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WELDING OF CAST IRON

The chemical composition and the special structure of cast iron require a few important basic rules to be observed when welding.
Grey cast iron is an iron-carbon-silicon alloy. The carbon can be present in two forms:

1. As a carbon-iron compound (Cementite, Fe3C)
2. As free carbon, in the form of lamellar graphite or graphite noduls.

If the metal cools down too quickly from the welding temperature, there is not enough time for the carbon to precipitate as free carbon, and a hard and brittle white cast iron is formed (cementite, ledebruite, martensite). This occurs especially when a filler metal of similar composition to the parent metal is used. To prevent this happening, the workpiece must be preheated and allowed to cool slowly after welding. The use of a dissimilar filler metal (cast iron electrode with nickel core) makes it possible to keep the heat input, and therefore the formation of hard transition zones, within narrow limits and to obtain welded joints of faultless mechanical quality.

Workpiece preperation

Clean base material of all foreign matter (e.g. oil, grease, rust).
Groove out cracks (angle of approx. 90° degrees) and drill a hole at each end.
For welding joints, bevel edges in V, X or U-form (angle of approx. 90° degrees), depending on material thickness. Grooving or bevelling can be carried out with grinding wheels, pneumatic tools, by hand filling or using Magmaweld EGOUGE gouging electrode.
Remove mill skin on both sides of weld joint, on a width of approx. 20 - 30 mm.
Avoid sharp corners of abrupt junctions.

Preheating and welding

• If welding is carried out with electrodes of similar material or with gas welding rods, preheat the whole workpiece to approximatelly 600°C.
• Allow to cool slowly, in a furnace, in hot sand or ashes.
• When "cold welding" cast iron, deposit only short runs, 20 to 30 mm long, followed by peening. The workpiece must not become more than hand-warm during welding.
• It is advisable to preheat workpieces of complicated shape to 300 - 350°C before welding, even if this is done with dissimilar electrodes.
• Cracks which do not run through the whole workpiece should be welded from the outside towards the inside.

Seal welding of grey cast iron

Absolutely tight sealing welds can only be obtained by welding on an entirely preheated workpice.