Due to the chemical composition and specific micro structure of cast irons, some rules should be considered prior to start welding. The gray cast iron is an iron-carbon silica alloy. The carbon may be present in the micro structure in two different forms: As a carbon-iron compound (cementite, Fe3C) or as a lamellar graphite or graphite ball. If the metal rapidly cools down from welding temperature to room temperature, the time necessary for the sedimentation of carbon as a free carbon may not be obtained and a hard and fragile white cast iron is formed (cementite, ledeburite, martensite). This particularly occurs if additional metal with a composition similar to that of the base metal is used. The workpiece should be pre-heated, and to prevent such a problem, it must be ensured that the workpiece cools down very slowly after the welding is finished. The use of additional metals with different chemical compositions (nickel alloy cast iron electrodes) ensures that the heat input and solid conversion areas remain within narrow limits and that the mechanical properties of the welding bead remains at sufficient levels.
The following rules should be strictly obeyed to get the best results.