Process 135 MAG vs Straight CO₂: What Buyers Should Ask
Key takeaways
- "MIG" is not a single process. ISO 4063 gives code 135 to metal active gas welding.
- M21 shielding gas is an argon-rich mix with a CO₂ fraction; pure CO₂ has its own designation.
- The argon fraction stabilises metal transfer, giving less spatter and a flatter bead.
- The practical result is less post-weld cleaning and fewer spatter inclusions near surfaces that must be coated or fitted.
Why the question matters
Most workshops answer "we use CO₂" or "we use mixed gas" without either party knowing what the difference is worth. It is worth something concrete: less cleaning labour, cleaner surfaces before painting and less chance of spatter where a bushing or a wear part has to sit.
Reading the codes
| Code | Meaning |
|---|---|
| 135 | Metal active gas welding (MAG), per ISO 4063 |
| M21 | Argon-rich mixed shielding gas with CO₂, per ISO 14175 |
| C1 | Carbon dioxide, per ISO 14175 |
So "process 135 with M21 gas" is a precise statement. It says the arc is shielded by an argon-CO₂ mixture rather than by pure CO₂, with enough argon to move metal transfer into a finer droplet regime.
What changes on the part
| Behaviour | Straight CO₂ | M21 mixed gas |
|---|---|---|
| Metal transfer | Larger droplets, less regular | Finer, more regular transfer |
| Spatter | Considerable | Reduced |
| Bead shape | Higher, more convex profile | Flatter, smoother tie-in to plate |
| Reinforcement on fillets | Higher | Lower and more even |
| Cleaning after welding | Grinding and chipping | Lighter dressing |
| Parameter window | Narrower | Wider — more tolerant |
There is a cost side too: argon blends are more expensive per cylinder than CO₂. In a workshop running hundreds of litres an hour that is a real line item, and it is one reason some suppliers stay with CO₂ despite the extra cleaning. Our view is that the cleaning labour and the paint preparation saved are worth more than the gas cost difference, particularly on units that will be blasted and painted in the same building.
What to ask your supplier
- Which process code do you weld with, and which shielding gas designation?
- Which wire standard do you buy to?
- Are the parameters written down per joint family, or left to the welder?
- Are actual values recorded during production?
- What happens between passes — is slag removal mandatory?
A supplier who can answer all five quickly writes procedures. One who cannot is relying on individual skill, which is harder to reproduce across a batch of forty and across a second order six months later.
SPEC BOX
| Field | Value |
|---|---|
| Process | 135 MAG, ISO 4063 |
| Shielding gas | M21, ISO 14175 |
| Wire | Solid wire, ISO 14341 |
| Thickness range addressed | 8–30 mm |
| Parameter recording | Required on run sheet |
FAQ
Does the gas change weld strength? It does not change the qualification logic; strength comes from joint design, filler and procedure. What changes is process stability, appearance and how much cleaning is needed.
Can you weld to other processes? Our production welding is process 135 MAG. If your part requires a different process, raise it at enquiry stage and we will confirm feasibility before quoting.
Why specify this instead of just asking for a good weld? Because "good" is not measurable and process details are. A written process can be checked; an adjective cannot.
Internal links: insights/shop-floor/mixed-gas-mag-welding-floor · insights/technical/how-to-read-a-wps · insights/technical/preheat-and-interpass-s355-vs-hardox
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