AGA-CW-Handbook-A432130-UK

6 Purpose of shielding gas
Helium and argon-helium mixtures can be used for root shielding when
it is necessary to get the gas to rise up in order to achieve root shielding.
As helium is lighter than air, it rises upwards and is safe to use as
a non-flammable gas.
1.2.5 Hydrogen
Hydrogen (H 2 ) can be used as a shielding gas component in the TIG
welding of austenitic stainless steels.
Added hydrogen provides a hotter and narrower arc, enabling higher
welding speed and better penetration. It also makes the fusion of the
weld beam and the base material smoother and reduces the oxidisation
of the weld.
In root shielding, added hydrogen is beneficial due to its oxide-reducing
effect. FORMIER ® 10, 10% of hydrogen in nitrogen, is an often used root
shielding gas. However, it is not recommended for use as the root shielding
gas for ferritic-austenitic (duplex) steels. Argon or pure nitrogen is
better suited to that purpose.
1.2.6 Nitrogen
Nitrogen (N 2 ) is used as a shielding gas component in the TIG welding
of stainless austenitic steels with nitrogen additions and superduplex
steels. In these steels, nitrogen is used as an additive with up to 0.5%
content to improve the strength of the steel and prevent spot corrosion.
Adding a few per cent of nitrogen in the shielding gas helps prevent the
nitrogen loss otherwise occurring in the weld during welding.
The effect of hydrogen and helium added to argon on arc voltage
Argon + X% hydrogen
Argon + X% Helium
FORMIER ® 10, 10% of hydrogen in nitrogen, is an often used reducing
root shielding gas.
It improves the root-side spot corrosion resistance on austenitic steels.
The same effect can be achieved in the welding of superduplex steels
by using pure nitrogen for root shielding.
Arc voltage
24
20
16
12
8
0 20 40 60 80 100
X % X% H 2
, HX 2 ,% X% He He
1.2.7 Nitrogen monoxide
Nitrogen monoxide (NO) added to MISON ® shielding gases reduces the
amount of ozone generated during welding. This improves the welder’s
work environment and reduces the irritation of mucous membranes
caused by ozone. Improvements in the work environment also improve
the welder’s ability to concentrate, productivity and welding quality.
The nitrogen monoxide in the MISON ® shielding gases also have a stabilising
effect on the arc in the MIG welding and soldering of stainless
steels and aluminium.
The higher the helium or hydrogen content in the shielding gas, the higher the
arc voltage. Heat transfer to the weld increases, which can be utilised as better
penetration and higher welding speed.