding technique.

Choosing From Different Types

One of the most important things to keep in mind is that there are different types of cast iron welding. These types are made from different materials, and not accounting for these differences and how they are welded can lead to cracks or accidents. This can cost hundreds or even thousands of dollars, not to mention the safety hazard it can represent.

First and foremost, you need to determine what type of cast iron you are welding and how you intend to work with it. As mentioned above, there are actually different types of cast iron welding to consider, and you don’t want to get your materials and methods mixed up.

The primary types of cast iron you can weld include:

  • Gray Iron: This is one of the most common types of cast iron available to weld with. It can be done, but as demonstrated, it isn’t easy and takes some training.
  • White Iron: This is so unwieldy that it’s essentially unweldable for all but the most accomplished welding whizzes, and even then you’d better be sure it’s absolutely worth it.
  • Ductile Iron: Rare, and slow to weld, but if you can manage to find it, this is one of the more easily weldable types of cast iron on this list.
  • Malleable Iron: The malleability of this type of iron is precisely why it’s essentially useless for welding purposes.

Which type of cast iron should you attempt to weld? That will depend on the type of project you’re working on and whatever the product manual calls for. Gray cast iron is by far the most common type of cast iron to be welded, and the one to keep in mind through the rest of this article.

This is also the only type of cast iron you should use until you get more experience working with it, at which point you can attempt to work with other types if you’re so inclined.

If you are welding with another type of cast iron, you’ll want to make sure that it can indeed work with your project.

Finally, if you’ve never welded cast iron before, it can get quite intense, so having someone with experience on hand to help is advisable.

Stick Welding Cast Iron Repair >> Check out the video below:

Choosing Your Welding Process

Now that you have chosen the right type of cast iron to work with (which, again, for our purposes is going to be gray cast iron, and it almost certainly should be for you), you now need to give some thought as to how to weld it.

It isn’t as though gray cast iron is like putty in your hand. You cannot just mold it to your will in a casual fashion. As mentioned, it is especially difficult to weld cast iron, and that job can be a lot harder if you aren’t using the right type of welder for the job.

In this case, that’s a TIG welder. While MIG welders are more commonly used for a lot of other welding jobs, TIG welding is superior for jobs such as welding steel, and such is the case with cast iron as well.

There are many reasons for this, not the least of which being the fact that TIG welders are much better at managing spattering for cast iron. TIG welders are actually better than not just MIG but Stick welders at this, in large part because they are able to avoid damage to the surfaces being welded.

Related reading: What´s The Difference Between TIG and MIG Welding?

Trying to weld a cast-iron surface with a MIG or Stick welder typically kicks up so much spatter that it can lead to serious damage – and given the aforementioned issues with cast iron cracking, you don’t want to exacerbate those issues any further. TIG welding should enable you to avoid that issue.

If that isn’t an option, you can try alternative measures, specifically oxyacetylene welding. The benefit of using oxyacetylene is that it can help manage the extreme heat necessary to weld cast iron effectively which, again, can lead to cracking and greater damage if you aren’t careful.

Then there’s brazing. This isn’t really welding so much as welding as it is the next best thing if you’re looking to join joints. Brazing involves joining two or more pieces together by melting filler metal into a joint, with the filler metal being kept at a lower melting point than the metal piece into which it is flowing and ultimately being attached.

Related reading: How Does TIG Welding Work?

That said because everything depends on how well these metals flow and stick together, you need to make sure that the surfaces are not rusty and are clean, lest any particulates contaminate, and complicate the brazing process.

Finally, there is MIG and Stick welding. Yes, we said that they aren’t the top choices due to their huge splattering issues, but if you choose consumables that mitigate that factor, they can do in a pinch. That said, TIG welding is still a much better choice all things being even. You also need to be extremely careful that you are using the right filler metal, or MIG and Stick welding can cause extreme cracking.

Choosing Your Filler Wire, and Electrodes

Next, you’re going to need to make sure you are using the correct consumables and other components. As mentioned above with MIG welders, this can make a big difference in how much splattering occurs and mitigate the risk of damage, but this is essential for TIG welders as well.

This is especially important as trying to weld cast iron is already like walking a tightrope, and your filler wire choice can make that high wire act even more perilous.

There are only a few select filler wires and electrodes that can work well with cast iron, so you’ll need to make sure that you’re working with a brand and model that explicitly says it’s okay for this purpose.

Most types of filler wires (and electrodes) that are not designed with cast iron in particular in mind cool too quickly, which is one of the ways that the aforementioned cracking can occur.

Related reading: What Welding Rod to Use for Cast Iron?

In addition, these types of consumables and filler wires can lead to the metals not properly fusing with the cast iron materials.

One key factor is determining the right type of electrode and consumable to use – nickel content. Nickel can be hugely helpful as it does not cool too quickly, meaning that you can avoid the problems associated with that.

However, manufacturers of consumable electrodes know this and take advantage of that fact to make these among the most expensive options on the market. Nickel has the additional drawback of not being very well suited for welding thicker sheets of metal.

Ideally, you want to work with a 55/45 ratio of nickel to iron. Not only is this more affordable than if you welded with just cast iron, but it is also easier and more machinable to work with.

That being said, it is still much harder to work with than a 99% weld, which has a lower threshold for shrinkage than iron rods, which minimizes the risk of cracking taking place.

If you weld with other types of iron, they can produce different colors than cast iron. Stainless steel, meanwhile, when fused with the austenite properties of stainless steel, does not harden as drastically or change as rapidly as with other iron consumables. It does expand and contract a lot more during the heating and cooling process, however, so you’ll need to account for that.

Finally, if you are brazing, you might look to brazing bronze. Using these materials can be a good way of avoiding cracks between the joints.

Cleaning and Tending to Weld Surfaces

As mentioned, you’ll need to make sure you keep the surface of the metal clean. Additionally, you must make sure the consumables themselves are not contaminated, so take the time to clean them before each use.

Peening can help with this. It is the process of tapping a soft weld with a ball-peen hammer (hence the name) while it cools so as to check the stability of the weld and help prevent cracking. That said, you should do this with care so as not to deform the weld.

Regulating the Temperature

One of the biggest debates in terms of how to weld cast iron is the method for preheating. The type of preheating that works best with cast iron welding is disputed, with some preferring minimal heat while others preferring other methods and higher and lower temperatures.

That said, preheating is essential, because cast iron is incredibly brittle compared to other types of metals, so you have to heat it slowly so you can work with it gently as it expands and contracts. Cast iron starts to change its properties around 1400 Fahrenheit. Welding at lower temperatures via minimal heat can help reduce the overall temperature and thus avoid this threshold.

However, while these lower temperatures can mitigate these concerns, the internal stress on a cast iron still takes place, which can take its toll over the lifespan of the weld. That said, this can produce a weaker weld than other methods.

Above all, it needs to be stressed that you need to let your cast iron weld cool nice and slowly. Do not quench the weld with water or blast it with a concentrated burst of compressed air.

These rapid changes are one of the most surefire ways of causing cracks and potentially destroying your entire weld. As a general rule, the slower your cast iron weld cools (and this can be days) the better.

Frequently Asked Questions

Here are a few common questions people often have about Cast Iron Welding:

Can I weld cast iron with a MIG welder?

Can you weld cast iron with a MIG welder? Yes, you can do it, Although many experts would advise against using a MIG welder to weld cast iron, it is quite possible to do. It is best to consider these tips if you plan to use your MIG to weld cast iron.

Cast iron comes in different qualities and ratios of iron, carbon, and steel and you should be sure of the cast iron you choose to weld. Engine blocks, engine parts, and even pistons are a safe bet, pieces from a quality source (you can make a good guess of the cast-iron quality due to what purpose the piece served.)

  • Use specialty Nickel-based electrodes as your first choice. This is expensive but will produce welds that are machinable. If costs are a factor, you may consider 55% Nickel rods.
  • An 80/20 mix of Argon and Carbon Dioxide gas is your best option, although this option may rust in the future
  • Using brazing wire is an option but try to avoid metals that will be used for loading or will experience impact.
  • Studding may provide greater mechanical strength to the weld, job dependent
  • Low and slow is the way to proceed when MIG welding cast iron
  • Preheat the iron and let it cool down slowly

The studding method can be used for major breaks in large casings. holes are drilled and taped over the prepared surface and screw steel studs into the threaded holes.

The trick is to leave 3/16″(5mm to 1/4″ (6mm) of the stud above the surface. Then weld the studs into the cast iron and cover the entire surface of the break with weld deposit. One the deposit is made the two sides of the crack can then be welded.

Before welding, you must:

  • Strip the item of all accessories and external loads
  • If cracked, drill a small stop hole at each end of the crack of about 1/8″ (3mm)
  • Clean the crack by milling or hand grinding to provide a V-shape channel with an opening of 60 degrees reaching the bottom of the crack. In thin sections, you may need backing to get a full penetration on your root pass.
  • Clean the area of all grease and contaminants Create a 15-minute burnout of 480 degrees Celcius (900 Fahrenheit)  and brush well to remove residue.

For preheating, it is always preferable to heat the entire cast iron casing rather than just a section. If cast iron is taken from room-temperature to welding temperature too fast it will crack from the stress.

Preheating may be done by using a blowtorch to heat the material in a pinch and the ideal pre-heat is between 260-650 degrees Celcius.

To aid the cast iron to cool slowly to prevent cracking, welders may wrap the object in an insulated wrapping blanket. If it is a smaller piece, it can be placed in a bucket of sand.

Does heating cast iron weaken it?

Does heating cast iron weaken it? No, the biggest stress on cast iron in the welding process is not the heat itself but uneven heat distribution in the welding process itself. Many times cast iron is blamed by poor welding techniques. The principle lies in the manner in which the cast iron expands and contracts during the heating process.

Restricted expansion while heating and restricted contraction when cooling are the two biggest factors involved in cast iron cracks. Unlike steel which has a tensile strength much higher than its yield strength, cast iron has equal yield and tensile strength.

Due to the carbon in cast iron, it will not yield to the contraction strain (stretch) before it breaks. Cast Iron does not stretch or bend. Providing heat of 1,200 degrees Fahrenheit allows the iron an artificial yield point, allowing it to stretch.

That being said, it is impossible to heat cast iron in a space that restricts expansion/contraction without causing stress or fractures. If a cast-iron object is secured by two sides while heated, it will not have the area to expand and might fracture. 

likewise, a cast-iron item that is heated in a localized potion that is surrounded by cooler metal, its expansion will be inhibited and it will crack. End pieces may be welded in an easier manner because they have sufficient area to expand without restrictions of the surrounding material.

 Hardening occurs when cast iron is heated beyond 1200 degrees and then cooled too quickly. Heat-retardant materials should be used to cool the cast iron slowly or if possible the item should be covered by sand to provide insulation while cooling.

So it is the inconsistent heat that leads to the cracking/hardening and sometimes with a cold weld can damage cast iron without a crack appearing. Cast iron can store stress a long time before it cracks

A critical temperature in most cast iron is about 1400 degrees Fahrenheit. While the arc does exceed these temperatures, the cast iron must not remain at these temperatures for extended periods.

Can you weld a cast iron with a stainless rod?

Can you weld a cast iron with a stainless rod? Yes, it is possible, however, stainless steel rods are not an ideal choice when it comes to welding cast iron. The high amount of chromium in the filler and the high carbon content of the cast iron make a very brittle chromium carbide.

Steel rods can be used for simple repairs and fillers and are handing when working with hard to clean surfaces

For cast iron welding a nickel-type electrode is needed;

  • Use Tech-Rod 99 stick electrode for single-pass welds,
  • Tech-Rod 55 for multiple pass welds
  • Where the weld is expected to rust, use the Lincoln Ferroweld stick electrode.

The best rods for casting are the Nickel and the 55% Nickle rods which are good in repairing thick sections and have a lower co-efficient expansion than the 99% nickel rod which means fewer cracks in the fusion line.

Can Cast Iron be glued?

Can cast iron be glued? No, gluing is not really a viable option for load-bearing cast iron. Bear in mind that glue is never an alternative in cast-iron pots and pans or any cast iron item that experiences heating. For smaller items and decorative features you may glue cast iron easily if you follow the following steps:

Choose your Epoxy adhesive

Epoxies are created by the polymerizing of a mixture of resin and hardener. When the resin mixes with a certain catalyst, curing begins. curing is the process of molecular chains react to chemicals and form covalent bonds, They are a great alternative to welding due to their excellent adhesion in metals.

Clean the surface properly and then wipe the item down with acetone. This degreasing is essential for the glue to set

Sand the surfaces that will be glued lightly with 120-to 200-grit sandpaper. This sanding will provide grip for the glue. 

Wipe with acetone again to remove any of the filings or powder left after the sanding.

Choose a cold-weld type, steel-enforced epoxy typically called a liquid weld. The cold weld will come in two tubes, the hardener, and the resin.

Mix the epoxy and the hardener in equal parts on a disposable tray (use a dowel or wooden stick.) Apply an even coat to one cast iron part and press it into the other.

Allow to set for about 25 minutes or according to the manufacturer and let the object stand for 24 hours. Because of the nature of the epoxy glue, the curing and bonding process is one that occurs over several hours, so it is best to be patient and wait.

Glues are an excellent alternative to mend cast iron chips or breaks, but they do degrade over time and will most probably have to be re-glued in the future.

Does cast iron get brittle with age?

Does cast iron get brittle with age? Yes, cast iron gets brittle with age because the longevity of a cast-iron object depends mostly on the stress and strains that it experiences and the conditions in which it is kept. Cast iron is an incredibly durable material, which is why it can be so tricky to weld. The longevity of a cast-iron object depends mostly on the stress and strains that it experiences and the conditions in which it is kept. 

Cast iron is not particularly susceptible to rust and corrosion as but is known for being sensitive to extremely high temperatures. Cast iron cookware has an extended life capacity beyond its steel and copper peers if it is looked after properly. It should be protected from heat extremes such as cold water while hot, and it may last a lifetime.

A case in point is the cast-iron piping found in many older homes that are still functioning after 100 years. This longevity is despite the pressure of constant water and extensive time in damp conditions underground.

Gray irons, the most common form of cast iron has a tensile strength of about 14 MPa (20,000 psi.) Cast iron has great compressive strength, up to four times their tensile strength.

That being said, cast iron is brittle in its fundamental structure. In order to hold such great compressive strength, cast iron sacrifices its tensile strength and its ability to stretch and bend easily.

Cast iron has a percentage of carbon, roughly 2.11% which lends it its brittle nature. But being brittle does not mean that cast iron is weak, cast iron can hold incredibly heavy loads and absorb vibrations from heavy machinery.

Our Conclusion

Cast iron welding is one of the most challenging methods by far. It has one of the smallest margins for error and is extremely unforgiving. The brittleness of the material adds to this difficulty considerably.  The slightest mistake or the tiniest hairline fracture could cause the whole thing to come crumbling apart.

All of this means that these welds take far longer to prepare than others. It is therefore imperative to follow the steps mentioned above as well as any other tips you may find for welding cast iron materials in a safe and effective manner to the letter. Take the time to make sure that you don’t rush and allow the metal to cool evenly and slowly.

By now, it should be clear why cast iron welding is considered to be one of the most difficult welds among professional and hobbyist welders. It should thus also be apparent why experience is so necessary when working with them.

That said, it has hopefully also been made clear why cast iron welding is also of great interest to so many. Rarer fruit is always enticing, and that’s certainly true with cast iron welding. It’s less common than other welds, and can likewise produce results that are uniquely powerful and aesthetically pleasing – as long as they are done right.

By following the steps outlined here and seeking out the assistance of experienced personnel if necessary, you can make sure your cast iron welding adventure leaves you with a result worthy of the effort.

David Harper

David is the Co-Founder and Senior Editor at David's an experienced fitter and tuner/welder who's passionate about helping others develop in life through new skills and opportunities.

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