AGA-CW-Handbook-A432130-UK

24
Shielding gas and quality
5.3.1 Mechanical characteristics
With the condition that the shielding gas is suited to the welded steel
and the filler material type used, it does not affect the mechanical
characteristics of the weld.
5.3.2. Corrosion resistance
One of the basic issues with stainless steels is to understand the effect
of the welding process on corrosion resistance.
If the carbon dioxide content of the shielding gas in MAG welding with
solid and metal-cored wires exceeds 3%, the result may be the harmful
carbonisation of the weld deposit.
Carbon reacts with the chromium in the steel, forming chromium
carbides at grain boundaries. Correspondingly, the chromium content in
areas near the grain boundaries decreases and corrosion resistance is
reduced (grain boundary corrosion).
However, most stainless steel today have very low carbon content or
they are stabilised, so the above will not usually be a problem when
stainless steels are welded.
Nitrogen is added as an alloy in some stainless steels to improve corrosion
resistance but also to increase their strength.
Examples of this are high-alloy austenitic and superduplex steels.
Nitrogen loss occurring during the welding of these steels can impair
their corrosion properties.
MAG spray arc welding, solid wire
Weld carbon content (% C)
Weld Sheet Filler material
0.040
0.030
0.020
0.010
0.000
0 2 4 6 8 10
Shielding Shielding gas carbon gas dioxide carbon content dioxide (% content CO2) (% CO 2 )
On the left, a root surface protected with the FORMIER ® 10 root shielding gas.
On the right, a root surface unprotected during welding.
In MAG welding, and to some extent, in TIG welding with filler material,
this can be rather easily compensated using a filler material with a
suitable composition. In TIG welding without filler material, the nitrogen
loss must be compensated by using a shielding gas containing nitrogen
(MISON ® N2).
The nitrogen monoxide (NO) added into MISON ® shielding gases does
not affect the corrosion resistance of stainless steels.
5.3.3 Root protection
In some applications, the root side of the weld must be protected.
Otherwise, an oxide layer is formed, containing chromium originating
from the metal underneath the layer.
The chromium content near the root surface is reduced, increasing the
risk of corrosion. Argon, nitrogen-hydrogen mixtures and argon-hydrogen
mixtures are used for root protection.
The effect of the carbon dioxide content in a shielding gas on the carbonisation of
austenitic stainless steel.
When the carbon dioxide content rises above three per cent, weld carbon content
of 0.03% is approached, above which the danger of grain boundary corrosion is
considered to increase.