Welding Cast Iron

When you combine carbon and iron, the result is the metal we call cast iron. It’s durable, resistant to wear and has been in widespread use for centuries. Even with the development and use of other – arguably more specialised and exotic – metals, this workhorse material continues to be used widely. Here’s some advice on welding cast iron from the R-Tech team… 

Why is cast iron welding difficult?

There are many different grades of cast iron and the weldability of these grades varies considerably. So-called grey cast iron is the most common; it’s the material that we’re dealing with in this article (the others are known as malleable cast iron and ductile cast iron).

Can you weld cast iron?

It’s difficult, but certainly not impossible. Despite its widespread use for so long, cast iron continues to have a reputation for being a difficult material to weld. This is largely due to its high carbon content (often 2.5–4%), and the hardening and brittleness caused by carbon migration during the welding process.

Without detailed metallurgical analysis it can be hard – if not impossible – to tell different grades of cast iron apart, let alone determine the brittleness of a particular metal. Yes, although it’s wrong to say all cast iron is brittle, many grades are. This has contributed to cast iron’s reputation for being highly sensitive and prone to cracking during welding.

MIG, TIG or MMA?

Depending on who you talk to and specific project details, you might be able to use MIG and TIG to weld cast iron in some circumstances. However, this usually comes with strong cautions. The general consensus among the R-Tech team is that manual metal arc welding (MMA) should initially be the go-to process for welding cast iron.

This is particularly so with repairs to castings. These form the majority of cast iron welding work (the brittleness of cast iron castings often leads to breakage). Typical projects? How about mending a turbocharger housing or motorcycle exhaust flange, repairing a lathe’s broken tail-stock, or fixing the leg of a cast iron stove?

MMA arc welding cast iron

Good practice and patience are essential. Here’s some useful advice for welding with cast iron:

• Control the expansion and contraction of your cast iron workpiece(s). It’s very important to pre-heat the cast iron gently to minimise thermal shock – particularly if you’re welding with cast iron rods. Don’t pre-heat cast iron beyond 760°C (1400°F). This will take it out of its austenitic phase, and into a critical temperature range where it will change further as it approaches its melting point. Wherever possible, heat the whole casting and weld using the lowest possible amperage to minimise local heating of the metal.
• As well as the point made above, it’s also helpful to keep the workpiece warm while welding and to cool it as slowly as possible afterwards. You may even consider bedding the welded casting in sand to help it cool slowly. Whatever you do, don’t use compressed air for cooling or throw cold water on the workpiece (it’s happened!). Cast iron really doesn’t like being cooled too quickly. If you hear ‘tinkling noises’ from the cooling work, it probably means there’s been some internal fracturing. That’s not good.
• Keep your welding rod vertical.
• Stagger your welding as much as you can. For instance, you may be able to weld in another area of the workpiece while your first weld cools. Welding a series of small segments like this, rather than attempting one long weld, helps prevent localised over-heating, build-up of residual stresses and cracking. A good rule of thumb? Avoid single runs of welding that exceed 10x the diameter of your welding rod.
• Clean your workpiece after every weld.
• For additional strength in a welded cast iron joint, consider drilling, tapping and studding the joint before completing the weld.
• Wherever possible, seal existing cracks to avoid them being extended. Creating a V- or U-groove around cracks and joints in cast iron helps ensure that the weld doesn’t just sit atop the crack – and helps you achieve maximum penetration and weld integrity. Using a U-groove with rounded edges may help avoid heat build-up which can aggravate carbon migration.
• Take your time when welding cast iron. Patience is always helpful when welding, but the particular challenges of cast iron make thorough preparation of your workpiece essential. Thoroughly remove rust, oil grease and any other contaminants, take your time and get the job right. 

What kind of welding rod do you use on cast iron?

One of the attractions of using MMA for cast iron is the wide range of available stick welding electrodes, including machinable and non-machinable variants. A Nickel (Ni) MMA welding rod for cast iron (sometimes called ‘cold cast rod’) often helps. They’re not cheap, compared with Ferro-Nickel rods, but they can make a big difference to your cast iron welding. Both types of cast iron rod incorporate specialised high-Graphite flux to limit undesirable carbon migration in and around the weld. 

MIG welding cast iron

Although cast iron can be welded with the MIG process and specialised flux-cored electrode wires, the resulting welded joint won’t be as strong as with MMA arc welding. As mentioned earlier, the R-Tech team recommends MMA stick welding as first choice for welding cast iron.

TIG welding cast iron

Opinions on the use of TIG welding for cast iron range from it being possible with specialised filler rods, to its complete unsuitability due to an open arc process such as TIG giving no opportunity for carbon migration. It’s not something the R-Tech team advocates. Again, their recommendation is to use MMA stick welding for cast iron.

Can cast iron be welded or brazed?

Cast iron can definitely be welded, and it can also be brazed – including with TIG brazing. Depending on the specific cast-iron welding project, R-Tech Welding Supplies’ Mike Gadsby suggests considering this as a jointing option:

‘TIG brazing, with a TIG welding machine, may offer a satisfactory alternative to other processes. With TIG brazing you use lower amperage to heat the workpiece, but without actually melting the cast iron. There’s no need to use flux, as you would do when brazing with oxy-fuel. That’s because you can use the gas shroud effect of the TIG torch (use argon) to shield the brazing zone. Run DC current at around 50% normal amperage and use something like Phosphor Bronze No8 (C11) or Sifalbronze No.32 rods. As usual, cleanliness is critical, so you must be very thorough with your preliminary deoxidising and degreasing. With the understanding that you won’t achieve the strength and joint integrity that welding can achieve, TIG brazing may be worth considering – remember to use a suitable filler rod, lower temperatures and fill-in with braze.’

Get more advice on welding cast iron

Whatever your cast iron welding project, you’ll benefit from helpful advice, a quality welding machine and lots of practice. Add quality consumables and you’ll be well on your way to mastering cast iron welding . If you’re a professional who’s already there, you’ll know just what we mean.

Once you’ve mastered the basics of welding cast iron, you may even extend your skills into more specialised areas such as welding cast iron to mild steel, welding wrought iron or, perhaps, having a go at MIG welding cast iron….

Let’s talk about welding iron

Whatever you and your machine will be welding with cast iron, remember that the R-Tech team is here for you. Of course they’ll help you choose anything from a kilo of TIG brazing rods to a top-of-the-range MIG or TIG welding machine capable of arc welding cast iron (MMA). However, they also like sharing valuable advice based on decades of experience – or simply talking about the challenges of welding cast iron for a few minutes.