Welding Cast Iron (#20)
When attempting to weld a cracked and/or broken cast iron part there are several things to consider. First, a die penetrant test should be conducted to check the extent of the crack. Once the length of the crack is determined, I like to drill a small (1/8˝) diameter hole at each end of the crack to prevent further propagation of the crack during the welding process.
Next, the crack must be fully “V” notched and ground smooth before welding can begin. I use a stick welder with a high nickel electrode, preferably with 85% nickel. This makes the weld softer and less likely to crack and easier to machine.
When a high carbon electrode is used the high carbon content of the cast iron (usually 2-4%) is pulled into the weld pool and that along with the high carbon content of the electrode makes the weld become very hard and possibly crack unless precise pre-heat and post-heat treatment procedures are followed. This is not always practical in a small shop.
It is always a good idea to do a simple pre-heat (even when using high nickel electrodes) with an acetylene torch getting the iron to about 250°F -500° F. If repairing a deep crack or complete break, do it in several passes to prolong the cooling rate. A cast iron weld using this procedure can be easily machined or ground smooth.
Kovach & Assoc. Performance Engine Building