Can You Weld Cast Iron? Your Complete Guide

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Welding cast iron is quite a challenge, but if you rigorously follow the recommended guidelines, you may be able to repair cast iron parts with little or no cracking. 

Discover why welding cast iron is so difficult, how to weld it, and which welding rods are best to use for cast iron.

The Biggest Challenge with Cast Iron Welding

Cast irons are iron-carbon alloys with 2-4% carbon content, about 10x the carbon amount in most steels. There are many cast iron types, but our guide focuses on grey cast iron because it&#;s the most commonly used. Keep in mind that it&#;s impossible to weld white cast iron, while all other types are weldable.

While cast irons are hard, strong and machinable, they have poor ductility, making them incredibly difficult to weld. Welding any metal causes expansion and contraction as the material heats and cools. However, since cast iron has low ductility, especially in the fusion zone, the residual stresses from expansion and contraction can cause weld cracking.

When you weld mild steel, a ductile material, as the weld and the fusion zone expand and contract, the surrounding material can &#;flex.&#; However, cast iron doesn&#;t deform before breaking - it just breaks. So, when the weld cools and starts pulling the surrounding cast iron mass, it can easily crack.

To make things worse, the welding process can create locally-enriched carbon zones, and on rapid cooling, the heat affected zone (HAZ) can become extremely hard and brittle. That&#;s why cast iron is susceptible to HAZ cracking and generally considered challenging to weld.

How to Weld Cast Iron

It&#;s possible to weld cast iron using oxy-acetylene and stick arc welding (SMAW). You can also join cast iron parts by brazing, but the bond cannot match the welding strength. 

Welders commonly use the stick welding process for cast iron. Its high-temperature arc allows fast travel speeds, and you can use lower preheat temperatures. In addition, stick welding can be used with a DCEN (electrode negative) polarity, which can help you reduce HAZ. Make sure to use a quality arc welder like the Kemppi Minarc Evo 140VRD, or the more powerful version, Kemppi Minarc Evo 180VRD. Your welder should have a stable and predictable arc output to avoid inconsistencies. Since cast iron is a sensitive material, you don&#;t want to use poorly optimised machines.

Prepare the Part for Welding

Removing all surface oil, paint, grease, and casting skin is crucial. In addition, remove the residual graphite from the surface using mineral spirits. Pay particular care to the weld area.

If the joint requires grooves, use a V groove with a 60-90° included angle. It&#;s helpful to create grooves around cracks and joints to ensure maximum penetration and avoid surface-only welds.

Controlling the Heat

Preheating the cast iron is the key step in avoiding weld cracking. You should preheat the entire casting slowly and evenly, if possible. As the part is heated, it will expand. As a result, it can easily crack if there is a thermal gradient over its volume. However, if the heat is applied evenly, there won&#;t be any localised spots resisting expansion that can crack.

Preheating will also allow the casting to cool slowly, decrease the thermal gradient between the HAZ and the rest of the part, and reduce the residual stresses. Typical preheating temperatures for cast irons are 260-650 degrees Celsius (500-°F). Never preheat above 760°C (°F).

Stagger the welds during the welding process whenever possible. Use the least amount of amperage and avoid making welds longer than ten times your electrode&#;s diameter. 

Allow the casting to cool slowly after welding to prevent residual stresses and cracking. You can put it in sand or cover it with specialised welding blankets to prevent rapid heat loss. 

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Welding Rods for Cast Iron

Nickel alloy welding rods (AWS A5.1 Eni-CL) are the most commonly used electrodes for welding cast iron, and for a good reason. Nickel electrodes create a ductile weld metal that remains soft and machinable. The welds will have reduced stresses and improved resistance to cracking.

Need Additional Help?

Welding cast iron is tricky, even for highly experienced welders. If you need further assistance in equipment and filler metal selection, reach out to the experts at Alphaweld. Give us a call on (08) or get in touch and our team will gladly assist you.

Welding Cast Iron has quite a few potential pitfalls, but if done correctly, it&#;s not difficult.

Why is Welding Cast Iron problematic

The key reason why welding cast iron can be problematic is the high carbon content.  During the welding process, this carbon migrates into the weld metal and/or the heat affected zone adjacent to the weld metal, causing elevated hardness/brittleness.  This is how Cast Iron gets its reputation for post weld cracking.

Process for Welding Cast Iron

Gas Welding heats the Cast Iron slower than Arc based processes and the flame is lower temperature than an arc.  This means Carbon migration is not normally a problem.  Use of a proprietary Cast iron Gas Welding rod is important.  The only real downsides to Gas Welding Cast Iron is the amount of heat needed if components are large.  Gas Welding is also a slow process.

Arc/Stick Welding is, arguably, the best all-round process for Welding Cast Iron, provided the correct welding rods are used.  Cast iron Welding Rods have a special Graphite rich flux, this graphite chemically ties up the Carbon in the Cast Iron, limiting migration into the weld metal and heat affected zone.  There are two common types of Cast iron Welding Rod, Ferro-Nickel and Pure Nickel.  Ferro-Nickel are typically 53% Steel and 47% Nickel.  Ferro-Nickel Rods are cheaper than pure Nickel and are ideal for welding Cast Iron to Steel.  Pure Nickel will produce a softer, more malleable weld deposit.  I would advocate using Ferro-Nickel, unless the job specifically requires Pure Nickel.

Mig Welding is, in my opinion, not a great way to weld Cast Iron.  Whilst there are specialist Flux Cored Wires available, unless you have a repeating application that you can create a procedure for, I would not advocate Mig Welding Cast iron.

Tig Welding is not considered a suitable process for Welding Cast Iron.  An open arc process such as Tig offers no opportunity to mitigate Carbon migration.

Welding Techniques

Gas Welding &#; There are no specific techniques that need to be deployed.

Arc/Stick Welding &#; All of the following are important:

1. Prep the job with a &#;U&#; shaped groove, avoid sharp corners as this can lead to heat build up which will exacerbate carbon migration.
2. Use the correct type of welding rod.
3. Ensure the component is AT LEAST at room temperature, adding a small amount of pre-heat will help, but you only need the component to be warm to the touch.
4. Limit the amount of welding done in one run.  As a rule of thumb, do not put down a continuous run that has a length greater than 10x the diameter of the welding rod being used.  Having said that, it&#;s ok to put down multiple runs in different parts of the component (see graphic in next item).  Eg, if repairing a 300mm crack with a 3.2mm rod, you can weld a run of 32mm, then do another 32mm run in another part of the crack.  Avoid letting the weld area get too hot, this is the purpose of short runs.
5. If you&#;re repairing  crack, run a bead across each end of the crack to avoid the crack spreading further.
6. Keep the welding rod vertical, not at an angle, like you would for most stick welding.
7. Don&#;t expect to weld cast iron quickly, the key is to take you time and do it properly.

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Graham

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