Shielding gas selection

Which mixture suits the material, the process and the thickness — with the EN ISO 14175 designation, so you can order it by name rather than by nickname.

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Carbon steel

Argon with 8–25 % CO₂ covers nearly everything. More CO₂ means a hotter, deeper-penetrating arc and more spatter; less means a cleaner bead face and a steadier arc, and it is what spray transfer needs — above about 20 % CO₂ spray transfer stops working at all. Pure CO₂ is cheap and penetrates well, but it only gives globular transfer and it spatters.

Stainless and aluminium

Austenitic stainless takes argon with a small active addition — 2 % CO₂ or 2 % oxygen — kept small on purpose: carbon picked up from CO₂ reduces corrosion resistance. Aluminium takes pure argon, or argon with helium once the section is thick enough that argon alone will not carry the heat.

TIG

Inert gas without exception. Any active component destroys the tungsten within minutes. Argon-hydrogen mixtures are used on austenitic stainless to raise the heat and clean the surface, and on nothing else — hydrogen in ferritic or duplex steel causes exactly the cracking that preheat is meant to prevent.

Reading the gas designation

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M21, M20, M12, I1 — what is in the cylinder

The label reads something like “ISO 14175 – M21 – ArC – 18”: the group, the components and the addition in per cent. The groups you meet most often in a shop:

• M21 — argon with more than 15 and up to 25 % CO₂. The classic 82/18 mix (Ar + 18 % CO₂) for carbon steel.
• M20 — argon with more than 5 and up to 15 % CO₂, e.g. Ar + 8 %: a softer arc, less spatter, good for thin sheet and spray transfer.
• M12 — argon with 0.5–5 % CO₂, in practice Ar + 2 % CO₂ for stainless; M13 — argon with 0.5–3 % oxygen.
• C1 — pure CO₂. I1 — pure argon, I3 — argon with helium. R1 — argon with hydrogen.

The group limits are set by EN ISO 14175, and gas suppliers describe their mixes the same way (Ar + 8 % CO₂ as M20, Ar + 18 % as M21, for example). Two cylinders of “mix” from different suppliers can therefore sit in different groups — read the code, not the trade name.

Which gas for what — the calculator’s settings at a glance

• Carbon steel, MAG, up to 3 mm: a lower-CO₂ mix (Ar + 8…18 %, M20 / M21), about 11 l/min.
• Carbon steel, MAG, over 3 mm: M21 (Ar + 18…25 % CO₂) or C1, about 14 l/min.
• Stainless, MIG/MAG: M12 or M13, 12 l/min up to 3 mm, 15 l/min above.
• Aluminium, MIG: pure argon I1 up to 6 mm (14 l/min), thicker — Ar + 30 % He, group I3 (18 l/min).
• TIG, any metal: argon I1 — 6 l/min up to 2 mm, 8 l/min up to 6 mm, 12 l/min above; R1 is acceptable on austenitic stainless.

Three examples from the shop

1. A gate from 40×40×2 box section. 2 mm steel, lots of short visible welds. A lower-CO₂ mix (M20, e.g. Ar + 8 %) means less spatter and less grinding before painting; flow about 11 l/min.
2. A frame from 10 mm plate. M21 with 18 % CO₂ as the standard; pure CO₂ gives deeper penetration and cheaper gas, but about 90 % of the melted wire ends up in the weld instead of 95 %. On 20 kg of wire that is 2 kg lost to spatter instead of 1 kg — plus the time to chip it off. Flow about 14 l/min.
3. A 3 mm stainless handrail. With MIG — M12 (Ar + 2 % CO₂), 12 l/min; with TIG — pure argon I1, 8 l/min, and a root purge inside the tube.

CO₂ or a mix — cost against quality

Pure CO₂ is the cheapest gas but not the cheapest weld. In CO₂ the arc needs roughly 2–3 V more at the same current, it is harsher and spatters more, and the bead stands higher. It pays where the weld is heavy and hidden and penetration is what matters. On visible work, thin sheet and spray transfer the mix pays for itself in grinding time. Wire feed and voltage for both gases are worked out by the wire feed speed calculator.

Typical mistakes when choosing a gas

• M21 on stainless. Carbon from the CO₂ carburises the weld and strips its corrosion resistance — use M12 or M13 on stainless.
• A CO₂ mix on aluminium. The pool oxidises and the weld comes out black and porous. I1 or I3 only.
• A MAG mix in a TIG torch. Any active component burns the tungsten away in seconds.
• Hydrogen on carbon steel. R1 is for austenitic stainless only; on carbon steel it risks cold cracking.
• Purging with “whatever is to hand”. Root purging uses argon or an N₂ + H₂ mix (4–8 l/min), not a CO₂ welding mix.

More on the metals themselves: welding stainless steel and welding aluminium; argon for TIG is covered on the TIG welding page.

Author: Dmitry Frolov, welder and metal fabricator Updated: 28 September 2026