Shielding Gas for MIG Welding Mild Steel On a Home MIG Welder

By Bill Byers / April 1, 2019
Shielding Gas For MIG Welding Mild Steel Title Image

The right shielding gas mix to use when MIG welding mild steel can be confusing to the home repair-ist or home hobbyist.

The best to use depends on the different mild steel gauges you are welding and you want to think about the results you want from the weld. These two things along with the amount of money you have to spend can push you towards different shielding gas mixes.

To help you, let me tell you about the common, most often used shielding gases for MIG welding mild steel and when best to choose which one to use.

What Gas Do You Use for MIG Welding Mild Steel?

1. C25 (25% Carbon Dioxide, 75% Argon)

2. 100% CO2 (Carbon Dioxide)

3. High Argon Mix (85% Argon, 15% Carbon Dioxide)

4. Even Higher Argon MIx (90% Argon, 10% Carbon Dioxide)

The Role of Shielding Gas in MIG welding Mild Steel

A shielding gas covering for your weld is fundamental to MIG (Metal Inert Gas) welding. ​ MIG welding also known as GMAW (Gas Metal Arc Welding), demands a shielding gas is used.

The shielding gas is exactly that - a shield from the surrounding atmosphere.  Shielding gas protects your weld from the Oxygen, Nitrogen and Hydrogen in the air. If the air was given free access to your weld, in the heat of the arc the atmosphere would cause porosity.

Porosity Example

Porosity Example

​Porosity are holes in your weld both on the surface and under the surface. Holes that causes your weld to fail.

That's not all a shielding gas does

A shielding gas:

  • Affects the stability of the electrical welding arc
  • Helps to control the weld penetration
  • The bead profile, it's shape and form
  • Helps control spatter
  • Helps control porosity
  • Influences how the finished weld looks

This means that it is important to choose your shielding gas well when MIG welding mild steel with your home welder. ​

Let us have a look at each of the recommended shielding gases for mild steel in more detail.

Reasons to Use C25 Shielding Gas When MIG welding Mild Steel

The most commonly used shielding gas for home and hobby MIG welding is C25 (75% Argon gas and 25% Carbon Dioxide).

It's not the cheapest of gas mixes but it is well liked because it gives you, the person MIG welding at home, a nice balance between cost and attractiveness of the finished weld.

It is a mix gas that is readily available at your local welder supply store. And not only that, is often available in the smaller gas cylinder sizes, perfect when you're doing the odd or occasional weld around the home or ranch.

Home workshop welding particularly for the beginner welder uses MIG welders that run on a 120 volt circuit. These MIG welders have a maximum welding output they are capable of and they typically weld with the short circuit transfer method. ​

If you are not sure what this means let me explain. It is literally how the arc melts the tiny pieces of your filler metal and transfers it into the weld pool.

Short Circuit Wire Transfer

Short Circuit Wire Transfer Method

C25 is a good multipurpose gas mixture that is a desirable balance between the very narrow weld profile produced when welding with 100% Argon and the volatility of welding with 100% Carbon Dioxide.

Argon is an Inert gas and so is much less reactive than Carbon Dioxide. Argon works well at lower voltages, provides a cleaner arc start and a smother melt of the welding wire into the base metals. This means it is better for thinner mild steel and low carbon metal gauges.

The Argon in the C25 gas mix stabilizes the Carbon Dioxide and results in a nice balanced weld.

Typical C25 Bead Profile

Typical C25 Bead Profile

​The surface profile of the finished weld tends to be flatter and neater. With the correct weld settings C25 produces little weld spatter. And clean up after welding mild steel or low carbon steel is minimal and easy.

Reasons to MIG Weld with 100% Carbon Dioxide Gas

When cost is an issue and the weld doesn't have to be the best looking then MIG welding with pure CO2 is a good option.

100% Carbon Dioxide, is widely available and is the cheapest gas to purchase at the welding supply store. Another plus point when you buy a tank of Carbon Dioxide gas is that you get more volume of the gas in the cylinder when compared to an Argon/Co2 mix. This is because CO2 is in a liquid state in the high pressure cylinder.

Freezing of their gas regulator as the Carbon Dioxide moves to a gas state is a complaint of some home MIG welders.

Carbon Dioxide isn't an inert gas like Argon although it is commonly used as a shielding gas in MIG welding. It is a more reactive gas with the electrical arc and produces a 'hotter' arc than the C25 mix.

The electrical arc when welding with Carbon Dioxide gas is less stable. It crackles and pops, it sounds like the arc stutters and starts, along with a fair bit smoke and fume as you weld. The result is more spatter, meaning more clean up of the finished weld is needed.

The gas will weld in short circuit transfer and globular mode, so it fine for the home welder user. But,

Carbon Dioxide really needs higher amperage out of the MIG welder to work well. On a low Amp (the lowest Amp settings a home MIG welder produces), the weld pool is too cool and the wire butts off the surface of the weld pool and low metal penetration is the result.

It is really not the gas to choose when when welding thinner metal gauges at low Amps. Choose C25 for that.

Carbon Dioxide is better used with the 20 gauge and thicker mild steel gauges.

And its great for a wider deeper weld penetration.

Typical C25 Bead Profile

Typical C25 Bead Profile

​YouTube Video Comparing the Weld Results of C25 Gas to CO2

Paul from weld.com compares in this five minutes 20 second video the weld profiles of C25 and Carbon Dioxide. It's worth seeing what an experienced welder produces from the different gases.

To my mind the Carbon Dioxide weld produced a thicker more proud weld profile compared to the C25 gas mix. Yes that is to be expected.

The Carbon Dioxide weld really wasn't too bad looking at all.  What do you think?

Best Gas for MIG Welding: C02 vs C25

​Video Credit: Weld.com

Reasons to Weld Mild Steel with a Higher Argon Gas Mix

It is true that the higher the percentage of Argon in the shielding gas mix the more you're going to have to pay for that cylinder of gas.

But is the cost worth it?

It depends on the MIG welder you have and the results you want to achieve.

A higher shielding gas mix of 85% Argon and 15% CO2 for welding mild steel is used when:

  • You want to weld fast.
  • You want a clean look on your weld with minimal weld clean up. Essentially you want less spatter.

Though it will cost you more for a tank of this gas mix. Before you go out and buy it is worth knowing that as well being used in the short circuit transfer method, this gas mix can put your MIG welder in spray transfer mode.

What is spray transfer?

This mode like the short circuit method is how the metal from your filler wire is transferred over to the weld pool by the electrical arc. The metal is transferred as the name implies in fine drops - sprayed into the weld pool.

Spray Wire Transfer

Spray Wire Transfer Methos

​A reasonably high voltage is needed from your MIG welder (towards the top end of the capability of a 140 Amp MIG welder running on a home 120 volt electrical circuit).

High Amps and a high percentage of Argon in your shielding gas mix will put a spray transfer capable MIG welder into spray transfer mode, giving you a clean looking MIG weld.

MIG Welding Mild Steel with a High Argon Gas Mix

And by a high Argon shielding gas mix I mean 90% Argon and 10% Carbon Dioxide.  This will be a costlier gas to buy when compared to the other gas mixes discussed so far.

Similar to the 80% Argon, 20% Carbon Dioxide reasons to use are:

A shielding gas that results in low spatter

It is a higher Argon gas mix with enough CO2 to give you good penetration of the base metals.

Good for heavier or thicker mild steel and low carbon gauges

A narrower bead profile when compared to the 80% Argon, 10% Carbon Dioxide

A smoother wetter weld puddle, you'll get a nice stable arc welding on mild steel and low carbon steels

This is a shielding gas mix for neat looking, low spatter welds.

A shielding gas mix that works for short circuit and spray transfer modes.

Why not 100% Argon For Welding Mild Steel?

You could weld with pure Argon for mild steel but most people don't. Because 100% Argon gas mix doesn't give the best weld profile on mild steel. 100% Argon on mild steel encourages the arc to produce a narrow thin weld profile.

Much like a thin finger of weld into the joint. It makes the weld difficult to position to get good melting and merging of the mild steel base metals with the filler wire.

The overall result is low weld penetration and much of the filler metal sitting proud on top of the weld.

As a more costly gas mix ​it is best to reserve your pure Argon for the metal it is best welding and recommended for, Aluminum, where it works well as a shielding gas.​

What to Bear in Mind When Choosing Your Shielding Gas for Mild Steel

Picking the right mild steel shielding gas depends on these 4 things.

1. How you want the weld to look and perform. Clearly you'll want a strong weld but if it is the best looking weld you want then go for a higher Argon shielding gas blend.

2. How much clean up you want to do when you have finished mild steel welding? Minimal or none then a high Argon shielding gas mix is for you.

3. How expensive is the gas? If you are cost sensitive then that may well steer you toward using 100% Carbon Dioxide. The look of the welds with CO2 are reasonable and if you're prepared to grind and clean up your weld, when needed, then that could be the perfect mix for you.

4. The availability of the gas mix you want in a convenient tank size. You may be restricted by what is available in your local area.

The order of importance of these 4 points will be dictated by what is the most important aspect for you.

Last Words

I hope this article has helped you in your search to find the right shielding gas for mild steel for your home MIG welder. I have tried to answer the question and give you some answers as to why a shielding gas is important.

The right shielding gas protects your weld and enhances the arc to produce the look of the weld. Shielding gas aids in the weld penetration as well as potentially minimizing weld clean up  - depending on the gas mix you choose.

The shielding gas is of course only one component to a good MIG weld, which starts with clean base metals.