Shielding Gas for MIG Welding

Argon was my first thought, but you should read this:

Selecting a shielding gas for joining stainless steel: The right ..

For welding of SS with MIG process, the ideal gases are pure argon, argon with 1 % Oxygen or argon with 2% Oxygen.

The characteristics of each gas used in a shielding gas mixture affect the way the gas will perform including the shielding efficiency, arc stability, shape and strength of the weld. Depending upon the application, the right balance of gasses in a mixture will produce optimum properties for the application and greater tolerance to voltage and current.

Being an inert gas, welding with pure argon, there is hardly any loss of alloying elements between the filler wire and weld deposit. Even elements like Titanium is transferred across the arc making type 321 stainless steel weld metal practical with this gas.

Though it is good for spray arc mode, in flat & horizontal welds on SS, the quick freezing character of the gas, does not allow good wetting of the toes and causes undercuts at the edge of the weld bead.

So, it is necessary to add active gasses to argon such as oxygen, or carbon dioxide to increase the the heat input for GMAW of SS and stabilize the droplet size.

Oxygen helps to stabilize the arc, improve the weld bead profile, edge wetting and minimise the tendency to undercut the materiel. Oxygen additions up to 1-2% prevents excessive loss of Manganese, silica and increase the temperature of molten metal transferred across the arc. The molten metal will have less surface tension than with pure argon, wetting the parent metal to flatten the weld bead profile.

Hence, argon with 1% oxygen gives improved are stability, produces a more fluid and controllable weld puddle, good coalescence and bead contour and minimises undercutting on heavier stainless steels.

Argon with 2% oxygen provides better arc stability, coalescence and welding speed than 1% oxygen mixture on thinner stainless steel materials.

For SS welding generally 1-2% oxygen mixture is recommended. With more percentage like 5%, surface of the bead becomes increasingly oxidised with consequent loss of Mn, Si, and Chromium.

Carbon dioxide could be used with FCAW process and not for MIG welding of SS.

Sridhar.