Which shielding gas for which job

Shielding gas does one job: it keeps the air away from the molten weld pool. Oxygen and nitrogen from the surrounding air dissolve into hot metal and come back out as porosity, brittleness and cracks. The right gas for a job depends on the base material first, then on the welding process, and only then on thickness.

The single most common mistake in a general workshop is using one cylinder for everything. An argon/CO₂ mix that works perfectly on mild steel will carburise stainless and will produce black, porous rubbish on aluminium.

Flow figures assume a standard nozzle in still air. Increase by about a third with a large gas cup or a gas lens, and screen the joint properly if you are working outdoors or near a fan.

Argon or CO₂ — which should you buy?

For MIG welding carbon steel, both work, and the choice comes down to what the job is for.

• 100% CO₂ is cheaper per cylinder and penetrates deeper, which suits heavy structural work where the weld will be painted or hidden. The trade-off is noticeably more spatter, a rougher bead and more clean-up time. CO₂ cannot produce spray transfer.
• 80% Argon / 20% CO₂ costs more but runs smoother, spatters far less and leaves a bead you can leave visible. It also supports spray transfer on thicker plate with a leaner mix. For most fabrication shops the saving in grinding time is worth more than the gas.
• Pure argon is for aluminium and for TIG. On carbon steel MIG the arc becomes unstable and the bead sits on top of the plate instead of fusing into it.

The mistakes that cost the most

1. Turning the flow up to cure porosity. Above roughly 25 litres per minute the gas becomes turbulent and drags surrounding air into the weld pool, so more gas makes porosity worse while emptying the cylinder twice as fast. Find the leak instead.
2. Any CO₂ near aluminium or titanium. Both need a fully inert shield. CO₂ breaks down in the arc and releases oxygen, which forms an oxide film faster than the arc can clear it.
3. Feeding gas to self-shielded flux-cored wire. Gasless wire makes its own shield from the flux, and an external gas stream blows that shield away.
4. Skipping the back purge on stainless. The root oxidises into a grey, crusty surface known as sugaring. It cannot be repaired from the outside and it fails inspection.
5. Old or oil-soaked gas hose. Rubber hose absorbs moisture and lets air in through pinholes you cannot see. It shows up as porosity that looks exactly like bad technique.
6. Welding under a fan. In our climate the fan is always on, and a column of shielding gas has no chance against it. Screen the joint or move the work.

Frequently asked questions

What gas do I use for MIG welding mild steel?

An 80% argon / 20% carbon dioxide mix is the workshop standard. The argon keeps the arc stable and the CO₂ adds the heat needed for good fusion. Pure CO₂ is a cheaper alternative with deeper penetration but considerably more spatter.

Can I use pure argon for MIG welding steel?

No. Pure argon does not carry enough energy into carbon steel, so the arc wanders and the bead sits on the surface without fusing properly. Carbon steel MIG needs an active gas component — either CO₂ in the mix or straight CO₂.

What shielding gas is used for stainless steel?

For MIG, argon with about 2% CO₂ or 2% oxygen. The CO₂ must stay low, because above roughly 3% the carbon migrates into the weld and destroys the corrosion resistance that the stainless was chosen for. For TIG, use 100% argon, and purge the back of the joint on any root that has to pass inspection.

What gas is used for welding aluminium?

100% argon, for both MIG and AC TIG. Above roughly 20 mm an argon/helium mix helps get heat into the joint, because aluminium conducts heat away faster than almost any other common metal. Never use any gas containing CO₂ or oxygen on aluminium.

Do I really need to back purge stainless steel pipe?

Yes, on anything that carries product or has to pass inspection. Without a purge the underside of the root oxidises into a rough grey scale. It cannot be reached to repair, it becomes a corrosion site in food and chemical service, and it will be rejected on a coded job. Purge until oxygen is below 0.1% using a meter rather than guessing by time.

Why is my weld porous?

In order of how often it turns out to be the cause: a leaking or perishing gas hose, spatter blocking the nozzle, a draught or fan blowing the shield away, flow set far too high, contaminated or damp base metal, and finally the wrong gas for the material. Turning the flow up is almost never the fix.

Where can I buy welding gas in Perlis and Kedah?

Bann Edar Sdn. Bhd. is an authorised MOX-Linde distributor stocking pure argon, argon/CO₂ mixtures, carbon dioxide, oxygen, acetylene, nitrogen and special mixtures, with cylinder exchange at Kangar, Perlis. Call 04-976 5570 or WhatsApp 019-470 9929.

Disclaimer. The figures and recommendations on this page are general guidance for planning and set-up only. They are not a substitute for the manufacturer’s data sheet, an approved welding procedure or the applicable standard, and they cannot account for the conditions of your particular job. You remain responsible for checking every figure before you rely on it. Bann Edar Sdn. Bhd. accepts no liability for any loss, damage or injury arising from the use of this tool.

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