One of the most difficult to weld material is the cast iron. The carbon content of cast iron is 2-4%. This is the reason why cast iron is very difficult to weld.
When welding cast iron, the casting must be either kept cool or be allowed to cool slowly.
Temperature is very important factor in welding cast iron. At 1450 degrees F, cast iron tends to crack. Though during welding, the temperature reaches 1450 degrees F or above, it should not hold at that emperature for a long period of time, otherwise, cracking will occur.
Normally, cast iron is preheated prior to welding. The joint edges are preheated up to at least lesser than the critical temperature which is 1450 degrees F. The joint edges is preheated slowly and uniformly. Rosebud torch is advisable to be used. When the joint is uniformly preheated at required temperature, stitch welding (at low current) will commence. Stitch weld shall be done as short as possible until the weldjoint is completed. After completion, cool the weld at very slow rate of cooling. Covering it with welding blanket will be useful. Note: Do not cool the weld at high rate (by using water), otherwise, cracking will occur.
Another way of welding is by not preheating the joint. The joint should be kept cool but not cold. The temperature should be above normal temperature which is at least 95-100 degrees F. during welding, the temperature shall be controlled to 95-105 degrees F range. Don't make it hotter than 105 degrees F. Stitch welding shall also be done as hsort as possible. After completion of welding, cool it down at low cooling rate. Do not increase the cooling rate by using water, otherwise, crack may occur. Covering it with welding blanket is also helpful.
Another important factor in welding cast iron is choosing the right electrode. Nickel-type electrode is commonly used for this job.
NOTE: This is just a guideline. The author shall not be held liable for any outcome of this guidelines once it's used in the industry. It is advisable that PQR shall be run and WPS shall be prepared in accordance to the above guidelines to check if the metallurgical properties of the weld is acceptable.