How to TIG Weld Cast Iron: With and Without Preheating

ByAndrew Valsamis Updated on TIG

You will learn what settings and consumables you need to TIG weld cast iron and the steps you must take with and without preheating, after reading this article.

Finally, as a bonus, we’ll see a great alternative to join or repair cast iron with the same TIG equipment.

Cast iron is hard to weld because it is brittle, and the welding heat can cause fast expansion and contraction. That often leads to cracking.

Therefore, it’s critical to avoid putting high amounts of heat in a short length of time in cast iron.

Weldpundit already has an article on how to weld cast iron for beginners with basic information.

Table of Contents

Why is TIG welding cast iron hard?

You can TIG weld cast iron, but it has some difficulties.

Cast iron is hard to TIG weld because this process demands perfectly clean joint and filler rods to avoid welding defects. Cast iron comes with numerous impurities both inside and on the surface. Furthermore, TIG welding is hard to master, and you need good experience before attempting to weld cast iron with it.

TIG welding (or GTAW) needs clean metal for acceptable welds. Even if small amounts of contaminants enter the puddle or the arc, they will cause defects such as porosity, inclusions, high arc instability, etc.

Cast iron, especially low-quality ones, are filled with inclusions and unwanted elements, for example:

  • Offen cast iron contains large amounts of sulfur and phosphorous.
  • To lower the costs, the manufacturers add dirty scrap metals into the blast furnace.
  • The casting method creates air gaps in the casting.
  • Various oxides form on the casting’s surface (casting skin).
  • Often oil and grease have penetrated the porous surface.

Perfectly cleaning old or low-quality castings is hard, time-consuming, and still can give you poor results. You will have better luck if you weld new and high-quality castings. Or change process and stick weld cast iron that is more suitable for dirty castings.

TIG welding is the hardest process to learn even for easy to weld metals such as mild steel. It is a manual process that needs both hands plus one foot if you use a pedal and excellent hand-eye coordination. You need to weld steel efficiently before you try to weld cast iron.

An advantage of TIG welding is that you can weld thin castings with a lower cracking risk than stick or MIG welding.

What settings do you need?

TIG welding cast iron needs almost the same torch setup and settings as steel. However, you want to set a lower amperage that will effectively melt the rod but will form a small puddle.

This way, you will prevent:

  • Deep penetration in the casting to avoid cracking from rapid expansion or contraction.
  • Too much carbon and other impurities from entering the puddle. This is important to keep the puddle as clean as possible when TIG welding.
  • A wide heat-affected zone (HAZ) will be narrow. The HAZ is the area next to the welded joint that gets harder and more brittle when you weld. It is where most of the cracks appear.

Of course, you must not set the heat too low. The puddle must be easy to control, and the weld metal must fill the edges.

Furthermore, a wider cup for better gas coverage is helpful because:

  • TIG welding cast iron needs nickel rods that give sluggish weld metal that doesn’t flow well.
  • Cast iron forms oxides and might still have residual impurities after cleaning that can restrict the weld metal’s flow.

You can also use a foot pedal to change the amperage while you weld according to your judgment.

Also, modern TIG welding machines can output pulsed current to achieve penetration with less heat.

These two can make welding cast iron easier but aren’t necessary for most jobs.

What TIG welding rod is the best for cast iron?

The best TIG welding rod for cast iron is the nickel-based ERNiFe-CI (or ERNi55). This rod doesn’t shrink much and has high tensile strength and ductility (elasticity). With it, you can weld most cast-iron types, somewhat dirty castings, and cast iron to steel. Finally, the weld metal has good machinability.

Nickel-based rods are the proper consumables for cast iron. This is because nickel weld metal doesn’t shrink much and stays ductile. This prevents pulling the cast iron, something that causes cracking. Nickel also resists carbon absorption.

The ERNiFe-CI rod has 55% nickel (Ni), a small amount of various other elements, and the rest is iron (Fe). It has high enough tensile strength (90ksi) to weld most ductile iron grades.

The ERNiFe-CI also has high ductility to weld cast iron to other steels such as carbon or stainless steel. It is suitable for multi-pass welds.

Furthermore, because nickel resists carbon absorption, the weld metal stays machinable, so you can drill it or cut it.

However, if high heat enters the casting, an amount of carbon might combine with iron and form carbides. Carbides are exceptionally hard and lower the metal’s machinability.

The next best rod is the ERNi-CI (or ERNi99) which is almost pure nickel with a tiny amount of alloying elements. Use it when you want 100% machinable weld metal since it is weaker and more expensive than the ERNiFE-CI.

The ERNi-CI doesn’t have enough tensile strength to weld most ductile irons, so it is more suitable for gray and malleable iron.

This rod cannot tolerate high amounts of sulfur, phosphorous and other impurities found in low-quality gray iron castings. As a result, the bead may crack longitudinal when it cools down. This is called hot cracking.

Another rod you can use is the ERNiCu that, instead of iron (Fe) it contains copper (Cu). It offers high ductility and corrosion resistance.

However, it might absorb carbon. To avoid this, ensure the lowest possible penetration because carbon and copper in the puddle lead to hot cracking.

With TIG nicker rods, you don’t need preheating if the casting is small or needs a minor repair.

Can you TIG weld cast iron with mild steel rods?

It is possible to TIG weld cast iron with mild steel rods such as the ER70S-6 but it is not recommended. Since there is no nickel in steel rods, carbon from the casting will migrate in the weld metal, making it extremely brittle with a high shrinkage rate. When the bead cools, or the casting comes under mechanical or thermal stress, it is very likely to break.

Cast-iron nickel rods are very expensive and not easy to find. For this, you can try and weld with typical steel rods and get acceptable welds for simple repairs on low-cost castings. Still, it’s not wise for critical welds, expensive or irreplaceable castings.

To lower the risk of failure, you need to have some experience in cast iron welding. You need to follow all the welding steps correctly without mistakes. Furthermore, steel rods need much higher preheating.

The weld metal will not be machinable. It will be impossible to drill or cut it. Furthermore, you may or may not be able to grind the bead depending on how much carbon is absorbed.

Can you TIG weld cast iron with stainless steel rods?

TIG welding cast iron with stainless steel rods is not recommended. Chromium from the rod and carbon from the casting will form carbides that are even harder than any cast iron. Furthermore, stainless steel has a much higher tensile strength and shrinkage rate that can pull and crack the casting.

Stainless steel rods, such as the ER309 or the ER312 are popular for joining hard to weld metals. But the ultra-high carbon content in cast iron is a problem. A bead that contains chromium carbides will shrink and crack when starts to cool down.

If you want to avoid the expensive nickel rods, you should at least acquire a couple of them. You can remove the flux of nickel stick rods and use them. With them, weld the surface of the joint. This is called buttering. After that, you can fill the joint with ER70S-6 or ER309 rods. Buttering will offer welds that will last, but it is suitable only for non-critical welds.

In the second part of the article, let’s see how to TIG weld cast iron with preheating step by step.

1. Identify the cast iron type

Gray iron is the most common type, but it is brittle with many impurities, making it hard to TIG weld. Malleable iron is ductile and easier to weld. Ductile or nodular iron has a low amount of impurities that help the TIG process. White cast iron is unweldable, it always cracks.

Furthermore, each type has many classes. Each class has a different hardness, tensile strength, ductility, etc, making welding cast iron more difficult. Identifying the class is hard and needs chemical analysis.

A photo of cast iron
Gray cast iron

Weldpundit already has an article about how to identify metals, but here are the basics to figure out what cast iron you have.

A spark test is an excellent method to identify metals, but you must have identified sample metals to compare them.

In a nutshell, the hard and brittle gray iron has short and reddish sparks, but the ductile irons have longer and yellowish sparks that look more like steel sparks.

If you have a broken part, gray iron has a characteristic dark-gray color. Malleable cast iron has a white, steely surface but gets gray towards the center. Nodular iron’s color is more similar to gray iron.

You can distinguish gray iron from ductile irons with a hardness test:

  • You can use a center punch and a hammer on it. If the crater that you created has a smooth surface, it is gray iron. If the crater has a burr, it is a malleable or nodular iron.
  • You can also try to drill the casting. If the chips are like powder, it is gray iron. If they are somewhat continuous, it is ductile iron. 
  • If the chips are long and very ductile, you might have cast steel.

If you cannot drill the casting, it is hardened by the manufacturer, the service conditions, or former welding. If so, it is too hard to weld it with success.

However, if it is localized, then you can remove the affected area before proceeding.

Since gray cast iron is the most popular, the rest of the guide will be about this brittle metal.

2. Clean the casting’s surface

This is a crucial step since TIG welding needs clean joints to offer good welds. As mentioned, dirt will destabilize the arc and restrict the weld metal’s flow and contaminate it.

Furthermore, hard-to-weld metals always need a clean surface to reduce the risk of failure, and cast iron always has impurity problems when manufactured. To make things worse, oil and other contaminants may cover the surface. Because cast iron is porous, oil can penetrate the surface. 

If the casting has absorbed oils, cleaning with acetone or similar products will not be effective. Use a mild degreaser that does not contain chlorine. Chlorine is poisonous by itself, but the welding heat and the arc’s ultraviolet rays convert chlorine to other hazardous gases.

Read the product’s description to verify that it is not chlorinated and the how-to-use instructions. Also, this Weldpundit article on how to protect yourself from welding fumes and gases will be helpful. 

The trade-offs of nonchlorinated products are that they are more expensive and very flammable.

Another more effective way to remove residual oil is by preheating the casting to 750 °F (400 °C) for 15-30 minutes. This way the oil, and other impurities will burn out. But it’s costly.

If your casting is free from oil, you must remove surface contaminants such as the casting skin, rust, paint, etc. You can use sandblasting for quick removal, but it can leave contaminants on the surface. If you use an angle grinder, don’t focus on one spot to avoid discoloration.

Clean both sides at least 1″ (2.5 cm) to bright metal to prevent impurities from entering the puddle. After sandblasting or grinding, use a file to remove residual carbon and other contaminants on the joint.

Finally, clean the joint and welding rod with acetone or, better yet, an alcohol-based cleaner to remove any remaining contaminants, including those that are invisible.

If you weld what seems like clean cast iron and porosity appears, it is because there are impurities deep inside the casting. They come to the surface under the influence of the welding heat. In a situation like this, you remove the bead and start over again until you get a bead without porosity. The buttering technique, described earlier, can seal these impurities, saving you the trouble of rewelding.

3. Prepare the joint

The best way to prepare a joint or remove a crack on cast iron is with a chisel and a hammer.

This way you:

  • Avoid contaminating the porous casting with abrasives that cause defects when TIG welding.
  • Keep the casting cold.
  • Do precise work since it’s a slow process.
  • It is safer than grinders. 

After that, you can use a die grinder with carbide burrs. Carbide burrs remove metal fast, without leaving residual abrasives or carbon.

You can use an angle grinder, but move it evenly over the area and avoid focusing on one spot. Grinding generates high amounts of heat that can locally harden the metal. When you finish, use a file to remove residual carbon and abrasive material on the joint.

Ensure that the groove will be as symmetrical as possible, and always try to weld in the flat position.

Nickel filler metal is sluggish, and each bevel you made needs to be at least 35 degrees. Furthermore, a wider groove will make it simpler to fit the TIG torch.

Removing cracks

You must remove all cracks because the expansion and contraction forces will make them bigger if you weld on or next to them.

To remove a crack:

  1. Drill two holes at least 1/8″ (3.2 mm) wide and 3/16″ (4.8 mm) beyond the ends of the crack. 
  2. Then, remove the crack by creating a V groove at a safe depth, up to 1/8″ (3.2 mm) from the bottom of the crack. There is no need to go deeper than that. The deeper the joint is, the more weld metal you need to fill it. That will put more heat into the casting.
  3. If possible, try not to leave an open root.

Broken parts

Because gray iron is rigid and brittle, broken parts fit perfectly with each other. Therefore, you can either:

  • Fit and tack weld the broken part. After that, you can create a groove, just like fixing a crack.
  • Create the bevels on both parts. Then tack weld the broken piece to the casting.

4. Preheat the casting to avoid temperature differences

Preheating prevents the fast expansion of the casting when you weld it and the quick cool-down after welding. It also prevents carbon migration in the weld metal. With preheating, you decrease the risk of cracking. 

Preheating adds another important advantage when you TIG weld. It removes impurities not only from the surface but inside the casting too.

You can use an oven or a flame torch. The typical preheating temperature of medium-sized gray iron castings is 500 °F (260 °C).

If you preheat with a flame torch, move it over the casting and concentrate on the thicker parts. After a while, use the temperature stick on the surface to check the temperature.

If the stick melts evenly all over the casting, you are ready to weld. If not, continue preheating as necessary.

You can also use an infrared digital thermometer, but they are not as reliable, especially on shiny surfaces.

When you preheat the casting, you must weld it in a reasonable amount of time to prevent it from getting cold. If you use the temperature stick and it doesn’t melt, reheat the casting.

You must avoid any cold drafts during welding to protect the preheating temperature. If the casting is big, insulate it with welding blankets or similar equipment.

If you hear tinkling noises, it means the casting is cracking. Chances are you didn’t avoid the temperature differences in the metal with enough preheating.

5. Weld short beads to avoid concentrated heat

When you weld brittle metal, you weld very short beads. A long one will put too much heat at a fast rate in the casting.

The general rule is to weld 1″ (2.5 cm) beads. However, the length depends on the casting thickness:

  • For a 1/4″ thick casting, you can weld a 1″ bead.
  • For a 1/4 to 1/2″ casting, weld a 2″ bead.
  • And for 1/2″ and thicker, weld a 3″ or slightly longer bead.

The heat that goes into the casting also depends on the way you place the beads. To reduce the cracking risk, you don’t place each bead right next to each other. Instead, you spread them along the joint to spread out the heat.

Below is an image with an example of how to spread the beads on the joint.

An image of symmetrical welding for cast iron
Symmetrical welding for cast iron

Some additional tips for better results:

  • The traveling speed greatly affects the heat that goes into the casting. For this, you should travel as fast as possible without creating any defects from it. 
  • Weld straight beads (or stringers) without weaving the TIG torch.
  • Avoid starting the arc on the casting, but in the joint or on the previous beads.
  • Always fill the bead’s crater. If there is a gap, the shrinkage forces can crack it.
  • If a crack appears when welding, stop and drill holes at both ends to stop it from expanding.
  • If your casting is irregular, weld the thicker areas first.
  • Put the working (or grounding) clamp on the casting for better connection.
  • Avoid all drafts since they will displace the shielding gas. TIG welding is very sensitive to atmosphere contamination. Also, nicker weld metal doesn’t wet out well, and lack of gas coverage worsens it.
  • If you think it’s necessary, use pulsed current or a foot pedal to control the amperage when you weld.

Avoiding turning red-hot the area you weld is a reliable indication that you do not overweld the casting.

6. Peen each bead to prevent shrinkage stress

Peening is when you tap all over the red hot bead with a small rounded hammer immediately after welding it. Peening will stretch the weld metal and reduce the shrinkage forces.

As a result, the bead will have lower residual stress and cracking risk. 

Start from the crater and use rapid blows that will lightly indent the weld metal. Hitting the bead too hard or too long will harden the weld metal and make things worse. That is called work hardening.

You may also use a rounded chisel or an electric needle scaler with round needles.

7. Post heat the casting to fix temperature differences

After welding, you must ensure the casting has at least the same preheating temperature, spread out evenly. Use the temperature stick again and reheat the casting if necessary.

8. Protect the casting so it can cool down slowly

The final step is crucial to preserve the metal’s characteristics and prevent cracking. The slower you cool down the casting, the better. 

For critical welds or valuable castings, you can delay the cool down rate even down to 50 °F (10 °C) per hour.

You must insulate the entire casting so it can cool down evenly until it reaches room temperature.

If you preheated the casting with an oven, use it again but to cool it down. Otherwise, you can cover it with welding blankets, similar fiberglass material, or dry sand.

In the third part of the article, let’s see how to TIG weld cast iron without preheating.

How to TIG weld cast iron without preheating

There are many situations when you might consider to TIG weld cast iron without preheating:

  • The casting might be too big or complicated to preheat correctly. 
  • The damage is short cracks or superficial wear, so only a low amount of heat will enter the casting.
  • You might not have the necessary preheating equipment at the moment. 
  • You cannot dismantle the part, and it will crack if you preheat it.
  • The casting is small and simple, and you want to avoid the trouble and the costs. 
  • You already have some experience welding cast iron.

You can TIG weld cast iron without preheating using nickel rods (ERNi99 and ERNi55), very short beads, and constant peening. The important step is to leave each bead to cool before welding the next one. However, no preheating will always make the weld weaker and more prone to cracking in the future.

Welding without preheating is also called cold welding. Keep in mind that stick welding is the best process to cold weld cast iron.

You also skip the post-weld reheating and the casting’s slow cool-down. This will save you effort, time, and money if the casting doesn’t crack.

The main concern is to minimize the heat input and the width of the heat-affected zone (HAZ) to the absolute minimum. To do this:

  • Use only nickel rods made for cast iron.
  • Use the lowest amperage to melt the rod properly and achieve low penetration.
  • Weld 1/2″ (1.25 cm) to 1″ (2.5 cm) short beads without weaving. 
  • Always peen the bead when it’s red hot.
  • After each bead, you stop and let the bead cool down until you can touch it comfortably with bare hands. That’s around 100 °F (38 °C), but not lower. This step is the most important when welding without preheating.
  • After that, you weld another short bead at a different area on the joint and repeat the cycle until you finish. 

You can also use pulsed current if your TIG welding machine supports it and a foot pedal if you have one.

Skipping the preheating step doesn’t mean you can weld cold cast iron. Welding in a cold environment will always give poor results. Instead, try to warm the casting to room temperature 75°F (24°C) before starting.

Keep in mind that lack of preheating is risky, and you should consider preheating when the casting:

  • Is very thick.
  • Has demanding service conditions.
  • Is expensive or irreplaceable.
  • Is damaged in the center. Cracks or broken parts at the edges can be welded easier with less or no preheating.

Finally, let’s see a great alternative to join cast iron with TIG equipment.

A great alternative to fusion welding

With TIG welding equipment, you can use copper-based rods and braze weld cast iron. Braze welding uses the same equipment as typical (fusion) welding and similar joint design. The difference is that the copper-based rods melt at a lower temperature than welding rods. This way the casting doesn’t melt.

The most popular braze welding rods are silicon bronze (ERCuSi or SiB) and aluminum bronze (ERCuAl). ERCuAl has higher tensile strength (70ksi) than ERCuSi (50ksi), but it is more sluggish and doesn’t wet out well.

Advantages

TIG brazing has numerous advantages for cast iron:

  • Since the casting doesn’t melt, there is no puddle. That makes braze welding far better to join cast iron, since it releases a lower amount of heat. The casting doesn’t get harder, and there is low development of shrinkage stress. As a result, there is a much lower chance of cracking.
  • This process needs even lower preheating than nickel rods, or no preheating at all.
  • You can join cast iron to other metals easier than nickel rods.
  • Braze welding rods are cheaper than nickel ones.
  • Finally, it is faster and easier than fusion welding.

If you have a TIG welding machine that can output alternating current (AC) (external link), you can use the aluminum bronze (ERCuAl) rod with the balance control setting on.

This is a significant advantage because the added reverse polarity (DC+) will clean the aluminum oxide that forms in the brazing metal. This oxide prevents the smooth wetting and makes the bead weaker.

The cleaning effect of AC will not only make the aluminum braze metal stronger but also prettier. The added strength allows TIG brazing to join or repair stronger or bigger castings with low cracking risk.

Disadvantages

Now let’s see the disadvantages of TIG brazing:

  • Because braze welding does not penetrate the casting, it lacks the strength of fusion welding. Even if both braze and welding rods have the same tensile strength, fusion will always give a stronger bond. If the cast iron you weld has demanding service conditions or is very big, it would be better to weld it.
  • Brazing weld metal has low-heat resistance. When brazing filler metal reaches 930 °F (500 °C), it loses a lot of strength and will not hold the joint together. Critical welds that reach temperatures of 400 °F (200 °C) or higher are best to fusion weld them.
  • Furthermore, TIG brazing needs an even cleaner joint to flow correctly, especially if you work with gray iron.
  • Finally, braze weld metal always have goldish color that might not be acceptable.

Weldpundit has an article that discusses when it is better to braze or weld cast iron in more detail.

More Weldpundit articles

How to MIG Weld Cast Iron: With and Without Preheating.

Can You Weld Cast Iron to Steel? And How to Do It.

Stick Welding Electrode Selection for Beginners: Type, Size, and Amperage.

Comparison Between Stick and Flux-Cored-S Welding Processes.

Can You Weld Magnetized Metal? And How to Demagnetize It.

Is Stick Welding Easy or Hard to Learn? And How Long It Takes.

Can You Weld at Home? Basic Considerations.