Welding cast iron is a difficult procedure. The main reason for this is delicate balance required in temperature changes in order to prevent to repaired iron from cracking. If the procedure is done correctly and carefully, most major cracks can be avoided. Smaller cracks near the welded areas can be dealt with afterward if a water-tight seal is desired. As with any type of welding, protective gear and strong caution should be used when repairing cast iron.
Slowly preheat the entire casting in the oven to a temperature approaching 1,100 to 1,200 degrees F. Do not exceed 1,400 degrees F, as temperatures in this range tend to make the iron more susceptible to cracking. Remove the casting from the oven.
Weld the casting with a reduced current, using segments of only about one inch. This will reduce the amount of residual stress during the repair, thus reducing the risk the iron will crack. Weld hastily, but do not over-weld–the more time the casting spends at critical temperatures, the more stress it incurs.
Immediately upon completion, place the casting back into the oven. Gradually reduce the temperature in the oven to cool the iron very slowly. Drop the temperature in the oven roughly 100 degrees every 20 minutes or so until the the casting is back to room temperature.
Examine the area welded for any tiny cracks. Use a standard sealing compound right away to seal up any areas that have started to crack.
Cast iron is very brittle, so temperature is critical. Keeping the iron at extreme temperatures for too long or cooling it too rapidly will greatly increase the risk that the casting will crack. It is crucial to use a short, deliberate weld under a low current, and to work quickly.