Welding of Cast Iron

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.

Preparing the Workpiece

The base metal should be cleaned up from all foreign materials (i.e. oil, grease, rust).
The cracks should be cleaned by opening (with an angle of approximately 90°) the welding bend and a stopper hole should be bored on both ends.
For welded connections the workpiece should be prepared by opening V, X or U welding bend (with an angle of approximately 90°) according to the material thickness.
The welding bend may be opened or carved by grinding method, with pneumatic tools or with OERLIKON E 900 mitering electrode. The surfaces on both sides of the welding bend (nearly 20 – 30 mm to the bend) should be cleaned by grinding.
Sharp edges and corners should be avoided.

Pre-Heating and Welding

If the welding is performed with electrode or oxyacetylene stick having the same chemical composition the whole workpiece should be homogenously pre-heated up to app. 600°C.
The welded piece should be cooled down slowly in furnace, hot sand or ash.
When performing "cold weld" on cast iron, the welding with short passes (20-30 mm) should be preferred and the welds should be immediately hammered. Overheating the workpiece during welding should be avoided.
It is recommended that the workpieces with complex geometry should be pre-heated to 300 - 350°C before welding even if nickel content welding electrodes are used.
The cracks that are not outside the workpiece should be welded outside-in.

Sealed Welding of Gray Cast Iron

The sealed welds are only possible on pre-heated workpieces.