A cast iron weld will very often contain hard spots, or will be hard in some places and soft in others. The chief cause of this is the chilling of the metal from the molten state.
When molten cast iron is suddenly cooled, the dissolved carbon remains in the iron as such, but when cooled slowly the carbon separates from the iron in the form of graphite. A fracture in a suddenly cooled metal would show white iron, and on testing with a file, would prove to be very hard; a fracture in slowly cooled metal would show gray iron and would be soft.
Sudden cooling of cast iron makes it hard. When a cold welding rod is plunged into the pool of molten metal under the flame, the rod chills the metal it comes in contact with, and causes small round hard spots in the metal. These hard spots make it difficult to machine or finish the weld. To overcome this problem, the rod should be red hot before it is brought in contact with the melted iron. As an example, when welding a tooth or boss on a gear wheel or other large casting, if the casting is cold and the filler rod is added, the metal cools immediately on being welded, and results in a hard, chilled weld, which lacks strength and is impossible to machine.
Hard, porous and unacceptable welds can also result from using an incorrect flux or an excessive quantity of flux, impurities in the filler rod, or rods low in silicon.