Welding gases do two completely different jobs, and most guides list them in one column as though they were interchangeable. Some gases shield the weld from the air. Others burn to make the heat. Oxygen appears in both lists, doing something different each time.
Two Jobs, Not One
An arc melts metal in the open air, and molten steel will take oxygen and nitrogen straight out of the atmosphere — which is what makes a weld porous and brittle. A shielding gas is there to push the air away from the puddle, and nothing else.
A fuel gas does the opposite: it burns, and the heat is the point. Oxy-acetylene has no arc at all. The two jobs need different gases and different equipment, and confusing them is the most common mistake in articles like this one.
Which gas does which job
| Gas | Job | Where you meet it |
|---|---|---|
| Argon | Shielding | TIG on everything, MIG on aluminium, and the base of most MIG mixes |
| Carbon dioxide | Shielding (reactive) | MIG on carbon steel, alone or blended with argon |
| Helium | Shielding | Blended with argon for thick material and aluminium |
| Oxygen | Both | The combustion half of oxy-fuel cutting, and 1 to 5% in some MIG mixes |
| Acetylene | Fuel | Oxy-acetylene welding, brazing and cutting |
| Hydrogen | Shielding additive | Small percentages with argon, on austenitic stainless only |
Which Shielding Gas for Which Job?
Shielding gas by process and metal
| Process and metal | Gas | Why |
|---|---|---|
| MIG, carbon steel | 75% argon / 25% CO₂ (C25) | The usual short-circuit choice: a stable arc and far less spatter than straight CO₂ |
| MIG, carbon steel, deep penetration | 100% CO₂ | Cheapest, digs deeper, spatters more. Short-circuit and globular transfer only |
| MIG, aluminium | 100% argon | Non-ferrous metals want a pure inert gas; argon gives arc stability and low spatter |
| MIG, stainless | Argon-based tri-mix | A small CO₂ or oxygen addition for wetting, without carburising the weld |
| TIG, almost anything | 100% argon | Affordable, widely available, excellent arc stability |
| TIG, thick aluminium or copper | Argon with 25 to 75% helium | Helium's higher thermal conductivity raises heat input, so faster travel and deeper penetration |
Two practical notes on that table. “MAG” is the same process as MIG with an active gas — CO₂ is not inert, which is why Europe distinguishes the two names and why the formal name for both is GMAW. And gas flow is not a dial to max out: Miller puts TIG flow at roughly 10 to 35 cubic feet per hour, and advises the lowest rate that still works, because too much flow becomes turbulent and pulls air in.
The Six Gases, One at a Time
Argon
Inert, so it reacts with nothing, and heavier than air — which is why it sits over a puddle in the flat position and why it needs more flow overhead. It is the default TIG gas on every metal and the base of most MIG mixtures, blended with carbon dioxide, oxygen or helium to change how the arc behaves. It costs more than carbon dioxide, which is the only real argument against it.
Carbon dioxide
The cheapest shielding gas, and a reactive one — it breaks down in the arc and releases oxygen, which is why straight CO₂ gives deeper penetration and more spatter than an argon blend. It works in short-circuit and globular transfer, not spray. On thick steel where appearance does not matter, it is hard to beat on cost.
Helium
Inert, and much lighter than air, so it drifts up and away from the joint and needs noticeably higher flow rates than argon. Its higher thermal conductivity means a hotter arc for the same current, which is why it is blended into argon for thick aluminium and copper. It also makes the arc harder to start, which is the other reason it is used as a blend rather than alone.
Oxygen
Two separate roles. In oxy-fuel work, oxygen is what makes the fuel gas burn hot enough to melt or cut steel. In MIG, one to five percent oxygen is sometimes added to argon to stabilise the arc and improve wetting. Beyond small percentages it oxidises the weld, which is the whole reason shielding gas exists.
Acetylene
The hottest of the fuel gases, burned with oxygen for welding, brazing and cutting. It is also the most dangerous gas in the shop: unstable under pressure, with a wide flammable range in air. Swapping the welding tip for a cutting tip turns the same torch into a cutter, which is why it is still standard equipment where an arc is not practical.
Hydrogen
Added to argon in small percentages for TIG on austenitic stainless, where it raises arc temperature and leaves a cleaner, brighter weld. Never use it on carbon steel or on martensitic and duplex stainless: hydrogen is the direct cause of hydrogen-induced cracking, which appears hours or days after the weld has cooled and looks fine when you finish.
The Rules That Come With the Cylinders
These are regulations, not tips. OSHA 29 CFR 1910.253 governs oxy-fuel gas welding and cutting, and three of its requirements are the ones people break.
Four cylinder rules worth knowing by heart
Three are quoted from the regulation. The fourth kills people quietly and is not in it.
- Acetylene never above 15 psig
- “Under no condition shall acetylene be generated, piped or utilized at a pressure in excess of 15 psig.” Above roughly that pressure acetylene can decompose explosively without any oxygen present at all.
- Fuel-gas cylinders valve end up
- the regulation requires fuel-gas cylinders to be placed with the valve end up whenever they are in use. Acetylene cylinders are packed with a porous filler and acetone; on their side, liquid acetone can be drawn into the hose.
- Oxygen stored away from fuel gas
- in storage, oxygen cylinders must be kept from fuel-gas cylinders by at least 20 feet, or by a noncombustible barrier at least 5 feet high with a fire-resistance rating of half an hour.
- Inert gas displaces air
- argon, CO₂ and helium are not toxic, they are asphyxiants. Argon is heavier than air and pools in tank bottoms, pits and trenches. A confined space that has had a shielding gas running in it is not safe because it smells fine.
Frequently Asked Questions
What gas is used for MIG welding?
For carbon steel, 75% argon / 25% carbon dioxide is the standard choice — a stable arc with much less spatter than straight CO₂. Straight CO₂ is cheaper and penetrates deeper but spatters more, and works only in short-circuit and globular transfer. For aluminium, use 100% argon.
What gas is used for TIG welding?
100% argon on almost everything. It is affordable, widely available and gives excellent arc stability. For thick aluminium or copper, a blend of argon with 25 to 75% helium raises the heat input for faster travel and deeper penetration, at the cost of harder arc starts.
Can you weld without gas?
With the right wire, yes. Flux-cored arc welding uses a wire whose core produces its own shielding as it burns, which is why it works outdoors where a shielding gas would simply blow away. Stick welding does the same thing with the electrode's flux coating. TIG and standard MIG both need gas.
Why is argon used more than any other welding gas?
Because it is inert, so it reacts with nothing in the puddle; it is heavier than air, so it stays over the weld; it gives a stable arc; and it is cheap enough to use in volume. Every other shielding gas is either a blend based on argon or a specialist choice for one job.
What pressure should acetylene be set to?
Below 15 psig, always. OSHA states that under no condition shall acetylene be generated, piped or utilized above 15 psig. This is not a margin-of-safety recommendation: above roughly that pressure acetylene can decompose explosively on its own, with no oxygen involved.
Sources
- 29 CFR 1910.253, Oxygen-fuel gas welding and cutting — the 15 psig acetylene limit, valve-end-up requirement and the 20-foot storage separation
- Miller, shielding gas for TIG welding — argon as the default, argon-helium blends, and the 10 to 35 cfh flow guidance
- Introduction to Welding, GMAW electrodes and shielding gases — gas selection by metal and transfer mode
- ANSI/AWS Z49.1:2021, Safety in Welding, Cutting, and Allied Processes — ventilation and confined-space requirements
The Short Answer
Three combinations cover most of what anyone actually does: C25 for MIG on steel, pure argon for TIG and for MIG on aluminium, and oxygen with acetylene when there is no arc. Helium, hydrogen and added oxygen are adjustments to those, not alternatives. The safety rules matter more than the selection: acetylene below 15 psig, fuel cylinders upright, oxygen kept 20 feet from fuel gas, and never trust a confined space that has had shielding gas running in it.