Tungsten inert gas welding, called TIG welding for short, uses a tungsten electrode in an oxygen-free gas field to join metals together. TIG welding makes strong, smooth welds, and it works on a variety of different metals. TIG welding is also suitable for joining different kinds of metal together.
Cast iron has a reputation for being difficult to weld, but TIG welding can join pieces of cast iron together. Because of its brittle nature, welds in cast iron often crack and fail. More than other metals, cast iron requires you to follow the steps closely to get a good weld. If you skip steps or get them out of order, you raise the risk of cracking. To TIG weld cast iron, follow these steps:
- Prepare the electrode by grinding it to a point and inserting it into the collet.
- Set up the welder for TIG welding per the manufacturer’s directions.
- Adjust the gas flow to protect the new weld from corrosion.
- Clean the joint to be welded thoroughly. Use a wire brush or grinder followed by acetone or another solvent. The weld won’t stick to dirty metal.
- Put the solvent away before welding. You don’t want to stop welding to fight fires.
- Connect the workpiece to the welder. You can connect the lead directly to the workpiece, or you can use a welding table connected to the lead.
- Put on safety gear. You need a welding mask, welding gloves, and a long-sleeved shirt or welding jacket. TIG welding generates severe heat and sparks, so you need to be protected.
- Preheat the cast iron to a temperature between 500 and 1200 degrees. Hot cast iron will hold the weld without cracking. Welding cold cast iron can lead to cracks.
- Make the weld. Grab a rod, strike a spark, and weld. Work slowly in short sections and don’t overheat the workpiece. Tungsten electrodes are hot enough to melt cast iron.
- Let the piece cool slowly to prevent cracking. Gently peen the weld as it cools to relieve stress on the joint.
It is possible to TIG weld cast iron, but it is not easy. To successfully TIG weld cast iron, you need to be very careful. Follow the steps below, and you can successfully weld cast iron. Don’t skip any steps, and don’t get them out of order. If you don’t follow the steps, the weld is likely to crack the cast iron and prevent any further welding attempts.
Equipment Needed to TIG Weld Cast Iron
TIG welding requires a TIG welder with a shielding gas supply, an electrode to carry the current, and a welding rod to make the joint. Unlike other forms of welding, TIG welding uses a tungsten electrode to carry current and a separate wire to serve as the welding metal. You need both hands to TIG weld, one for the electrode, and one for the rod.
The process used for TIG welding is prone to corrosion while the welds are hot. Using a shielding gas protects the weld from oxygen and keeps it from corroding. The best shielding gas mixes for TIG welding cast iron are either pure argon or 75% argon / 25% CO2. If you don’t use the shielding gas, the weld will be brittle and prone to cracking.
Pro tip: Don’t TIG weld in a tightly enclosed space. If argon or argon/CO2 build up where you are working, you can suffocate. The first sign that argon or CO2 has built up too much is passing out, followed quickly by death. Poor ventilation and TIG welding don’t mix.
The electrode carries electric current from the welder to the joint. The current becomes the arc that heats the welding rod to make the weld. The best electrodes for TIG welding cast iron are thoriated tungsten rods. Choose a 1% thorium rod, marked EWTh-1 with a yellow tip, or a 2% thorium rod marked EWTh-2 with a red tip for welding cast iron.
The welding rod is the material that melts to join the pieces together. Unlike arc or wire welding, TIG welding uses electrodes to carry current separately from the welding rod. You have to hold the tungsten electrode in one hand and the welding rod in the other.
The best type of rod for TIG welding cast iron is nickel because it has similar thermal expansion properties as cast iron. This helps minimize cracking along the weld when the piece is heated. This is particularly important if the weld will be exposed to high temperatures, such as in an engine block or cast iron stove.
The drawback of nickel rods is that they are costly. If you are welding a piece that won’t be exposed to high temperatures, aluminum-bronze rods work well for cast iron and are much cheaper than nickel. Mild steel rods like 7018 or 6013 can also work to TIG weld cast iron.
Of course, you also need standard welding supplies to clean the workpiece, prepare the electrodes, and peen the joint. The other items you need include:
- Bench grinder for preparing the electrode before you weld
- Wire brush for cleaning the work surface so that the weld will adhere to the surface
- Acetone or another solvent to remove all traces of grease, paint, and other contaminants from the weld surface
- Angle grinder for grinding material out of cracks
- Protective equipment to keep you safe and secure while you weld
- Propane torch to preheat the workpiece
- Ball peen hammer to peen the weld and prevent cracks. You don’t need a big hammer; gentle taps are all that is required
There are four major parts of the process for welding cast iron: setting up the welder, preparing the joint, welding, and cooling the workpiece.
Setting Up the Welder
Cast iron is fussy about welding, and once you have it ready to weld, you can’t waste time setting up. Get your welder prepared to go before you do anything else so you can start the weld as soon as the piece is ready.
Step 1: Prepare the Electrode
The electrode should have an excellent point to allow for the best control of the arc. If your rod doesn’t already have an excellent point, use the bench grinder to put a point on the electrode. Once the tip is ready, insert the electrode into the collet on the welding torch.
Step 2: Set Up the Welder
Once the electrode is ready, adjust the settings on the welder for cast iron. Since welding units are different, consult the manufacturer’s directions to set up the welder based on the joint type and material thickness of your project.
Start with the lowest recommended amperage for welding cast iron. Overheating the workpiece can cause cast iron to crack, so it is best to be conservative with the heat you use. Only build up to higher amperage if the lowest setting won’t melt the rod.
Step 3: Set Up the Gas
The best mixes for TIG welding cast iron are pure argon or 75% argon/25% CO2. This is another area to follow the guidelines in the manual. Using too little gas will cause the weld to oxidize, reducing its strength. Using too much gas won’t cause problems with the weld, but argon is expensive, and you don’t want to waste it.
Make sure your shop has adequate ventilation for TIG welding. TIG welding in a poorly-ventilated area is genuinely a life-threatening endeavor. Because CO2 and argon are heavier than air, they will pool in low spots rather than diffusing through the air. You can drown in argon and never realize the danger.
Prepare the Work Piece
Once the welder is ready to go, you can start on the workpiece itself. Cast iron is often used in very dirty applications, like wood stoves and engine blocks. You need to get all the gunk off the piece before trying to weld it. If the material is greasy or dirty, the weld will stick to the gunk instead of the metal.
Step 4: Clean the Cast Iron
Use a wire brush to clean the piece thoroughly. Scrub it hard to get all the dirt, soot, and crud off all the surfaces to be welded. Once you have brushed it clean, wipe it down with acetone or another solvent to remove all traces of paint or grease.
For some hairline cracks, it is advisable to use a thin blade on an angle grinder to cut out the material on the edges of the crack. If you can’t get into the crack with a wire brush and acetone, you need to reach for the grinder. Plunge it into the crack to get edges that are clean and will hold a weld.
Step 5: Put Away the Solvent
Ok, so this isn’t a part of the welding process, but do you really want to start using high heat close to something as flammable as acetone? The arc from welding, heat from a propane torch, and even heat from cast iron can trigger a reaction in acetone fumes.
Take a second to put away the solvent before you strike a spark. A fire in your shop makes it very hard to weld anything.
Step 6: Set Up for the Weld
Align the pieces to be welded correctly and clamp them in place. Connect the welder to the workpiece or connect it to the welding table and put the workpiece on the table. You need to connect the piece to the welder to complete the circuit and get a spark. Once everything has been set up, you can start welding.
Making the Weld
Now that the welder and the workpiece have been set up, you ready to start welding. Cast iron is fussy about welding, so you need to have everything ready before you begin welding.
Step 7: Suit Up
Before you start to work, put on your protective equipment. You need to wear a welding mask and welding gloves. You should also wear a long-sleeved flame-retardant shirt or a leather welding jacket to protect your arms from sparks. Welding is hot, dangerous work. It will be much more hazardous if you don’t wear the right protective gear.
Step 8: Preheat the Cast Iron
Cast iron is extremely brittle. It will crack if part of the workpiece gets hot, and the rest does not. To prevent cracks, preheat the entire piece to be welded with a propane torch. The whole work piece should be at least 500 degrees before you start. Don’t let it get over 1200 degrees, or it will begin to melt.
Light the propane torch and play the heat over the entire workpiece until the whole piece is hot enough to weld. Some welders even use gas grills to heat small parts thoroughly. Once it is hot, you need to be ready to start welding right away. If it starts to cool before the weld is complete, stop and heat it again. Keep it hot until you are done.
Step 9: Weld
Grab your electrode and rod, crack a spark, and start welding. Hold the electrode about an inch from the joint surface with one hand and insert the tip of the rod into the spark with the other hand. Build a small weld pool of melted welding rod, then start to draw out the material along the joint.
Use the tip of the welding rod in the welding pool to draw it out along the joint. As you work, the rod will gradually melt and be consumed by the joint. You will have to coordinate both hands to get a smooth bead. Keep pushing the rod into the ark and withdrawing it, then advancing the ark and repeating.
A bit of Advice When Welding Cast Iron:
- Since cast iron is sensitive to overheating, work in small sections. You need to keep the temperature of the workpiece between 500 and 1200 degrees. Weld about an inch of joint, then stop and let the area cool. You can work on a different area while it cools, or you can just take a short break.
- If you overheat the cast iron, it will start to melt. The melting point of cast iron is about 1400 degrees, lower than the melting point of either iron or steel. Get it too hot, and it will flow out of shape. Since the electrode in TIG welding is over 3000 degrees, melting your cast iron is possible if you linger in one place for too long.
- Conversely, you need to keep the cast iron from cooling off too much as well. If you do too much welding on cold cast iron, the temperature difference between the welded area and the rest of the piece will cause severe cracking. You need to be very careful about the temperature of the cast iron while you weld.
Step 10: Peen and Cool
You can protect from cracks by peening (flatten with a hammer) the weld bead as it cools. Take a small hammer and tap the bead flat while it’s still hot. Peening spreads the weld out and helps it maintain contact with the joint. This relieves stress around the weld and prevents cracking.
Once you have completed the weld, let the piece cool slowly to prevent cracking. If it cools too quickly, the weld and the surface will contract at different rates, and cracks will form along the joint. Burying the piece in sand or otherwise insulating it can slow cooling and reduce cracks.
Some welders put the newly-welded piece on top of a wood stove with a nice fire. They leave it there until the fire has gone completely out and cooled. This is an excellent slow cooling process.
Welding Cold Cast Iron
It is possible to TIG weld cast iron without preheating, but it is challenging. To weld cold cast iron, you must keep the cast iron from getting hot while you weld. To do this, work slowly in tiny sections. It is almost like stitching—put in a very short bead, then stop and let it cool or work on another area of the weld. Cold welding has a low probability of success and a high chance of cracking the cast iron.
Types of Cast Iron
There are two different types of cast iron. Most cast iron is called “gray cast iron” because, when it cracks, the edges of the crack look gray. Gray cast iron can be welded. White cast iron looks white or silver when it cracks.
Gray cast iron is the most common type of cast iron. Engine blocks, exhaust manifolds, wood-burning stoves, and skillets are made from gray cast iron. It is somewhat brittle and may crack when exposed to high-temperature variations. Gray cast iron can be welded if you are careful about it.
White cast iron is less common than gray. It is much harder than gray. It is used in industrial applications such as pump sleeves, bearing faces, and mill parts. White cast iron is quite brittle and cannot be welded.