Welding cast iron using a tungsten inert gas (TIG) welder is possible, but will challenge even the most seasoned welder. Cast iron contains a lot of carbon; anywhere from 3 percent to as much as 8 percent. This carbon content makes cast iron very brittle and extremely difficult to weld. Welding cast iron requires a precise process and careful temperature monitoring to ensure a quality weld and prevent cracking in the cast iron itself.
Preparation
Remove rust and oxides from the surfaces to be joined using a grinder. A smooth surface will help with weld formation. Before welding, preheat the parts to at least 500 degrees F, and higher if possible. If the parts are too cool when welding begins, the thermal shock at the weld location will cause the cast iron to crack. If extensive welding is required, the part will need to be continually heated to compensate for cooling occurring on the edges of the part. Cast iron is extremely sensitive to temperature changes, so maintaining a constant temperature across the part during the welding process is important.
Welding
Select a filler rod material based on the final application. For pieces that will not undergo significant thermal cycling, aluminum bronze makes an excellent filler material. For pieces that will require post-welding machining, or will experience extended thermal cycling, such as engines, use a nickel filler rod. Use a low current to weld, to prevent residual stresses from building up. Create several short welds instead of one continuous weld to prevent excess heating by the arc. Striking the weld with a ball peen hammer while the weld is still red can help to relieve the stresses in the weld caused by material shrinkage.
Cooling
It is critical that the cast iron parts are allowed to cool slowly after the welding is completed. If cast iron cools too quickly, it will become hardened. Hardened steel becomes even more brittle and difficult to work with, so it is important that the part is cooled slowly over 12 to 24 hours.This may require the part to be held at temperature in an oven and have the temperature step down over time, or hold the hot part in an insulating material, such as sand, until it comes to ambient temperature.
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