AGA-CW-Handbook-A432130-UK

30 Shielding gases for stainless steels
7.2 Which shielding gases are suited to different steel grades
Because the microstructures of different steel grades are different, their
sensitivities to the different components of shielding gases are also
different.
For more information on this subject, see Chapter 5.
7.2.1 MIG/MAG welding
The carbon dioxide and oxygen content of the shielding gas must not be
too high in order not to oxidise the weld surface too much.
However, a certain amount of oxygen or carbon dioxide is required in
the MAG welding of steel to stabilise the arc.
MISON ® 2 and MISON ® 2He are shielding gases containing 2% carbon
dioxide.
They are recommended for regular-grade stainless steels (ferritic,
austenitic and duplex).
The helium added to the MISON ® 2He shielding gas improves penetration
and provides a more fluid, well spreading molten pool.
When high-alloy austenitic and superduplex steels are welded, we
primarily recommend using the inert shielding gas MISON ® Ar.
Recommended gases are MISON ® 18 (18% CO 2 ) and MISON ® 25 (25% CO 2 ).
The use of carbon dioxide is also possible with some wire types.
However, this will result in more spatter and welding smoke, both of
which are less than desirable with stainless steels.
7.2.2 TIG welding
The most multi-purpose shielding gas for the TIG welding of stainless
steels is MISON ® Ar.
Due to the nitrogen monoxide added to the shielding gas, the arc is
more stable than with argon.
The MISON ® H2 shielding gas containing 2% hydrogen can also be used
for the TIG welding of austenitic stainless steels contains.
MISON ® H2 provides a less oxidised weld, higher welding speed and
better penetration and fusion between the weld and the base material.
However, it is not suitable for ferritic and ferritic-austenitic steels such
as duplex and superduplex.
Nitrogen is used as an alloy material in high-alloy austenitic and superduplex
steels.
The small amount of nitrogen monoxide added to the shielding gas provides
a more stable arc, less spatter and better penetration than argon.
MISON ® 2He is also a suitable shielding gas for these steel grades.
It has the downside of a clearly more oxidised weld surface.
Rutile cored wires require a shielding gas with a higher carbon dioxide
content than the abovementioned gases.