Linde BOC RAPTOR 160 MIG Operating Manual page 17

of Ohms Law that states that as the electrical
resistance increases if the voltage remains stable
then the current must fall.
To bring the welding current back to 250 amps
it is necessary to increase the wire feed speed,
effectively increasing the amount of wire being
pushed into the weld pool to make the weld. It is
this affect that produces the 'higher deposition
rates' that the flux cored wire manufacturers claim
for this type of product. Unfortunately in many
instances the welder has difficulty in utilising this
higher wire feed speed and must either increase
the welding speed or increase the size of the weld.
Often in manual applications neither of these
changes can be implemented and the welder
simply reduces the wire feed speed back to where
it was and the advantages are lost. However, if the
process is automated in some way then the process
can show improvements in productivity.
It is also common to use longer contact tip to
workplace distances with flux cored arc welding
than with solid wire MIG/MAG welding and this
also has the effect of increasing the resistive
heating on the wire further accentuating the drop
in welding current. Research has also shown that
increasing this distance can lead to an increase in
the ingress of nitrogen and hydrogen into the weld
pool, which can affect the quality of the weld.
Flux cored arc welding has a lower efficiency than
solid wire MIG/MAG welding because part of the
wire fill contains slag forming agents. Although
the efficiency varies differs by wire type and
manufacturer it is typically between 75–85%.
Flux cored arc welding does, however, have the
same drawback as solid wire MIG/MAG in terms
of gas disruption by wind, and screening is always
necessary for site work. It also incurs the extra cost
of shielding gas, but this is often outweighed by
gains in productivity.
Self-shielded Operation
There are also self-shielded consumables designed
to operate without an additional gas shield. In this
type of product, arc shielding is provided by gases
generated by decomposition of some constituents
within the flux fill. These types of wire are referred
to as 'self-shielded'.
If no external gas shield is required, then the
flux fill must provide sufficient gas to protect the
molten pool and to provide de-oxidisers and nitride
formers to cope with atmospheric contamination.
This leaves less scope to address performance, arc
stabilisation, and process tolerance, so these tend
to suffer when compared with gas shielded types.
Wire efficiencies are also lower, at about 65%, in
this mode of operation than with gas-shielded
wires. However, the wires do have a distinct
advantage when it comes to site work in terms of
wind tolerance, as there is no external gas shield
to be disrupted.
When using self-shielded wires, external gas
supply is not required and, therefore, the gas
shroud is not necessary. However, an extension
nozzle is often used to support and direct the long
electrode extensions that are needed to obtain
high deposition rates.
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