Smootharc 180 Multiprocess, Operating Manual

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9 BOC Smootharc 180 Multiprocess operating manual
The wire feed unit takes the filler wire from a spool, and feeds it
through the welding torch, to the arc at a predetermined and accurately
controlled speed. Normally, special knurled feed rolls are used with flux-
cored wires to assist feeding and to prevent crushing the consumable.
Unlike MIG/MAG, which uses a solid consumable filler wire, the
consumable used in FCAW is of tubular construction, an outer metal
sheath being filled with fluxing agents plus metal powder. The flux fill is
also used to provide alloying, arc stability, slag cover, de-oxidation, and,
with some wires, gas shielding.
In terms of gas shielding, there are two different ways in which this may
be achieved with the FCAW process.
• Additional gas-shielding supplied from an external source, such as a
gas cylinder
• Production of a shielding gas by decomposition of fluxing agents
within the wire, self-shielding
Gas shielded wires are available with either a basic or rutile flux fill,
while self-shielded wires have a broadly basic-type flux fill. The flux
fill dictates the way the wire performs, the properties obtainable, and
suitable applications.
Gas-shielded Operation
Many cored wire consumables require an auxiliary gas shield in the same
way that solid wire MIG/MAG consumables do. These types of wire are
generally referred to as ‘gas-shielded’.
Using an auxiliary gas shield enables the wire designer to concentrate
on the performance characteristics, process tolerance, positional
capabilities, and mechanical properties of the products.
In a flux cored wire the metal sheath is generally thinner than that of
a self-shielded wire. The area of this metal sheath surrounding the flux
cored wire is much smaller than that of a solid MIG/MAG wire. This
means that the electrical resistance within the flux cored wire is higher
than with solid MIG/MAG wires and it is this higher electrical resistance
that gives this type of wire some of its novel operating properties.
One often quoted property of fluxed cored wires are their higher
deposition rates than solid MIG/MAG wires. What is often not explained
is how they deliver these higher values and whether these can be
utilised. For example, if a solid MIG/MAG wire is used at 250 amps,
then exchanged for a flux cored wire of the same diameter, and welding
power source controls are left unchanged, then the current reading
would be much less than 250 amps, perhaps as low as 220 amps. This
is because 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
Extended self shielded flux cored wire nozzle