AGA-CW-Handbook-A432130-UK

22 Shielding gas and quality
By reducing the CO 2 or O 2 content of the shielding gas, the burning loss
of alloy materials in the filler material is reduced, increasing the yield
strength and tensile strength. The differences between mechanical characteristics
in argon-carbon dioxide mixtures in the range of 8–25% are
so small that they usually do not have a practical significance. However,
when pure carbon dioxide is used, the difference may be significant
compared to the above.
Extensive research has shown that the small NO addition in the MISON ®
shielding gases does not affect the mechanical characteristics of the
weld.
Weld toughness with different shielding gases and filler materials
PZ 6103 metal-cored wire (AWS A5.20: E71T-G)
PZ 6104
nickel-alloy
metal-cored wire (AWS A5.29 : E71TG-Ni1)
PZ 6111 rutile cored wire (AWS A5.20: E71T-1)
brittleness and porosity, in particular with unalloyed, low-alloy and
non-austenitic high-alloy steels.
In certain conditions, adding hydrogen to the shielding gas is beneficial.
In the TIG welding of unalloyed and low-alloy steels, productivity can
be improved and surface oxidation reduced by using the MISON ® H2
shielding gas, which contains 2% hydrogen. This requires that the base
material is not very thick and the internal stresses do not rise very high.
Hydrogen content of weld
11
22
33
very low
average
low
high
44
The fatigue resistance of the welded joint strongly depends on the
geometry of the welded joint. In MIG/MAG welding, the weld shape
´can be affected by the selection of a shielding gas. With gas mixtures,
a smoother fusion between the weld and the base material is achieved
than with carbon dioxide. The peak stresses in the structure (notch
effect) are smaller and the fatigue resistance of the welded structure is
better.
In structures with fatigue-inducing loads, certain requirements are set
for the fatigue resistance and the weld fusion with the base material. If
the fusion is poor, grinding or TIG treatment is required on the welded
joint, increasing costs. Oxide inclusions in the weld affect the fatigue
resistance even if the weld is grinded or polished. Oxide inclusions can
act as the nucleation of the crack.
The higher the carbon dioxide or oxygen content of the shielding gas,
the larger the number of oxide inclusions in the weld deposit.
The large amount of hydrogen dissolved into the weld can cause
55
66
0 5 10 15 20 25 30 35
ml H 2 /100 g weld deposit
1
3
5
Rutile electrodes
Submerged arc
Solid wires
Cored wires
Alkaline electrodes
TIG welding with MISON ® H2 as shielding gas
When unalloyed and low-alloy steels are TIG welded with MISON ® H2 used as the
shielding gas, the hydrogen content of the weld is comparable with rutile cored
wire welding.
Better fatigue resistance with argon mixtures
2
4
6
By using argon mixtures, a better fusion between the weld and the
base material can be achieved, while also improving the fatigue resistance
of the weld.
CO 2 Welding speed 40 cm/min
Ar + 20% CO 2 Welding speed 47 cm/min