Can You Weld Cast Iron?

Cast iron is not as commonplace as it used to be, but there are still millions of tons of it in service as valves, machinery frames, and even bridges. This is due to its abundance in the 20th century but also because it is so durable.

One issue many people face is the ability to repair cast iron parts if they crack or become damaged through use. In this article, we’ll discuss how to weld cast iron.

Can You Weld Cast Iron?

Generally, yes, but it needs extra steps and care when compared to steel. There is one exception: white cast iron is not considered weldable.

Regardless of type, all cast irons need complex procedures and specialist welding consumables to create a successful joint.

The Short Answer Explained

Gray cast iron is the most common type and can be welded with specific post and pre-heat treatments and specialist consumables. Grey cast iron has a dull grey finish, which is sometimes black. It produces fine powder-like shavings when drilled.

White cast iron, as mentioned, is widely considered unweldable. Its crystal structure and composition make the formation of iron carbides possible. White cast iron is bright and silvery and shows a prominent crystalline structure when broken.

Malleable iron can appear similar to white cast iron, showing a whitish deposit when broken, but it is far more weldable. This is thanks to its far lower impurity and carbon content.

Why Cast Iron Is Difficult to Weld

Cast iron is difficult to weld because it contains around 10x more carbon than regular steel. This makes the material very hard-wearing but also brittle and subject to cracking.

This is thanks to the high carbon content, causing high crack sensitivity and extreme localized hardness.

Best Welding Methods for Cast Iron (Stick, MIG, TIG, Brazing)

Most welding methods can be used to weld cast iron, with the correct consumable type. However, some are preferred over others. Listed below are the welding types, listed in order of preference:

1. MMA welding – MMA or stick welding is usually considered the best method, thanks to the availability of specific electrodes, and the relatively low heat input
2. MAG welding – A nickel consumable is often used, alongside a 20% CO₂ 80% argon shielding gas. This is generally more accessible, but gives less control than MMA
3. TIG Welding – Whilst precise and clean, TIG has a higher heat input than MAG or MMA, so it is not the most ideal method. However, with careful heat control and adequate heat treatment, it can be done
4. Oxy-Acetylene welding – An older method, which allows for better control of the HAZ, limiting martensite formation, but it requires high skill levels and preheat temperatures of 600°C+
5. Brazing – Whilst not technically welding, brazing can give a strong enough joint, with far lower risk of cracking, as the heat input is generally lower

Preheating, Cooling, and Filler Rod Selection When Welding Cast Iron

Both preheating and controlled cooling are essential when welding all types of cast iron, and filler rod selection is very important too; normal steel electrodes/wire are not suitable.

Cast Iron Preheating

Preheating reduces the temperature differential between the weld pool and the surrounding metal. When welding regular low-carbon steel, the resulting stress induced by the temperature differential is eased by the material stretching.

Cast iron is not malleable and cracks instead of stretching; this is why preheating is so important.

Choosing the correct preheat temperature is vital and first requires the identification of the cast material.

Cast Iron Filler Rod Selection

Rods specifically for cast iron are commercially available and tend to be high in nickel content. 99% nickel rods produce the best quality welds that can be machined afterward.

50-55% nickel electrodes are cheaper than 99% but can still produce satisfactory welds and are ideal for welding cast iron to steel.

In a pinch, steel rods can be used but need even more caution and heat control than the above, having the potential to produce extremely hard deposits that are very difficult to machine.

Is Welding Cast Iron Worth It

While it’s true that welding cast iron takes a lot more prep, skill, and control than normal materials, in some cases, it is invaluable. Particular cases, such as repair or reinforcement of vintage or legacy machinery, spring to mind, where no other easy solution is available.

In the case of replaceable parts such as valves, flanges, or pipework, where either a single item or a better entire system can be installed, the difficulty of welding cast iron is probably not worth it.