©
COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON
II weld metal.
Corrosion resistance in stainless steel is reduced as the carbon content
increases. To counteract this
possibility, a less reactive mixture of 90% helium - 7-1/2% argon
- 2-1/2% CO2
is sometimes chosen. This combination, known as a trimix, provides
good arc stability
and wetting. 2.4.7.2
Spray Arc Transfer - Pure argon produces
a deep constricted penetration at the center
of the bead with much shallower penetration at the edges (Figure 16). Argon
performs well
on nonferrous metals, but when used on ferrous metals, the transfer is somewhat
erratic with the
tendency for the weld metal to move away from the center line. To make argon
suit- able for
spray transfer on ferrous metals, small additions of 1 to 5% oxygen have proven
to provide remarkable
improvements. The arc stabilizes, becomes less spattery, and the weld
metal wets out nicely. If the
percentage of argon falls below 80%, it is impossible to achieve
true spray transfer.
2.4.7.2.1 Pure helium
or combinations of helium and argon are used for welding nonferrous
metals. The bead profile will
broaden as the concentration of helium increases. 2.4.7.3
Pulse Spray Transfer - The selection
of shielding gas must be adequate enough to support
a spray transfer. Material type, thickness, and welding position are essential
variables in selecting
a particular shielding gas. The following is a list of recommended gases:
Carbon Steel Argon/CO2/O2/He
(He less than 50%) Alloy
Steel Argon/CO2/O2/He
(He less than 50%) Stainless
Argon/O2/CO2
(CO2 max.
2%) Copper, Nickel,
& Cu-Ni Alloys Argon/Helium
Aluminum Argon/Helium
2.4.8
Electrodes
- The solid electrodes used in GMAW are of high purity
when they come from
the mill. Their chemistry must be closely controlled and some types purposely
contain high levels
of deoxidizers for use with CO2
shielding. 2.4.8.1
The electrode manufacturer draws down
the electrode to a finished diameter that, with
GMAW, is usually quite small. Diameters from .030" thru 1/16" are common.
2.4.8.2 Most
steel GMAW electrodes are copper plated as a means of protecting the
surface. The copper inhibits
rusting, provides smooth feeding, and helps electrical conductivity.
2.4.8.3 Information
on types and classifications will be covered in a future lesson.
