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Cast iron may only be welded by specially designed electrodes having particular
chemical, metallurgical and mechanical properties. These electrodes are mainly
divided in two groups: one gives a weld metal composition which is completely different
than that of the base metal.

The first group of electrodes have two types; non-alloyed steel core and cast iron
(gray cast iron or spheroid al cast iron) core. These electrodes should be welded after
preheating the work piece to 150 - 170 ºC. The degree of preheating is determined
by factors such as size of the work piece, complexity of shape and whether the welding
area will be machined or not after completion of the welding process. In some special
cases where the piece has small dents or cracks, it can be repaired with these
electrodes without preheating, provided that the shape of the piece is suitable and
that the area is not to be machined, as such welds would be too hard for normal
machining procedures.

The second group of electrodes includes pure nickel, nickel-based and
copper-based electrodes. Today nickel and nickel-based electrodes have a wide
range of application in welding cast iron parts or joining cast iron parts with other
metals or alloys. These electrodes may be used without preheating, although preheating
is always preferred. The weld metal produced by nickel based electrodes has the
same colour as that of the cast iron parts, whereas, tin bronze and aluminium bronze
electrodes, which could also be used in the same way, give a weldment with a
distinctive brass colour.

When welding with this type of electrodes (Nickel-Copper alloys), lowest possible
welding currents should be selected to minimize the mixing of the filler metal with that
of the base metal. These electrodes are used for second degree joints and special
Hardfacing applications on cast iron.

Cast iron electrodes are standardized in AWS A5.15, and DIN 8573.

Cast irons are not suitable for plastic deformation. Therefore, the expansion formed
by the heat input in the welding area produces stresses resulting in the enlarging of
existing cracks or leading to new cracks. When repairing cracks the edge preparation
should better be done with machining rather than grinding, as the vibrations produced
by grinding could also enlarge the existing cracks. Before starting the operation, holes
should be drilled to both ends of the cracks to prevent the spreading of the cracks.
Welding should be continued with alternating short passes to prevent a heat
concentration in one point, which would produce high internal stresses. That is also
why preheating, if not a must is always a help.

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