What Is 75/25 Welding Gas?

75/25 welding gas is a shielding gas blend of 75% argon and 25% carbon dioxide (CO₂), used mostly for metal inert gas (MIG) welding of carbon and mild steel. Welders and suppliers also call it C25.

The American Welding Society (AWS) classifies the blend as M21-ArC-25 in its A5.32 shielding gas specification, though older procedures may still list it by the legacy designation SG-AC-25.

Like any welding shielding gas, 75/25 keeps oxygen and nitrogen from the surrounding air away from the molten weld.

What Does Each Gas in a 75/25 Mix Do?

Argon gives 75/25 its smooth arc and low spatter, while CO₂ makes the arc hotter so the weld reaches further into the steel.

Argon is inert, so it shields the weld pool without reacting with it. On steel, though, argon alone doesn’t produce sound MIG welds. A small share of an active gas like CO₂ or oxygen is needed to stabilize the arc, which is why pure argon falls short for MIG welding steel.

CO₂ breaks apart in the heat of the arc and recombines at the weld, releasing extra energy into the metal. The result is deeper fusion into the base metal than argon provides.

Why Is 75/25 the Go-To Gas for MIG Welding Mild Steel?

75/25 is the default for MIG welding mild steel because it runs short-circuit transfer smoothly, leaves less spatter and a cleaner bead than straight CO₂, and handles thin steel with less risk of burning through.

Short-circuit transfer is the lower-heat mode most MIG machines run for everyday work. The wire touches the weld pool, shorts out, and deposits a small drop of metal, over and over, many times per second.

Less spatter means less grinding before painting. For an auto body shop or a trailer builder, that’s labor saved on every part.

Miller Electric notes that straight CO₂ can run too hot for thin steel, where 75/25 is the better fit. Sheet metal, brackets, and light tubing all benefit.

On thicker plate, welders can stick with 75/25 and build the joint up in several passes. Many fabrication shops use 75/25 for most of their steel work.

Can You Use 75/25 for Spray Transfer?

75/25 isn’t a practical gas for spray transfer with solid wire, because true spray transfer generally needs at least 80% argon in the blend.

Spray transfer sends a steady stream of fine droplets across the arc at higher voltage and current. It lays down metal faster and penetrates deeper, which suits thick plate welded in the flat or horizontal position.

Argon/CO₂ blends with more than 20% CO₂ are used for short-circuit welding, while blends with less CO₂ handle spray and pulsed spray. Common spray gases for steel include 90% argon/10% CO₂ and 98% argon/2% oxygen.

Turned up past short-circuit settings, 75/25 usually produces globular transfer instead. Large, irregular drops cross the arc and throw more spatter. Some welders can force it close to spray with enough voltage, but they report an unstable arc that’s hard to control.

Gas-shielded flux-cored wires can run a smooth, spray-like arc on 75/25, because ingredients in the flux change how the metal crosses the arc.

Many of these wires carry a dual AWS classification, such as E71T-1C and E71T-1M. The “C” means the wire is rated for 100% CO₂, and the “M” means it’s rated for a blend of 75% to 80% argon with the balance CO₂. Check the wire’s classification before switching gases, since a wire rated only for CO₂ wasn’t tested with a mix.

AWS D1.1, the code for structural steel welding, doesn’t pre-approve short-circuit MIG procedures. Structural fabricators running 75/25 in short-circuit mode have to qualify each procedure with test welds before using it on the job.

Can You Use 75/25 for Aluminum, Stainless Steel, or TIG Welding?

75/25 is the wrong gas for aluminum and for tungsten inert gas (TIG) welding of any metal, and it’s a poor choice for most stainless steel work.

The CO₂ in 75/25 oxidizes aluminum, so MIG welding aluminum calls for pure argon or an argon/helium blend. The right gas for welding aluminum depends on the welding process and the job.

TIG welding depends on a tungsten electrode that CO₂ contaminates, so TIG torches run pure argon or argon/helium. Here’s more on what goes wrong when you run a 75/25 mix through a TIG torch. On austenitic stainless steel, TIG welders can also use an argon/hydrogen blend, as our guide to the best gas for TIG welding stainless steel explains.

On stainless steel, 75/25 will lay a MIG bead. The CO₂ adds carbon to the weld, though, and that carbon reduces the corrosion resistance buyers pay stainless prices for. MIG blends made for stainless keep CO₂ to a few percent or replace it with helium or oxygen.

How Does 75/25 Compare to 100% CO₂ and 90/10 Argon/CO₂?

75/25 sits between straight CO₂ and 90/10 (90% argon and 10% CO₂) on nearly every measure: it spatters less than CO₂ but more than 90/10, penetrates less than CO₂ but more than 90/10 in short-circuit mode, and usually costs more than CO₂ but less than 90/10.

Straight CO₂ makes sense for heavy repair work where gas cost matters more than bead appearance. Our breakdown of CO₂ vs. argon blends for MIG welding steel covers that tradeoff in detail.

A 75/25 cylinder holds compressed gas, so its gauge pressure drops steadily as the cylinder empties. A straight CO₂ cylinder holds liquid, and its gauge stays steady until the liquid runs out, so the gauge can’t show how much CO₂ is left.

The higher price of 90/10 pays off in shops that switch between short-circuit and spray work on the same machine. For other metals and processes, see our guide to choosing the best welding gas.

How Should You Store 75/25 in Cold Weather?

Keep 75/25 in a heated storage area when you can, and let a cylinder that’s been out in the cold warm to room temperature before welding, because the CO₂ in the blend can condense and separate from the argon when it gets cold enough.

Argon can’t be liquefied by pressure alone above its critical temperature of -122°C (-188°F). CO₂’s critical temperature is 31°C (88°F), so pressure can turn it liquid at ordinary shop temperatures. In a full cylinder, a cold spell can push some of the CO₂ out of the mix.

A separated cylinder can deliver an argon-heavy mix first and a CO₂-heavy mix later, so the arc changes partway through the cylinder. Gas suppliers are aware of this layering, and some use special filling methods to keep argon/CO₂ blends uniform.

Warming lets the CO₂ return to a gas, but it may not blend back evenly on its own. If a cylinder that sat in the cold welds inconsistently, call your supplier rather than trying to remix it. Rolling or dragging a cylinder to mix it isn’t safe.

Don’t speed up warming with a heater or torch. Cylinders should never get hotter than 125°F.

When 75/25 Is the Right Cylinder to Order

For short-circuit MIG and dual-classified flux-cored wire on carbon steel, 75/25 lets most fabrication shops, body shops, and repair crews run one gas for nearly every job. A second gas only becomes necessary once spray transfer, aluminum, or stainless steel shows up on the schedule. 

If you’re sorting out which jobs need which blend, WestAir’s team can help before your next order.

Tyler O'Brien

Tyler is a results-driven marketing professional specializing in the industrial gases and equipment industry, bringing his 10 years of technical expertise and digital marketing acumen to the complex industrial gas B2B environment.