Instructions / Assembly

55
GMAW
www.lincolnelectric.com
E
lectrode diameters as large as 3/32” (2.4 mm), but usually less
t
han 1/16” (1.6 mm), are used with relatively high currents to
create the spray arc transfer. A current of approximately
300 - 350 amps is required for a 1/16” (1.6 mm) electrode,
depending on the shielding gas and type of stainless wire being
u
sed. The degree of spatter is dependent upon the composition
and flow rate of the shielding gas, wire feed speed and the
characteristics of the welding power supply. DC+ is used for
most stainless steel GMAW and an argon with 1-2% oxygen gas
m
ixture is recommended. Suggested welding guidelines for 200
a
nd 300 series stainless steels in the spray transfer mode are
given below. On square butt welds, a backup strip should be
used to prevent weld metal drop-through. When fitup is poor or
copper backing cannot be used, drop-through may be mini-
m
ized by short-circuiting transfer welding the first pass.
When welding with a semiautomatic gun, forehand (“pushing”)
techniques are beneficial. Although the operator’s hand is
exposed to more radiated heat, better visibility is obtained.
For welding plate 1/4” (6.4 mm) and thicker, the welding gun
should be moved back and forth in the direction of the joint and
at the same time moved slightly from side to side. On thinner
metal, only back and forth motion along the joint is used. The
more economical short-circuiting transfer process for thinner
material should be employed in the overhead and horizontal
position for at least the root and first passes. Although some
operators use a short digging spray arc to control the puddle,
the weld may be abnormally porous.
Power supply units with slope, voltage and inductance controls
are recommended for the welding of stainless steel with short-
circuiting transfer. Inductance, in particular, plays an important
part in obtaining proper puddle fluidity.
The shielding gas often recommended for short-circuiting welding
of stainless steel contains 90% helium, 7.5% argon and 2.5%
carbon dioxide. The gas gives the most desirable bead contour
while keeping the CO
2
level low enough so that is does not
influence the corrosion resistance of the metal. High inductance
in the power supply output is beneficial when using this gas
mixture.
Single pass welds may also be made using argon/oxygen and
argon/CO
2
gas mixes. However, arc voltage for steady short-
circuiting transfer may be as much as 6 volts lower than for the
helium based gas. The colder arc may lead to lack of fusion
defects. The CO
2
in the shielding gas will affect the corrosion
resistance of multi-pass welds made with short-circuiting trans-
fer due to carbon pickup.
Wire extension or CTWD (contact tip to work distance) should
be kept as short as possible. Backhand welding is usually
easier on fillet welds and will result in a neater weld. Forehand
welding should be used for butt welds. Outside corner welds
may be made with a straight motion.
A
slight backward and forward motion along the axis of the joint
s
hould be used. The following charts summarize the welding
guidelines recommended for stainless steel.
Short-circuiting transfer welds on stainless steel made with a
shielding gas of 90% He, 7.5% Ar, 2.5% CO
2
show good corro-
sion resistance and coalescence. Butt, lap and single fillet welds
i
n material ranging from 0.060 inch to 0.125 inch in 304, 310,
3
16, 321, 347, 410 and similar stainless steels can be made
successfully.
The pulsed arc process, as normally used, is a spray transfer
process wherein one small drop of molten metal is transferred
across the arc for each high current pulse of weld current. The
h
igh current pulse must be of sufficient magnitude and duration
to cause at least one small drop of molten metal to form and be
propelled by the pinch effect from the end of the electrode to the
weld puddle. During the low current portion of the weld cycle,
the arc is maintained and the wire electrode is heated, but the
heat developed is not adequate to transfer any metal. For this
reason, the time duration at the low current value must be limited
otherwise metal would be transferred in the globular transfer
mode.
Wire diameters of 0.035” and 0.045” (0.9 and 1.1 mm) are most
commonly used with this process. Gases for spray pulsed arc
welding, such as argon with 1% oxygen are popular, the same
as used for axial spray arc welding. These and other electrode
sizes can be welded in the spray transfer mode at a lower
average current with pulsed current than with continuous weld
current. The advantage of this is that thin material can be welded
in the spray transfer mode which produces a smooth weld with
less spatter than the short-circuiting transfer mode. Another
advantage is that for a given average current, spray transfer can
be obtained with a larger diameter wire than could be obtained
with continuous currents. Larger diameter wires are less costly
than smaller sizes, and the lower ratio of surface to volume
reduces the amount of deposit contamination.
The electrode diameters for gas metal arc welding are generally
between 0.030” and 3/32” (0.8 and 2.4 mm). For each elec-
trode diameter, there is a certain minimum welding current that
must be exceeded to achieve spray transfer. For example,
when welding stainless steel in an argon/oxygen atmosphere
with 0.045” (1.1 mm) diameter stainless steel electrode, spray
transfer will be obtained at a welding current of about 220 amp
DC+. Along with the minimum current, a minimum arc voltage
must also be obtained. This is generally between 22 and 30
volts.
Electrodes come on spools varying in weight between 2 and
60 lbs. Also available are electrodes for welding the straight
chromium stainless steels and austenitic electrodes that contain
more than the usual amount of silicon. The latter have
particularly good wetting characteristics when used with the
short-circuiting transfer process.