Welding cast iron is really hard. Even professionals who weld on it often will have some of their welds fail due to the nature of the metal. You could use the perfect technique with the right amount of heat and a matching filler metal, and it’ll still crack.

As its name suggests, cast iron is formed by pouring molten pig iron (iron ore from a blast furnace) into a mould (or cast) and letting it cool and solidify as one whole piece. That means, for the most part, if you’re welding cast iron, it’ll most likely be for a repair job, usually because it’s cracked.

We’ve covered everything you need to know about cast iron, what’s so difficult about it, and how to increase your chances of success when welding it.

What You Need to Know Before Welding Cast Iron

Cast iron has a 2% – 4% carbon content, which is about ten times the amount of carbon in most steels. The higher the carbon count, the harder the cast iron, but more carbon also makes it more brittle.

The high carbon count means the cast iron is already brittle, but welding on it often increases the amount of carbon in the weld zone. That makes it even more brittle as it’s being worked on, increasing the chances of the weld failing.

There are several different types of cast iron, each with a different combination of additional alloys and other metals that make it up. Their alloy mixes can also depend on where the cast iron was manufactured, so trying to find a filler material that matches is a challenge.

In general, cast iron is a dirty metal because of its mishmash of internal metals and embedded contaminants, making it prone to porosity and cracking.

It’s also relatively easy to confuse cast iron, cast steel and cast aluminium, so you want to double-check what material you’re repairing.

Why Is It Hard to Weld Cast Iron?

Welding cast iron is hard for three specific reasons:

•    Its high carbon content
•    Its very low ductility 
•    It’s dirty and contaminant-prone

These factors combined make cast iron incredibly prone to cracking, more so than any other material, making it one of the most difficult metals to weld.

High Carbon Content in Cast Iron

The carbon content in cast iron is usually present in the form of ‘free carbon’ or graphite, which is elemental carbon in an uncombined state.

Cast iron has excessive free carbon, which diffuses into the weld and heat affected zone (HAZ) when welded and creates iron carbides (cementite). Cementite (Fe3C) is a compound of iron and carbon and is classified as a ceramic in its pure form.

These iron carbides change the microstructure of the weld and are incredibly hard, brittle, and have almost no ductility at all.

The formation of iron carbides in the weld is a primary cause of cracking in cast iron welds, so reducing the diffusion of carbon in the weld is essential. Preheating, post-heat treatment and using the right filler are also crucial for producing a successful cast iron weld.

Low to No Ductility

Ductility is a metal’s ability to deform without breaking under tensile stress. In other words, how much the metal can stretch, bend, or move before it fails. The more ductile it is, the more it can bend.

The high carbon content in cast iron makes it strong, but it also makes it brittle. This brittleness means it’ll break before it bends. Because cast iron isn’t ductile, it doesn’t handle heating and contraction very well.

If you weld cold cast iron, you create a spot of immense heat, and when this hot weld begins to cool, it contracts. The problem is that because cast iron isn’t ductile and can’t ‘move,’ the shrinkage of the weld causes it to crack.

As the cooling weld pulls on the surrounding base material, the cast iron doesn’t move with it and instead cracks. In comparison, mild steel accommodates the contraction of a weld and moves with it.

Post-weld cracking in cast iron almost always occurs in the HAZ or along the toes of the weld (in the partially melted zone ‘PMZ’) rather than in the weld itself. These areas are the most at risk of cracking because they are where the iron carbides tend to develop.

If the toes of the weld and the HAZ have become hardened and brittle and a cooling weld pulls on them as it contracts, they’ll crack.

To reduce the chances of your weld cracking, you’ll need to preheat and post-heat treat the cast iron.

Dirty Base Material

Besides the high carbon content and non-existent ductility that leads to cracking (both major factors in its welding difficulty level), cast iron is incredibly dirty.

When cast iron is made, the pig iron often has scrap metals and other alloys added to it, so its manufacturing process can cause issues. As well as containing iron and carbon, it can also contain manganese, silicon, chromium, nickel, copper, molybdenum, other steels and limestone.

It also includes high levels of phosphorus and sulphur (which contribute to a high carbon level). These are considered impurities and make it difficult to weld without it cracking.

On top of its general make up, if you’re welding on something like an engine block, it can also be oil impregnated (full of oils), which would have been absorbed during its lifetime and any cyclic loading (going from hot to cold to hot repeatedly).

You can clean the outside of the joint all you want, but as you melt the base metal, contaminants can float to the surface and leave you with porosity and poor welds.

Identifying the Cast Iron

Before you start welding, the first thing you want to do is make sure it’s cast iron and not cast steel or cast aluminium.

The easiest way to tell if it’s cast aluminium? It won’t be magnetic.

To tell the difference between cast iron and cast steel, however, can be a bit trickier. You can check what you’re working with in a few ways.  

Colour Check

The first and least reliable way is by looking at its colour. Cast iron is grey with hints of silver, while cast steel is usually primarily silver. Picking by the metal’s colour isn’t a great idea, though, as both their colours can change depending on the additional alloys.

Grain Structure

The next way to check is by looking at the grain structure. Your piece will need to be broken to see this properly, but the graphite in cast iron makes its grain structure (grey and flaky) distinct from cast steel. 

Drill Test

The next way is a drill test. Because of cast iron’s grain structure, when drilled, it’ll produce a dark, graphite-like material that looks like chips or small flakes of graphite. In comparison, when cast steel is drilled, it produces a bright and shiny spiral or curl.

Spark Test

The next test you can do is a spark test. There is a distinct difference in the type of spark produced when you take a grinding wheel to cast iron or cast steel. To do a spark test, run your grinder along the edge of the metal.

If it’s cast iron, the spark will be almost invisible until the far end, where the colour becomes brightest. It looks like the spark has appeared or originated in the air (outwards). In comparison, a cast steel spark will be bright orange the whole length of the spark, which appears right from the abrasive disc and extends outwards.

File Test

The final test you can do is a file test. The file should bite right in and start filing on cast iron and cast steel. However, if you then tack weld your metal, the file will slide right over the top of cast iron and have no effect. (To create a tack, quickly zap the metal with a TIG welder and no filler.) The tack weld will be too hard for the file because cast iron hardens when it’s welded (due to the internal carbon). In comparison, cast steel will continue to file as normal.  

Once you know it’s cast iron, you’ll need to figure out what kind it is. There are four kinds of cast iron, some of which are easier to weld than others, and some are unweldable:

• White cast iron
• Grey cast iron
• Ductile (nodular) cast iron
• Malleable cast iron

White Cast Iron

White cast iron is free of graphite (elemental carbon) and instead contains iron carbides that make the microstructure very brittle. It’s generally unweldable and will result in cracking every time.

Grey Cast Iron

Grey cast iron is the most common type of cast iron. It’s made up of iron, carbon, manganese, and silicon. When it’s cast, the carbon forms graphite flakes (in either a ferrite or pearlite crystalline structure), giving it its uniform dark grey colour. Grey cast iron is easier to weld on; however, the graphite flakes dissolve during welding and make the weld and HAZ very hard and brittle.

Ductile Cast Iron

Ductile cast iron, also known as nodular cast iron or spheroidal graphite cast iron, contains graphite, like grey cast iron. However, unlike grey cast iron, which contains graphite flakes, ductile cast iron has a unique microstructure.

The graphite occurs in the form of nodules or spheroids rather than flakes (which cause stress concentration points). The graphite forms as nodules due to the addition of nodulising elements like magnesium and cerium.

The formation of the nodules makes ductile cast iron softer, and it can deform before it breaks (hence the name ‘ductile’), making it more weldable than grey cast iron.

Malleable Cast Iron

Malleable cast iron is white cast iron that has been heat-treated to reduce its carbon level and, therefore, brittleness. Malleable cast iron also contains nodules of graphite, which makes it more ductile than white cast iron, so it’s weldable. However, heating it to above 925°C while welding will recombine the carbon and iron and turn it back into white cast iron.

How Do You Tell the Difference Between Them?

The best way to determine which kind of cast iron you have is to check the original specification. You can also do a metallographic or chemical analysis to identify what you’re working with.

They do have visual differences that you can also use to identify the type of cast iron, although it’s not the most reliable.

Grey cast iron will appear a uniform grey along a fracture. White cast iron will show a whiter colour along a fracture. Malleable cast iron will show a whiter, thin outline around a grey centre. Ductile cast iron will also appear whiter along a fracture but is much more weldable than white cast.

To check if it’s ductile rather than white, you can hit it with a hammer. If the metal bends, rather than breaking, it’s ductile cast iron.

Methods for Welding Cast Iron

You can weld cast iron with any welding process, although stick welding offers the best results.

Stick Welding Cast Iron

Stick welding is generally the first choice when it comes to welding cast iron. It lets you concentrate your heat while moving quickly, so you can minimise how much base metal is being melted. The less base metal melted means less carbon is being dissolved into the weld, reducing the chances of embrittlement and cracking.

Stick welding also usually produces the best results on cast iron because of the filler materials you can use. The electrode you choose will depend on what the cast piece is being used for, whether you need to colour-match and if it will be machined post-weld.

There are two main types of electrodes for cast iron: iron-based or nickel-based. Iron-based electrodes are more likely to produce iron carbides, so they’re more suited to minor repairs or when the metal needs to be colour-matched.

On the other hand, nickel-based electrodes produce a more ductile weld and reduce the chances of cracking but are a lower-strength weld metal. The most common electrodes you can use are:

•    ENiFe-Cl (55% nickel) 
•    ENi-Cl (99% nickel)

HYPERARC Ni55 Electrodes

HYPERARC Ni98 Electrodes

Pure nickel (ENi-Cl) electrodes are one of the best for welding cast iron. The high nickel content makes the weld material ductile and less likely to crack, but it can be expensive.

Nickel iron (ENiFe-Cl) electrodes are not as ductile as pure nickel electrodes, but they’re still a great option that is less likely to crack. The addition of iron in the electrode helps to increase the strength of the weld metal.

MIG Welding Cast Iron

MIG welding cast iron can be done, but you’ll need a nickel-alloy filler wire and an 80% argon / 20% CO2 gas. You’ll also need to use the short-circuit transfer method of MIG welding to keep your heat input low and your HAZ small.

Pulse MIG is also highly recommended when welding cast iron for its minimal heat input while still maintaining proper penetration.

You can use both ENiFe-Cl and ERNiFeMn-Cl MIG wires to weld cast iron. However, these can be difficult to find in stores as they’re not very common wires.

You don’t want to use ERNi-1 MIG wire, even though it contains pure nickel. As well as the nickel, it also includes 3% titanium, which will combine with the carbon and form titanium carbides in your cast iron.  

Another less common option is silicon bronze. It works well for repairing cast iron while MIG welding as silicon bronze melts at 926.6°C (1700F), and cast iron usually melts at 1204.4°C (2200F). Therefore, your heat input can be lower, reducing the carbon diffusion while maintaining a proper weld on the metal.

TIG Welding Cast Iron

TIG welding cast iron can be done, but like all TIG welding, the final result will depend on your skill level. TIG welding concentrates the heat and has a slow travel speed, so a lot of people don’t recommend it for welding on cast iron.

However, there are several ways to combat that, including using pulse TIG, adjusting your AC waveform or using a foot pedal for heat control.

On the other hand, TIG welding gives you the best visibility of your weld pool, complete control over the placement of the filler metal and allows for the best precision.  

Like with stick and MIG welding, you want to use nickel-based filler rods.

ERNi99 (ERNi-Cl) filler rods, like the 99% nickel electrode, are almost completely pure nickel and offer a very ductile weld that isn’t as prone to cracking.

ERNi55 (ERNiFe-Cl) filler rods, like the 55% nickel electrode, offer a mix of weld strength and ductility.

ERNiFeMn-Cl filler rods are recommended for high-strength cast irons, although they may be difficult to find in stores. 

Quick Tip

You can remove the flux from a 99% nickel stick electrode, sand it down and then use it as a TIG rod instead.

Flux-Cored Welding Cast Iron

Welding cast iron with flux-cored wire isn’t recommended for manual cast repairs. It can be done, but it’s highly specialised and generally only suited to automated welding on thick cast iron parts.

There’s also only one filler material available that will work successfully, which is ENiFeT3-Cl. This wire is similar to ENiFe-Cl MIG and TIG filler materials, but it contains a higher level of manganese, which helps it resist hot cracking.

Braze Welding Cast Iron

Brazing welding cast iron is an option; however, it’s more suited to plugging leaks than anything structural or anything that will be under load.

Brazing works by melting a filler material into the base metal but doesn’t melt the base metal itself. The filler material has a lower melting point, so it uses capillary action to adhere to the base metal rather than melt with it.

Because the base metal isn’t being melted, there’s little to no risk of diluting carbon into your weld and embrittling your metal. The problem with brazing is that the joint strength is low, especially in comparison to a joint that has been stick welded.

If you’re brazing your weld with an oxy-fuel torch, you’ll need a copper-zinc filler metal and a flux to keep the joint from oxidising. For filler metals, you can use:

• RBCuZn-A
• RBCuZn-B
• RBCuZn-C
• RBCuZn-D

Which one you pick will depend on how much strength you want the joint to have and whether you need to colour match the cast iron. For flux, a borax-boric acid type of flux is generally used for copper-zinc fillers.

Keep in mind that zinc can be highly toxic, so be careful not to overheat your filler metal, as it can evaporate. Using a respirator or PAPR helmet or having a fume extractor and working in a well-ventilated area is recommended.  

TIG brazing is also possible by welding at low amps to avoid melting the cast iron and using a filler metal with a lower melting point than it (like aluminium bronze). The addition of the argon gas means you won’t need to use a fluxing material to shield the joint.

How to Weld Cast Iron

For the most part, welding cast iron is done the same way you would weld any other material, using whichever welding process you choose. The biggest difference is the prep and post-work involved.

Cleaning the Cast Iron

As with all welding, cleaning your metal before welding on it is essential. Cast iron, in particular, needs to be thoroughly cleaned of external contaminants because there are plenty of internal ones that are going to make welding difficult. Removing the surface ones just makes the job a bit easier.

That includes removing any casting skin, rust, paint, oil, and grease. Grinding and using acetone or a grease remover will both work to clean your metal. The acetone/grease remover will help remove any residual surface graphite before you start the weld.

An engine degreaser will work to remove a lot of buildup for parts like an engine block that have been in service and are particularly dirty. If the part is oil-impregnated, like an engine block would be, you’ll also need to bake out those embedded oils, which is separate from the preheating process.

To bake your piece of cast iron, it needs to be kept at 370°C for about 30 minutes. Then, wipe it down before you preheat and weld it.

Prepping the Cast Iron

Once your joint is clean, you’ll need to prep it. When you’re repairing a casting, you’ll need to bevel the joint, and you want to remove any kind of sharp corner on the joint and bevel. You want a smooth, roughly 30° bevel all the way along the joint.

The best way to achieve this is to use clean, hard carbide tools to get the necessary groove. The bevel should go almost all the way through the base metal for the best possible fill.

When you’re repairing a crack in the cast iron, you’ll need to drill a hole on either end of it. Drilling holes at both ends prevents the crack from propagating and spreading any further once it’s been welded. You’ll need to bevel out the crack once you’ve drilled both ends.

Preheating Your Cast Iron

With a cleaned and prepped joint, you can preheat your cast.

Cast iron needs to be preheated because it is prone to cracking. Preheating reduces the thermal shock, and by controlling the expansion and contraction of the metal, it reduces the residual stresses, minimising the chances of it cracking.

Cast iron can’t handle rapid heating (or cooling), also known as thermal shock. Localised heat (as created by an arc) causes the metal to expand and contract at a different rate than the colder metal around it, which results in cracking.

Preheating the metal keeps the thermal gradient at more even levels when you start welding. The difference in temperature between preheated metal and a welding arc is much less than between cold or room-temperature metal and an arc.

You want to preheat your piece somewhere between 260°C – 650°C. Anything hotter than 760°C is a critical danger zone for iron. You don’t want to get it that hot. When you preheat the cast iron, it should be done slowly and evenly.

You can preheat your cast iron part in a couple of ways, depending on how big it is.

If it will fit, a good way to preheat the entire cast part is by placing it in an oven or a BBQ. That way, you can leave it to sit and gradually build up to the desired temperature. If it’s too large for either of those, you can use an oxy torch or a map and propane gas torch.

Note

If you place your part in a BBQ, it can leave contaminants behind that’ll bake into it, so it likely won’t be food-safe afterwards.

Ideally, you want to heat the entire cast part, but sometimes that’s not feasible. If you can’t get your part in a BBQ or oven, try to heat as much as possible, especially around the area being welded. It’s a good idea to heat about 50-100mm around where you’ll be welding.

You can use a temperature stick or a handheld infrared thermometer to check that your part has reached the right temperature.  

Preheating vs Non-Preheating

Technically speaking, you don’t have to preheat your cast iron. However, having said that, it is much harder to weld successfully, and it’s much more likely to crack.

You don’t want to weld on cold metal. At a minimum, it should be room temperature. To get the best result without preheating, bring your cast iron part to around 35°C. Even though you’re still heating the part, this isn’t considered preheating your metal.

You should also only use a nickel filler material if you plan to cold weld, as they’ll cope with the weld shrinkage better than anything else.

Selecting the Right Filler Material

Your choice of filler rod will depend on the type of welding you’ll be doing. For the most part, you’ll want a nickel-based filler material that matches the process you’re using. Specific filler rods have been discussed above based on how you want to weld your piece.

Another thing to consider when choosing a filler material is whether your part needs to be machined after it’s welded. If it’s going to be machined, you’ll want to get a filler material with a high level of nickel. Due to its ductility and malleability, it’s the most suitable for machining.

You generally want to avoid steel fillers, as mild steel promotes iron carbide formation, and using a stainless filler can produce chromium carbides, which are also very hard.

Welding the Cast Iron

Welding cast iron is a slow process.

The best way to weld cast iron, whether you’ve preheated it or not (but especially if not), is to do short welds and to hop around the part, letting it cool between passes if necessary. Welding in small sections and skipping around will result in less distortion, relieve some of the stress and decrease the chance of cracking. You don’t want to weld more than about 25mm before stopping and letting it cool or moving to weld somewhere else.

You also want to have your amperage set as low as possible. You still need enough heat to melt your filler metal, but if your stick electrode has a range of 50A to 70A, you should be closer to 50A to reduce your heat input into the cast iron.

While you want to keep your heat input down, you need to maintain your cast iron’s preheat temperature. It might mean you need to hit the piece with an oxy torch as you weld, which is known as the interpass temperature.

If your interpass temperature exceeds 650°C, let the part cool down before you make any more welds. Don’t assist the cooling with compressed air or water, as rapid cooling will cause cracking.

Cast iron is a dirty metal internally, thanks to the mishmash of added alloys, as well as contaminants like oil. Heating it up and welding on it makes all of those impurities float to the surface. That, and its tendency to crack, is why welding on it can be so difficult.

If you notice porosity or a lack of fusion, which will often happen as bits float to the surface, you can stop, grind it back, and then continue.

Peening

Each time you finish a weld bead (even if it is only an inch long), you’ll need to peen it. To peen a weld, you need to hit it with a hammer. Peening the metal forces it to expand back out while it’s trying to shrink, which minimises the amount of stress in the metal and helps prevent cracking.

There are specific peening hammers with a ball peen, which directs the force back out to the metal, although a chipping hammer can also work. You’ll need to peen the weld while it’s still hot, which is why it needs to be done as you finish each weld. Hit the weld enough that it’s all been covered and the stress reduced before moving on to the next weld.

Post-Weld Heat Treatment

Once you’ve finished your weld and the cast iron part is holding together, the final step is to cool it down slowly.

You can post-weld heat treat your cast iron in several ways. You can use an insulating blanket, like a welding blanket, or you can place it into dry sand, or you can use the same method you used to preheat it and chuck it back in the oven or BBQ.

The key is that it needs to be able to cool slowly to minimise the stress and keep it from cracking as the metal contracts. 

Welding cast iron can be done; it’s just a much fussier, slower process that takes practice and some patience. With the correct metal prep, the right filler, minimal heat input, the proper welding technique and post-weld treatment, you’ll have a much better time trying to weld cast iron.