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Purging with Nitrogen in the Welding of Austenitic Stainless Steels (AWS Homepage)
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SPECIFICATION FOR WELDING SHIELDING GASES
A7.1.1 SG-A (Argon). Argon is a chemically inert
gas which is used both singularly and in combination with
other gases to achieve desired arc characteristics for the
welding of both ferrous and nonferrous metals. Almost all
arc welding processes can use argon or mixtures containing
argon to achieve good weldability, mechanical properties,
arc characteristics and productivity. Argon is used for welding
of nonferrous materials such as aluminum, nickel, copper,
magnesium alloys, and reactive metals, which include
zirconium and titanium. The low-ionization potential of
argon creates an excellent current path and superior arc
stability. In the GMAW process, argon produces a constricted
arc column at a high current density which causes
the arc energy to be concentrated in a small central area
of the weld pool. The result is a depth of fusion profile
which may have a distinct finger-like shape. Argon is also
used for single-side melt-through welding with or without
consumable inserts.
A7.1.2 SG-C (Carbon Dioxide). Carbon dioxide is
an active gas used primarily for GMAW and FCAW. The
heat of the arc dissociates the carbon dioxide into carbon
monoxide and free oxygen. This oxygen will combine with
elements transferring across the arc to form oxides which
are released from the weld pool in the form of slag and
scale. Although carbon dioxide is an active gas and produces
an oxidizing effect, sound welds and acceptable
mechanical properties can be achieved in many, but not
all, metals and alloys. An electrode having higher amounts
of deoxidizing elements is sometimes needed to compensate
for the reactive nature of the gas. Carbon dioxide can
be used for solid electrode GMAW with short circuiting
and globular transfer and FCAW of carbon and stainless
steel. Carbon dioxide cannot be used for spray transfer
with GMAW.
The popularity of carbon dioxide is due to common
availability as well as its lower cost per unit volume. The
lower cost per unit of gas does not automatically translate
to lowest cost per foot of deposited weld and is greatly
dependent on the welding application. The final weld cost
with carbon dioxide shielding gas is influenced by bead
contour, electrode spatter, and spatter removal. The lower
deposition efficiency for carbon dioxide caused by fume
and spatter loss will influence the final weld cost.
Argon is often mixed with carbon dioxide to improve
the operating characteristics. If mechanical properties are
to be maximized, a carbon dioxide and argon mixture is
often recommended.
A7.1.3 SG-N (Nitrogen). Shielding gases containing
nitrogen are not recommended for welding carbon steel.
Nitrogen will combine with other elements at high temperatures
which is why it is not recommended as a primary
gas, but is used in combination with other gases for selected
applications. Nitrogen is often used as a gas to protect the
weld root from atmospheric contamination. Nitrogen root
shielding of stainless steel welds may cause problems in
those applications where control of the ferrite content is
critical. Increased nitrogen content of the weld may reduce
the ferrite level. Small additions (≤3%) of nitrogen have
been combined with argon for GMA and GTA welding of
duplex stainless steel.
A7.1.4 SG-He (Helium). Helium, a chemically inert
gas, is used for weld applications requiring higher heat
inputs. Helium may improve wetting action, depth of
fusion, and travel speeds. It does not produce the stable arc
provided by argon. Helium has higher thermal conductivity
and a wider arc column than argon. The higher voltage
gradient increases heat input compared with argon, promoting
increased weld pool fluidity and better wetting action.
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