How to Weld Cast Iron: The Basics

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Frank Wilson

Frank Wilson

Senior Editor

You see cast iron everywhere, from kitchens to bathrooms to outdoor areas to bedrooms. Unfortunately, if you’re a welder, it is a pain to work with as it’s brittle and needs high heat output, or it might break.

The carbon in the cast iron makes it less malleable and more brittle. Some metals twist and deform under high heat, but cast iron is unpredictable, and cracks can appear without warning.

Although cast iron welding is challenging, once you have the heat range perfected and understand the basics, you’ll be well on your way to great repair jobs with cast iron.

Last Updated on March 22, 2021

Types of Cast Iron Alloys

To make sure that you don’t mess up the job, it’s vital to know what kind of cast iron alloy you’re working on. There are four different types of cast iron—grey, white, nodular, and malleable. 

The main structural difference is that grey cast iron has graphite underneath, and white has cementite below the skin, making it harder to weld. Nodular and malleable are more recent additions and are the easiest to weld.

Grey Cast Iron

The more pliable of the alloys, grey cast iron is also the most common. It isn’t as strong as steel, nor can it withstand the same shocks. However, it does have commensurate compressive strength as steel, which is how well the metal holds up under compression.

White Cast Iron

The cementite gives this type of cast iron a whitish overtone. Like its grey counterpart, white cast iron has many small fractures, but it is very hard, making it resistant to abrasions. 

Nodular Cast Iron

Invented in the mid-1940s, ductile or nodular cast iron has other compounds, like magnesium and cerium, added to it. These other compounds inhibit graphite growth, which keeps this type of cast iron more malleable than others. 

Malleable Cast Iron

When white cast iron is subjected to specific heat processes, it becomes malleable cast iron and is more workable than usual. The entire process takes two days, after which the metal has to cool completely before being welded again.

Ways to Weld

Besides having several types of cast iron alloys, there are also multiple methods for welding them. If you want to know how to weld cast iron, you must learn how to master the techniques. 

TIG Welding

TIG welding stands for Tungsten Inert Gas and, out of all three methods, is the least effective for welding cast iron, but it is one of the easier techniques to learn. TIG welding uses two rods to create a welded seam of metal and isn’t as capable when welding thick sheets of metal. 

MIG Welding

MIG (Metal Inert Gas) welding uses a wire to create sparks across the metal, melting and feeding into the weld pool. MIG welding is an excellent choice for cast iron, as it is reliable and works well with thicker metal. 

ARC or Stick Welding

This type of welding requires a high level of skill, but it is incredibly reliable and creates unbreakable seams. ARC welding creates a fast and strong weld, but beginners may struggle to successfully master this technique.  

How to Weld Cast Iron

Once you know which cast iron alloy and what method you’ll use, you’re well on your way to learning how to weld cast iron. Cast iron weld jobs only have a 50% success rate since the cast iron carbon could make it brittle. Therefore, heat management is the most important factor in the entire welding process, and pre- and post-heating are essential.

Here are the steps to weld cast iron effectively and successfully.

Step 1: Clean Surface Thoroughly

You must ensure that your surface is pristinely clean, or else you risk a failed weld. Before firing up your welder, perform a thorough cleaning, removing grease, paint, oil, or anything else that may get in the way of a successful weld.

Step 2: Preheat the Area

Preheating is key to a successful weld. It drives the gas bubbles out of the surrounding area, strengthening the work and reducing porosity. 

When working cast iron, you need to avoid the range of 150˚F to 550˚F, which means you are always working at very high temperatures, above 550˚F and below 1400˚F. Preheating the area around where you plant to weld will soften these areas and get them to the appropriate heat level.

Step 3: Take off the Skin

Cast iron usually has a casting skin, a thin layer of metal with a different microstructure than the bulk of the rest of the metal. You have to remove this layer to deliver a good weld. 

Step 4: Weld Low and Slow

When welding cast iron, you must move slowly and use a low current to prevent admixture and residual stress. Move 1” at a time, giving each welded area time to cool before moving on to reduce any residual pressure. Once you’ve completed your seam or repair, brush the area with a slag brush to clean it up.

Many welders use the beading technique, in which they weld a bead of repair metal onto the crack or repair area and slowly draw out the bead with a Peen hammer to flatten it and relieve residual stress. 

Step 5: Let Cool Completely

You not only have to cool your welded cast iron completely, but you also have to do it slowly, or else cracks will appear in the facade of the piece on which you’re working. Many welders use an insulated blanket to wrap the welded cast iron to slow down the cooling process, and others bury the piece in dry sand.

What to Avoid When Welding Cast Iron

If you are using the beading method to make repairs on cast iron, be careful not to weave and only work in 1” increments. Flatten the bead with an appropriate hammer and wait until this portion is cooled enough to touch with your bare hands and not burn them.

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Final Thoughts

Welding cast iron is a slow, laborious process that you cannot rush. With the proper tools and the right knowledge, you can tackle the difficult task of welding cast iron. Some know-how, the right equipment, and a lot of patience can help you become an expert cast iron welder.