©
COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON
II
profile
(See Figure 16), penetration, and speed of welding. In our discussion, we
will deal with the
more common choices used for the various transfer processes.
2.4.7.1 Short
Circuiting Transfer - Straight carbon dioxide (CO2)
is often used for short circuiting
arc welding because of its low cost. The deep penetration usually associated
with CO2
is minimized because of the low amperage and voltage settings used with
this process. Compared
to other gas mixes, CO2
will produce a harsher arc and therefore, greater spatter
levels. Usually, this is minimized
by maintaining a short arc length and by careful adjustment of
the power supply inductance. The
temperatures reached in welding will cause carbon dioxide
to decompose into carbon monoxide and
oxygen. To reduce the possibility of porosity caused
by entrapped oxygen in the weld metal,
it is wise to use electrodes that contain deoxidizing elements,
such as silicon and manganese. If the current is increased above the short
circuiting range,
the use of carbon dioxide tends to produce a globular transfer.
2.4.7.1.1 Mixing argon
in proportions of 50-75% with carbon dioxide will produce a smoother
arc and reduce spatter levels. It
will also widen the bead profile, reduce penetration, and
encourage "wetting". Wetting,
i.e., a uniform fusion, along with joining edges of the base metal
and the weld metal, minimizes the weld
imperfection known as undercutting (See Figure 17). FERROUS
METALS NON-FERROUS
METALS CO2
ARGON + CO2
ARGON + O2
ARGON HELIUM
BEAD PROFILE FIGURE
16 FIGURE
17 UNDERCUT
WETTING 2.4.7.1.2 The
75% Argon/25 CO2 mixture
is often chosen for short circuit welding of thin sections,
whereas the 50-50 combination works well on thicker sections.
2.4.7.1.3 It should
be noted that shielding gases can affect the metallurgy of the weld metal.
As an example, a combination of argon
and carbon dioxide may be used for welding stainless steel,
but as the carbon dioxide breaks down, excessive carbon may be transferred into
the
