How To Weld Cast Iron With A MIG Welder

Cast iron material is something that we use pretty much everywhere, from the kitchen to the workshop. Despite being a reliable metal, if you need to repair anything made of cast iron, it’s much tougher to do compared to other metals.

This particular metal contains high carbon of almost 4%, and it becomes more hard and brittle during the welding process due to carbon migration. Most DIYers, as well as beginner-level professionals, find it difficult to weld cast iron. However, you can still weld on a cast-iron surface using the MIG welding process.

If you are okay with how-to guides, it’s another article that will enhance your skill regarding this matter.

How To Weld Cast Iron With A MIG Welder

What Type of Metal Is Cast Iron?

Compared to stainless steel, cast iron is a less malleable material that is low in tensile strengths. The metal is made of iron, steel, and carbon, so it’s a part of the iron family.

You can easily form casts and molds using irons if it’s in the molten state. When exposed to excessive temperature, cast irons crack due to tensile load.

So, the only way to weld this metal is by expanding it by heating or when it’s stressed.

Why Is It Complicated Welding Cast Iron?

Under intense heat, cast irons cannot hold its tensile strength. Thus, if you apply mild heat or in other words, pre-heating the metal is one of the ways to prevent cracks.

Due to carbon composite, which is 2%-4%, the metal is brittle, and it cracks easily. So, when you apply high heat, the carbon inside the metal emits graphite flakes.

Thermal stress makes it quite difficult to process while using wire feed welding. It seems hard, but not impossible to weld cast iron at all.

Welding Cast Iron Using MIG Welder

Before you start the welding process, there are a few things that need to be taken care of as it’s cast iron that you will be welding.Check best 220v mig welder reviews and best 110v mig welder reviews to know more about mig welder.

Pre-heat and Post-heat the Metal

Cast irons become brittle, if it’s welded using wire feed MIG process. The thermal contraction is the thing that affects the tensile strength of cast iron. Thus, when the metal is exposed to excessive heat, it will shrink and crack.

So, quickly shifting the metal to welding temperature from room temperature will cause excessive stress, and cause the metal to crack. Similarly, if you try to cool the welded iron cast down too fast, it will also cause stress and fracture.

You can pre-heat cast iron using the blowtorch. Many DIYers get the job done using a car engine. If you want to go professionally, you will need filler rods for preheating the iron and braze it properly.

Whatever method you follow, make sure you pre-heat the cast iron with 260 to 6500C. If you go above 780, it will cause the metal to overheat and cause cracks on it.

Also, be sure to wrap the metal piece with a wrapping blanket when you post-heat it. You can also try using a charcoal BBQ and place the metal inside a bucket filled with sand and place the metal on the top of a fire wood oven. It will cool down the iron cast slowly when the fire will be no longer there.

Let’s Begin the Welding

Before you start the welding process, keep in mind that you need to go slow and low to be able to weld cast iron without damaging the piece.

Slowly deliver a low current, similar to stitching. Weld one inch at once, and do it in different areas on the iron piece. Give a 45-seconds gap within segments to make sure the metal isn’t overheated, and there is minimal residual stress. Heat management is more crucial thing here.

When you are done welding, see if there is any crack or slag hole in the metal piece. You can handle leaks using a compound sealing. Finally, use a slag brush to clear the excess slag to make the surface smooth.

Now, cool down the cast iron piece using your preferable post-heating method, let the metal cool down gradually.

Tips to Follow to Weld Cast Iron Using MIG

Even though many professionals don’t recommend using MIG to weld cast iron pieces, you can do it effectively following the tips below.

  1. The best MIG wire to weld the metal is special nickel wire. However, it’s quite expensive, if you don’t want to spend much on it, get MIG wire of stainless steel material.
  2. You can use 80% argon and 20% carbon dioxide to make welding work more effectively. Using these gases will make the weld rust by the time. But, cast iron is naturally prone to corrosion, so there will not be an issue adopting this opinion.
  3. Don’t use any brazing wire as it weakens the weld. It’s not suggested to use such metals.
  4. Studding can give the weld mechanical strength. Do it if it’s possible for you.
  5. Don’t go above 700 degrees Celsius while preheating the metal. Weld by dividing a few spots, and let the piece cool down between welding.
  6. If the cast iron has oil or some kind of stain on it, be sure to heat the edge until it turns a dull red color.

Best Rod to Use to Weld Cast Iron

99% of nickel rods are quite expensive and considered as the most premium option out there. It can provide the best machinable welding on low to medium phosphorous cast objects.

55% of nickel rods are an inexpensive option. Using it will provide decent machinable results. Professionals use it for repairing thicker sections in most cases. Compared to the 99% grade, its coefficient of expansion is much lower and causes much fewer cracks on its fusion line.

Steel rods are the cheapest option available for welding cast iron. For simple fillers and repairs, steel electrodes do a decent job. As these aren’t machinable, you will need to put extra effort into grinding on hard welds. Compared to others, these rods are even user-friendly for arc welding.

Final Words

Hopefully, now you got the idea of how to proceed for welding cast iron with MIG welder without ruining the metal piece.

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