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Welding Cast Iron

Thursday, 19th March 2020
Graham (Tech Advisor)
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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.

I hope you found this blog article about Welding Cast Iron useful, if things work out well for you, please feel free to post some pictures of your achievements on our Facebook Page

Please let me know what you thought of this article by leaving a comment.  Don’t worry, your email address won’t be added to a database or shared and you won’t receive any unsolicited email.



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9 responses to “Welding Cast Iron”

  1. Jim Barnett G0JGB says:

    is the new cmt technique suitable for cast iron ?

    • Graham says:

      Hi Jim
      I regret I’m not familiar enough with the Cold Metal Transfer process to definitively answer your question. I’m inclined to think it would not be suitable, because of it’s use of an open arc, but my suggestion would be to speak to a specialist like Fronius for a conclusive answer.
      Sorry I couldn’t be of more help
      Kind Regards

  2. marc Wheatley says:

    I am looking for supplies for arc welding cast. I live in Derbyshire. Dose BOC sale them.

  3. Steve says:

    What are the chances of being able to mig braze an engine block, mane the pros and cons

    • Graham says:

      Hi Steve
      You would need to get the whole engine block hot to have a sporting chance. Using Mig on Cast Iron is risky because arcing on Cast Iron can cause the formation of White Iron, which is incredibly hard and brittle. If you are going to try it, place a piece of steel on the block and arc up on that, then run off onto the block. You will need to cut the piece of steel off afterwards.
      Personally, I would Braze it with an Oxy/Fuel Torch, or Stick Weld it.
      Hope that helps, good luck.

  4. davidokeefe says:

    Years ago gas welded some thick cast iron plates pre heated plates can’t remember temp but was quite high pre heat.after repair was finished I placed Weldmet in a box with a fine sand covering it fully and this helped to cool slowly but I would think filler materials have advanced a lot now great column graham

    • Graham says:

      Thanks for the comment David
      Filler materials have advanced, but your method is still sound, especially where repairing older cast iron parts that may not be made from the best quality material. 300⁰C is a good pre-heat temp, but as you clearly know, any pre-heat helps as it reduces thermal shock.

  5. Wilfred Moses says:

    That’s really an informative article.
    However one more thing to add is that the welder’s hand should be steady and should be kept with a constant speed throughout the weld.

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