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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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