Shielding gases of TIG MIG and FCAW welding of stainless-steel duplex steel

Main Shielding gases for Welding

If one has to select the most common used shielding gases in welding, then any person involved in welding will name:

  1.  Carbon dioxide,
  2. Argon,
  3. Helium and
  4. Oxygen.

Each type of shielding gas has its specific characteristics (Density, inert or reactive, ionization potential) and production related factors such as factors such as cost, available labor and the weld properties desired ā€” all of these factors to be considered when selecting which shielding gas is best for a given welding application.

shielding gases in welding

Click here to read to know the various shielding gases used in Welding & their purposes.

Why needs the shielding?

The primary tasks of a shielding gas are to protect the molten pool from the adverse influence of the atmosphere, i.e. from oxidation (affect mechanical and corrosion properties of the stainless steel) and other gases such as nitrogen & hydrogen absorption (cause porosity), and to stabilize the electric arc by providing a ionization path. The choice of shielding gas to be used can also influence the characteristics of the arc such as type of metal transfer mode, penetration depth and profile, etc.

Shielding gas also plays a significant role in defining weld penetration profiles, helping maintain arc stability and reaching the desired mechanical properties in the finished weld. Shielding gas can also affect the transfer of the filler metal from the arc to the weld joint, which in turns contributes to the efficiency of the welding process and the quality of the weld. Other important factors that shielding gas help determine include the weld bead appearance, and weld toughness and strength.

Shielding gases for MIG/ FCAW welding of stainless steel and duplex stainless steel

The primary shielding gas for MIG welding of stainless steel and duplex stainless steel are inert gases such as argon (Ar) or helium (He), or a mixture of both depending on the purpose. Nevertheless, small additions of oxygen (O2) or carbon dioxide (CO2) can further stabilize the arc, improve the fluidity, weld penetration and also improve the quality of the weld deposit due to associated properties of these secondary additive gases. Carbon pick up can be a problem, and this limits the amount of carbon dioxide that can be used. For low carbon (ā€œLā€)-grade stainless steels this is limited to about 3% but for other grades up to 5% can be used.

Click here to Learn What is Stainless Steel, its types & properties

Stainless steel mig welding

For stainless steels there are also gases available containing small amounts of hydrogen (H2) to increase the productivity of the welding process as hydrogen produce more heat in the welding arc.

Trimix gas or three component shielding gas that contains a blend of 85 to 90 per cent helium (He), up to 10 per cent argon (Ar) and 2 to 5 per cent carbon dioxide (CO2) is used commonly when welding GMAW welding of stainless steel. A common tri-mix blend contains 90 per cent He, 7-1/2 per cent Ar, and 2-1/2 per cent CO2

Shielding gases for TIG and plasma welding of stainless steel and duplex stainless steel

There are a number of shielding gases mixtures that can be used for TIG welding stainless steel than for steel, but not all gas mixtures are suitable for all material types, i.e. hydrogen containing mixtures are suitable for welding only austenitic types and nitrogen containing mixtures for duplex types of stainless steel.

The primary shielding gas for TIG welding is argon or helium (Both are inert gas), or a mixture. In some cases, nitrogen (N2) and/or hydrogen (H2) is added in order to achieve special properties. For instance, an addition of hydrogen can be used for many conventional stainless steels to increase productivity. Nitrogen being an austenitic stabilizer is added in the shielding gas when welding duplex stainless steel to promote the austenitic phase in the weldment.

Ferritic stainless steels and duplex stainless steels must not be welded with shielding gases containing hydrogen. Alternatively, if nitrogen is added, the weld deposit properties can be improved. Oxidizing additions are not used because they destroy the tungsten electrode by oxidizing the tungsten and can also cause tungsten inclusion in the welding, a defect seen as white spot in the RT film.

Click here to Learn How to weld stainless steel, stainless steel to carbon steel.

Table below shows the various shielding gases and their properties such as density, thermal conductivity, ionization potential and chemical reactions.

gases for welding

link to Properties and characteristics of Welding Shielding gases

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