©
COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON
II
Although self shielding electrodes
have been developed for welding low alloy and stainless steels,
they are most widely used on mild steels. The self shielding method generally
uses a long electrical
stick-out (distance between the contact tube and the end of the unmelted elec-
trode) commonly from one to four inches.
Electrical resistance is increased with the long extension,
preheating the electrode before it is fed into the arc. This enables the
electrode to burn
off at a faster rate and increases deposition. The preheating also decreases
the heat available
for melting the base metal, resulting in a more shallow penetration than the gas
shielded process.
2.5.1.1 A
major drawback of the self shielded process is the metallurgical quality of the
deposited weld metal. In addition
to gaining its shielding ability from gas forming ingredients
in the core, the self shielded electrode
contains a high level of deoxidizing and denitrifying alloys,
primarily aluminum, in its core. Although the aluminum performs well in
neutralizing the affects
of oxygen and nitrogen in the arc zone, its presence in the weld metal will reduce
ductility and impact strength at low
temperatures. For this reason, the self shielding method is
usually restricted to less critical
applications. 2.5.1.2
The self shielding electrodes are more
suitable for welding in drafty locations than the
gas shielded types. Since the molten filler metal is on the outside of the
flux, the gases formed
by the decomposing flux are not totally relied upon to shield the arc from the
atmosphere. The deoxidizing and
denitrifying elements in the flux further help to neutralize the
affects of nitrogen and oxygen present
in the weld zone. 2.5.2
The
Gas Shielded
Process -
A major advantage with the shielded flux cored electrode
is the protective envelope formed by the auxiliary gas shield around the molten
puddle. This envelope effectively
excludes the natural gases in the atmosphere without the
need for core ingredients such as aluminum.
Because of this more thorough shielding, the weld
metallurgy is cleaner which makes this process suitable for welding not only mild
steels, but also
low alloy steels in a wide range of strength and impact levels.
2.5.2.1 The
gas shielded method uses a shorter electrical stickout than the self shielded
process. Extensions from 1/2"
to 3/4" are common on all diameters, and 3/4" to 1-1/2" on
larger diameters. Higher welding
currents are also used with this process, enabling high deposition
rates to be reached. The auxiliary shielding helps to reduce the arc energy
into a columnar
pattern. The combination of high currents and the action of the shielding
gas contributes
to the deep penetration inherent with this process. Both spray and globular
transfer are utilized with the gas
shielded process.
