What Is the Best Gas for MIG Welding Mild Steel?

MIG welding is very popular with many welders because it provides a cleaner finish for mild steel welding. MIG welding allows artists, home-hobbyists, farmers, motorsports enthusiasts, and DIY welders to make most types of fabrications and maintenance welds with ease.

The process relies on using a shielding gas to protect the arc and weld puddle. Typically, welders use CO2, argon, or a mixture of these two for welding mild steel because they provide the best shielding for projects.

But what is the best gas for MIG welding mild steel?

Two types of gas combinations are preferred in MIG welding for Mild Steel; 100% CO2 and a 25-75 Carbon-Argon mix.

25% Carbon with 75% Argon gas is the most commonly used shielding gas for home and hobby MIG welders. This mixture isn’t cheap but welders prefer it because it gives you a nice balance between cost and quality of the finished weld.

This mix of shielding gas widely available at local welder supply stores. You can buy it in different sized cylinders and it is easy to carry around in the trunk of your car. This gas mix is perfect if you are planning to do occasional welding around the home or ranch.

Before we go into more detail on this, though, as well as other gases commonly used, there are some other things you need to be aware of first.

Gases used in MIG welding

MIG Welding is carried out with an arc generated through a continuous solid wire electrode. The electrode is fed through a welding gun and creates a weld pool on the metal surface that joins two base materials.

The arc is protected through a shielding gas also sent through the welding gun. The shielding gas also protects the weld pool from contamination.

Different gases can protect the weld pool from environmental contamination. They can be differentiated either as inert or non-inert gases.

Inert gases

Inert gases, aka noble gases, are highly resistant to chemical changes under normal environmental conditions. This means the shielding arc and weld receives the best protection with inert gases.

The most popular inert gases are Argon and Helium. Both gases are normally used in MIG and TIG welding.

Many new welders ask, ’Can I use Argon for welding mild steel?’

The answer is yes.

It is one of the most popular shielding gases in the industry, besides CO2.

Argon is generally used on its own, at 100% or mixed with one or two other gases for a deeper penetration into the metal. Argon shielding helps make a wider but shallow weld penetration.

It is great for keeping the arc stable. Helium can burn much hotter than Argon and create a deeper weld. However, it is more expensive than Argon.

Since inert gasses are more resistant to chemical reactions, they create much less spatter during welding than semi-inert or non-inert gasses. Helium greatly reduces the porosity of the weld.

However, it uses much more power than Argon and requires more caution because it can get hot and cause burnouts and overheating.

Most welders mix both Argon and Helium with other less expensive gases to keep costs down. Mixing is also necessary since pure Argon or Helium application is only advised for nonferrous metals like aluminum and copper.

Semi and non-inert gases

Semi-inert gases offer moderate resistance to chemical changes and protect the arc and weld. Although they are not as effective as inert gases, they are much cheaper and mixed with argon to keep costs low.

Carbon dioxide is a semi-inert gas often used in MIG welding for steel fabrication.

Carbon is inert enough that it can be used on its own for many projects. It is also mixed with Argon in a ratio for more delicate projects.

The main benefit of using CO2 is that it allows much deeper penetration of the arc into the welding material. When used in a low proportion to Argon, like a 75-25 Ar/CO2 mix, it creates very nice welds while reducing porosity for the finish.

The one disadvantage of CO2 is that it creates a harsher arc that causes more spattering, which usually requires more cleaning around the weld later.

Oxygen is another gas used in MIG welding for shielding. Oxygen is non-inert and generally used in small proportions to stabilize the arc.

Its main purpose is to increase weld penetration for thicker metals and stainless steel projects.

If used in a large proportion, oxygen can be detrimental for welding because it will increase the oxidization of the metal. This is why it is only used in small percentages, like 1 – 5 percent in the mix.

What is mild steel?

Mild steel is a type of low carbon steel with 0.05% to 0.25% carbon composition, based on weight. Mild steel is not alloy steel, so it does not contain large amounts of other elements besides iron.

This steel is generally more ductile, machinable, and weldable than high carbon and stainless steel.

However, this also makes it nearly impossible to harden and strengthen mild steel through heating and quenching. The low carbon content also means it offers tensile strength less than high carbon and stainless steel.

What other gases are good for MIG welding mild steel?

I already mentioned the ideal and most common gas combinations for MIG welding mild steel above (100% CO2 and a 25-75 Carbon-Argon mix). But are there other options?


The second gas commonly used in mild steel welding is 100 percent CO2. This gas is used when the cost is an issue and you don’t need the weld to have the best look. 100% Carbon Dioxide is widely available in welding supply stores and it is the cheapest gas you can buy.

When you buy a tank of Carbon Dioxide gas, you get more volume of the gas in the cylinder compared to an Argon/Co2 mix. This is because the gas is stored in a cool, liquefied state.

Benefits of argon-carbon mix and carbon 100

Both these gas mixtures offer various advantages and disadvantages for users. Which one to use depends on your specific purpose of welding.

Argon-carbon mix for shielding

The greater the percentage of Argon mixture in your shielding gas, the better quality your projects will have.

A 75-25 mixture will allow you to work quickly and give your finished projects a much cleaner look. It is preferred when working on delicate projects with thinner metals or welds that will lie on top surfaces for your structures.

Some people also increase the Argon ratio and go with 85%- 25% mix because it gives them a smoother bead finish.

However, there are two problems with a high Argon mix.

First, it is very expensive. If you are a professional welder, make sure your costs don’t go over the prices you charge for your service.

The second problem is that higher Argon concentration drops the arc penetration rate in the material.

This is why welders don’t use pure Argon gas for welding. Shielding gases composed purely of inert gas, like Argon, are not good for MIG welding mild steel as they usually cause an ugly, inconsistent weld.

100% carbon for shielding

If cost is something you are concerned about and your welds don’t need to have the best-looking finish, then you could use pure CO2 for MIG welding mild steel. It is considerably cheaper than an Argon mix and easily available at supply stores.

Carbon Dioxide isn’t an inert gas like Argon but it offers enough chemical protection commonly used as a shielding gas in MIG welding. It reacts with the arc and produces a ‘hotter’ aura than the C25 mix.

This creates deeper penetration in the connecting metals and forms a stronger, bigger bead.

The electrical arc isn’t very stable when used with pure Carbon Dioxide. This causes the arc to crackle and pops more, creating spatter.

It also creates a moderate amount of smoke and fumes as you weld.

Higher spatter means more cleaning is required after you finish work.

Pure carbon should not be used when welding thinner metal gauges at low Amps because the arc can blow holes in the frame.


There is no best gas for mild steel welding. When working on a welding project, you will need to evaluate the goals you have to choose the correct shielding gas for your specific purpose.

Important factors include the cost of the gas, quality of the finished weld, how you want to prepare the material, thickness of the base material, and how much post-weld clean up is possible. Your productivity and time to spend in welding should be considered as well.

Most professional welders keep multiple tanks of shielding gases on hand and use them alternately depending on the project they are working on. This helps ensure you get the best welding gas for different projects and adjust as you go along.

Related questions

What gas is best for MIG welding?

Most welders agree that a mix of 75% Argon, 25% Carbon gives the best mix for mild steel welding. This shielding gas helps keep the flame steady and reduces spatter while still providing deep enough penetration.

Can I use argon for MIG welding mild steel?

Yes. You can use pure Argon for MIG welding mild steel. However, you are likely to get an unattractive finish that is tall and narrow. Using 100% Argon will weaken, then undercut and also lose ductility.

About Pierre Young

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Hey, I'm Pierre Young a qualified AWS Certified Welder. I got into welding in 2009 as a side hustle. Ever since then, I've been doing all kinds of welds - both for business and pleasure. While immersing myself in this wonderful hobby, I've learned from hands-on experience what welding gear works and what doesn't. Welding Headquarters is the site where I share everything I've learned.

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