How To Weld Cast Iron

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Learning how to weld cast iron is in general a difficult process as there are a variety of cast iron types with no two pieces the same. Most welding of cast iron is in the repair of cracked or broken castings due to the brittle nature of the metal. Other repairs to cast iron pieces include the building up of worn parts and flanges that have been eroded or coroded. These build up repairs can be done with specific cast iron welding electrodes or bronze. After the building process up the piece is  machined to its desired size.

Pre-Heating

It is generally prefered to pre heat cast iron before welding, typically to around 600 to 1200 degrees f. There are temperature crayons avaliable that melt at a certain temperature so you can keep the casting at a consistent heat. Preheating the casting before welding will slow the cooling rate of the weld, and the area around the weld.
Pre-heat the entire casting if possible and after welding, to reduce the tendancy to crack, allow the part to cool very slowly. Common practice for cooling include burying the part in sand, or wraping it in a insulating blanket.

Weld using a low current, and reduce the weld to short segments of about 30mm long, peening of the weld can help reduce the risk of cracking.

How to weld cast iron without pre-heating

In many cases the size of the casting, will require that the repair be made without pre-heating. When this is the case, the part needs to be kept cool, but not cold.
Heat the casting to around 100 f, this is not so hot as you cant put your hand upon it. Again short use welds of about 30mm, run welds in the same direction, peen the weld and allow the weld to cool to the casting temperature. Do not cool using water or blowing air.

 Cast Iron Electrodes

Various electrodes are avaliable depending upon the weld to be done, with specific rods for machining and general repair work.

How to weld cast iron with bronze.

This is generally used for small part repairs. The area to be welded should be prepared by filling and be free from contaminants. Pre-heat the weld area to a cherry red and sprinkle liberally with cast iron flux. Applt the bronze filler rod, again using plenty of flux.

Resources

How to spot weld