Cast iron can be welded, although it is better to preheat the piece to be welded beforehand. Preheating a cast iron piece to a temperature of 500 to 1,200 degrees Fahrenheit prevents cracking from significant thermal differences.
Preheating the cast iron piece to such high temperatures delays the cooling of the weld. The cast iron should be cooled very slowly to avoid cracking from thermal contraction. Wrapping the piece in an insulating blanket or burying it with dry sand can slow down the cooling process to avoid cracking it.
Low-temperature cast iron welding is also possible. This method calls for raising the temperature of the piece to around 100 F and then making long welds to spread out the spike in heat.