Author Topic: How to weld cast iron with a MiG welder.  (Read 37458 times)


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How to weld cast iron with a MiG welder.
« on: July 26, 2006, 02:40:59 AM »

Sometimes we are going to come across Cast Iron that needs welding up, an example being the frost damage to the barrel of my Lister when I bought it which I talk about elsewhere on here.

I am not a boilermaker, I am an engineer, which means I can weld, and means I do know about materials and metallurgy. I would not even attempt to weld a pressure vessel, that is a job for a boilermaker.

Many cast iron items that could be welded should not be, ever, by anyone, no matter what method is used, Lister cast iron components that should never ever ever be welded include the flywheels and con rods, because one stores vast amounts of energy and the other transmits vast amounts of energy.

Things that CAN be welded are things not subject to rapid motion, and not subject to great stresses, good example are the aforementioned frost damaged barrels, and things like car exhaust manifolds.

There are lots of ways of repairing or welding cast iron, these include metal stitching, metal spraying, and straight welding with preheat and delayed cooling amongst others, generally speaking you will find people recommend the one that they can do in exchange for money.

Sometimes this doesn’t help, sometimes the expense of doing things this way is crazy, and sometimes the associated expense, because it means you have to strip something to take it to the welder.

There is another way...

Saying “Cast Iron” is a bit like saying “Human Being”, everything from the newborn black (screw political correctness) infant through the teenage oriental to the elderly white man are all “Human beings” and we haven’t even got into what sex they are yet.

This is the first thing to understand, that cast iron is not an element like Titanium, nor is it a fixed mixture of things like Table salt, and the only way to tell what particular mixture or grade of Cast iron you have in any given thing (do NOT assume that any two items on the same engine have the same grade of iron) is to subject it to costly and time consuming materials analysis.

Your most useful tools are your eyes, your fingertips, common sense and experience, if you lack these ask someone who has, I do NOT propose to teach you about cast irons.

However, you can make a good guess at the grade from the application, cast iron used for engine bases, engine blocks, even pistons, is likely to be weldable, particularly if it comes from a known quality source such as Listers themselves.

The problems with welding cast iron are two fold in the main.

   1. When the weld material itself cools and contracts it is likely to cause fractures elsewhere.
   2. When then weld region temperatures rise and fall it is likely to cause carbon migration.

Number one is easy enough to visualise, cooling welds pull with tons of force, so it is obvious this can crack a brittle material like cast iron.

Number two is not so easy, a weld is molten metal, when the weld cools you are in effect recasting a small portion of the material, and how a casting is cooled is as important as what mixture of elements it contains in determining its properties, this is why preheating and delayed cooling are touted, it limits the migration of carbon and thus the creation of weld material that is literally as hard as an engineers’ file. e.g. too brittle to be any use and too hard to grind.

There is another way, all you need is a standard (not gasless) MiG welder, standard steel MiG welding wire, and standard Argon/CO2 welding gas.

    * Set the MiG welder to use the slowest feed rate and lowest current for the wire (0.6 or 0.8 mm wire is ideal) to maintain a nice weld, test on a piece of mild steel.
    * THOROUGHLY clean, grind, file, dress up and dry the area to be welded.
    * Weld in very small stitches, no more than 5-10 mm long at a time.
    * Leave a gap of at least 20 mm in between each stitch.
    * Do NOT try to weld the full depth in one pass, do root, then #1, then fill.
    * Leave enough time in between each stitch for the last one to draw all the heat away into the surrounding metal, DO NOT SPIT to test and this cools rapidly, use your finger, too hot to keep your finger on is OK, burnt finger is too hot, uncomfortably warm is about right, because ideally you don’t want the whole thing to cool.
    * Use a small and weedy angle grinder or better still a die grinder to take back the excess weld in between stitches and look for any porosity or pits.
    * Continue welding until done.

As a rough guide two pieces of frost damaged barrel making a hole bigger than the palm of my hand, done in situ without disturbing the barrel or anything else, took me about 4 hours. The only disadvantage was all welding was vertical, which didn’t really make any difference.

I dunno how much 0.8 mm MiG wire or gas I used, but one 115 mm grinding stone did all the dressing, and the job itself was done with a Clarke 120e Mk2

As you can see from the picture below, a damn near perfect repair, structurally as sound and as strong as the original casting, no local material hardening and no local pre-stressing from weld contraction, done in situ without the need to strip anything, for a total cost in consumables of maybe at the worst £10 including Ar/CO2 gas / wire / grinding disk / electric, and 4 hours of my time.

The secret is slow, steady, patient, and don’t get the work hot.

If you want to know anything about MiG welding can I recommend you spend some time over at which is a thoroughly excellent site.

As far as MiG welders themselves go, you can pick decent ones up on E-bay for £100, which is less than they cost to hire for a weekend. I’d recommend buying a half decent second hand one instead of a new hobby one. Hobby means the little Sips and everything that is gasless or does not take a full size reel of wire. As far as gas goes, pub bottle of CO2 is fine for mild steel, but I would recommend “proper” argon / carbon dioxide for cast iron.

Note, you CAN get specialist MiG wires, e.g. for aluminium, for various grades of stainless steels, and even for cast iron, just as you can get specialist stick electrodes. Your call if they are worth it.

I personally think ALL stick electrode welders will generate too much heat to weld this way, you’d have to preheat and delay cooling, and that goes for Oxy-acetylene and brazing too. The secret of the MiG is the very low amount of welding heat that you can generate on the low settings with small wire.

« Last Edit: July 26, 2006, 02:45:20 AM by GuyFawkes »
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Re: How to weld cast iron with a MiG welder.
« Reply #1 on: July 26, 2006, 06:54:48 AM »
guy:  good post with good well reasoned and laid out approach

an alternative:

exceptionally good results can be had with a nickel based rod and a stick welder as well.

the key to success is based on the following factors

a. properly prepared and cleaned castings, this means V out the cracks, make sure they are clean and dry, (more on clean later)

b. useing nickel rod, run a quick pass at moderate heat, dont worry about how it looks, because this is a cleaning pass.
   the reason i call this a cleaning pass is because cast iron is porous and oil and other liquid impurities will boil out of the pours and fowl the weld area, after the cleaning pass the crack will have to be reground out, V it well.

c, now go back at near the rated amperage for the rod you are using, and weld in short stitches of approx 1/2 inch length, flip your helmet up quickly and using your chisel end of the chipping hammer clean the slag and work the weld well, nice sharp blows perpendicular to the crack, this relieves the stresses set up in the weld. i usually strike the weld upwards of 50 times, not hard, just deliberate, you will  see the small indentions made by the chisel point.

d. continue 1/2 inch stitches and work the welds, as described in (c) above.

the time it takes to properly work the weld will allow the casting to cool and there is not much heat left to cause additional stresses to be built up.

i have used this method to salvage, cylinder heads, crankcases, winch cases (which are under extreme pressures) with excellent results.

one of the problems with welding cast is the weld zone shrinking and pulling the cast and creating a new crack, all the chisel peaning will relieve this contraction by actually expaning the weld zone.

the key is preparation, and taking your time, do it right the first time and it will be a good repair that will last as long as the original part.

after you are done, grind to suit for appearance, (bondo and paint if you are a purist)  :)

bob g,,
(useful forums), for all sorts of diy info