What Is the Best Gas for MIG Welding Mild Steel?

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It isn’t just professional welders who utilize MIG welding; it is also DIY enthusiasts, artists, motorsports fans, farmers, and many others. The fact is, MIG welding provides a cleaner finish when you’re working with mild steel, and it makes most types of maintenance welds and fabrications a lot easier. Typically, MIG welding relies on some type of shielding gas so that the weld puddle and arc are protected, and most of the time, this means argon, CO2, or a combination of both of these. Why? Because when it comes to mild steel, these gases provide the best shielding for a variety of projects.

Just the Basics

If you’re wondering about the best gas for MIG welding mild steel, there are two combinations generally preferred: a solution that is 100% CO2, and a mixture of 25% carbon and 75% argon. The latter isn’t exactly inexpensive, although it isn’t outrageous either, but it is very popular with home and hobby welders because it provides them with a good balance between the quality of the finished weld and the cost of the gas. In other words, they can get great results in the end and not spend so much money that it’s cost-prohibitive to use this type of gas.

The good news is, you can get this mixture of shielding gas at most welding supply stores, and it comes in many different sized cylinders. It is one of the smartest mixes to choose if you only weld occasionally at your home or ranch. That being said, the bottom line is that there is no one “best” gas to use if you’re mild-steel welding, but you can make the right decision if you first consider:

  • The goals you’ve set for the project
  • The amount of post-weld cleanup that is possible
  • The overall cost of the gases used
  • The way you wish to prepare the material
  • The thickness of your base material
  • The overall quality of the end product

In simple terms, this means that what works for you might not work as well for the next welder, so you’ll need to look at the big picture before deciding which gases to use when you’re MIG welding mild steel. In fact, each project you take on may in fact use different gases, depending on the factors mentioned above.

Inert Versus Non-Inert Gases

Before we get to specifics regarding the gases that will work well with MIG welding mild steel, let’s take a look at the difference between the gases that are able to protect the weld pool from any type of environmental contamination. They are as follows:

  • Inert (noble) gases. These gases are very resistant to the chemical changes that can occur during normal environmental conditions. The two most popular inert gases are helium and argon, and both can normally be used in TIG and MIG welding. So yes, argon can be used when you’re welding mild steel. With inert gases, there is less spatter during the welding process, as well as less porosity with the weld. That being said, you’ll need to use caution with inert gases because they get super hot and can cause both overheating and burnouts.
  • Semi- and non-inert gases. Semi-inert gases are not as effective as inert gases but are less expensive. They also tend to be mixed with argon, which keeps their costs low. An example of a semi-inert gas is carbon dioxide. CO2 is also used in MIG welding and allows for deeper penetration of the arc into the welding material. But it tends to create a harsher arc and results in more spattering, which can be messy. Oxygen is a non-inert gas and is often used in small proportions so the arc is more stabilized.

What all this means is this: when you’re researching the best gas for MIG welding mild steel, a lot of factors come into play, one of those being cost. In addition to 100% CO2 and a 25/75 carbon-argon mix, you have other options as well with this type of welding project. For now, however, let’s take a look at these two gas options.

Carbon Dioxide at 100%

When cost has to be considered and you need something inexpensive, 100% carbon dioxide is a great option. It is widely available and inexpensive, but it should still be used only for projects where the weld doesn’t need the absolute best look. Carbon at 100% has both pros and cons. First, it offers enough chemical protection that it can be used as a shielding gas in MIG welding. It creates deeper penetration in the connecting metals, and the bead formed is stronger and bigger. 

Still, it creates a moderate amount of smoke and fumes while you’re working and a lot of spatter, which of course means that your cleanup time is going to increase once you’re done working. In addition, you shouldn’t use pure carbon when you’re welding thinner metal gauges at low amps. Why? Because the arc can blow holes in the frame, and this is never a good thing.

Argon-Carbon 75/25 Mixture

Simply put, the more argon in the mixture, the better quality your projects will turn out to have. This 75/25 mixture means you can work quickly and produce a much cleaner look. For delicate projects that have thinner welds or metals, this mix is usually preferred over other types of gases. This being said, it is recommended that the percentage of argon not be any higher than 75%, for two main reasons:

  • It is very expensive, which needs to be considered when you’re a professional welder working on jobs for others.
  • It can drop the arc penetration rate in the material.

These are but a few reasons why pure argon gas isn’t used for welding, especially for MIG welding mild steel. The weld they produce is usually inconsistent and doesn’t look very good, which is why the 75/25 argon-carbon mix is generally recommended.


While you can use many types of gases for MIG welding mild steel, there are numerous reasons why pure carbon dioxide and a 75/25 argon-carbon mixture are two of the most popular. The above information lists just a few reasons why this is so.

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