A shielding gas in combination with a wire electrode is critical to the aesthetics and quality of a weld. These gases also aid in arc stability and shaping the penetration profiles and mechanical properties of welds. How much gas ensures that all these happen as expected? It is a question I hear a lot, especially from those who have been new to MIG welding. A quick answer is here.
The gas flow rate which is measured in cubic feet per hour (CFH) gives us a rough idea about how much gas to have prior to a MIG welding job. Typically, you need 15 to 40 CFH of gas for metal thickness ranging from 1/8 – 3/4 inch. Also, note that CO2 lasts longer than other gases.
The above reply can be too brief for a lot of enthusiasts who are about to adopt MIG welding seriously while trying to be knowledgeable. In fact, gas requirements cannot be explained in so few words. So, I’ll start with the basics of shielding gases in MIG welding only to let you know how much of a single gas or mixture suffices for MIG welding different metals and alloys.
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Recommended Gases for MIG Welding
In MIG welding, you can use either a single shielding gas or a mix of two gases in certain proportions depending on the particular application. Carbon Dioxide (CO2), Argon, Oxygen, and Helium are the most common choices.
- CO2 is perhaps the most popular reactive gas used by MIG welding professionals due to its low cost, deep penetration, and the ability to be used without getting mixed with an inert gas.
- Argon provides excellent weld quality as you weld non-ferrous metals such as aluminum, titanium, or magnesium.
- Oxygen comes in handy when you expect fluidity of weld pool, arc stability, and penetration while working with mild carbon, stainless, and low alloy steel.
- Helium is ideal for projects that involve thick and non-ferrous metals alongside stainless steel, and there are expectations for deep and wide penetration.
Some of these gases shouldn’t be used in their pure forms making the mixtures rather necessary than optional. While there are several combinations, you can use any of these as each brings unique advantages to your work,
- 95% Argon + 5% CO2 / 75% Argon + 25% CO2 / 98% Argon + 2% CO2
- 98% Argon + 2% Oxygen / 92% Argon + 8% Oxygen
- 25% Helium + 75% Argon / 75% Helium + 25% Argon
- 90% Helium + 7.5% Argon + 2.5% CO2
There is a reason why I’ve mentioned a thing or two about these mixtures. When you’ll use pure CO2 or another single gas, you can easily find out how much of that gas is required by your project. As you try one of these mixtures, you have to consider the percentages in the mixture to find out how much of each gas is required.
Now, let me talk about the available gas tanks and their capacities which also help with the estimate in some way.
Common Bottle and Cylinder Sizes
When it comes to containers used for high-pressure gases like carbon dioxide, helium, oxygen, hydrogen, and nitrogen, we see steel cylinders, each with a uniform size. These cylinders typically contain 20 to 300 cubic feet of shielding gas. You might have seen some letters, or volumes, or both on those containers, which indicate their sizes.
For example, R indicates 20 SCF or standard cubic foot. V indicates 40 SCF. There are other letters such as Q, D, S, K, and T which indicate 80, 125, 150, 200, and 300 SCF respectively. Aluminum cylinders come in more limited capacities or sizes. Again, tanks used to contain low-pressure gases like argon come in more varied capacities or sizes.
An argon tank that is represented by R/20 contains 21 CF, and the one labeled with RR/40 contains 44 CF. Similarly, other sizes like Q1/60, Q/80, S/125, S/150, K/250, and T/330 indicate 65, 83, 125, 155, 251, and 335 CF respectively.
If you’re busy with a basic welding project, you want to choose a small tank that gives you at least 1 hour of welding time, either continuously or with breaks. But I guess you are more interested in knowing the amount of gas required instead of the duration that a tank or cylinder lasts for.
A typical DIY project requires only an R tank which contains 20 SCF of gas. On the other hand, a small MIG welding project at a workshop can be handled with a tank containing 80-150 CF of gas. For larger projects, you’ll be looking at a 150, or 200 CF tank. If you have to handle a huge volume, I mean industrial applications, you need a 300 CF tank or the largest available size which comes cheaper than a few small tanks.
Metal Types and Thickness
You might wonder why I’m speaking of the types of metal and their thicknesses to choose for MIG welding. These two factors also help you determine how much gas your welding tasks will require.
While running a MIG welding project, choices of metals are limited to carbon steel or mild steel, stainless steel, and non-ferrous metals such as aluminum, titanium, and copper along with some nickel based alloys.
Metal thickness ranges from 0.125 inch to 0.5 inch or slightly thicker with 0.25 inch being the standard thickness. The thicker the metal, the more shielding gas you’ll need. But how do you calculate how much you’ll need? I’ll explain a specific term called CFH which you may have heard already.
Additional Read: Best Gas For MIG Welding Mild Steel
How Long Does MIG Welding Gas Last: CFH and PSI
CFH represents the flow of shielding gas required by a MIG welding job. This rate ranges from 25 CFH to 30 CFH depending on metal thickness and other factors with 20 CFH being the most ideal rate. CFH is important for another reason that suppliers use this very number to sell shielding gases.
Sometimes, a higher flow rate results in porosity and spatters while under circumstances like drafty or windy conditions, a higher flow rate is essential, not avoidable. The flow rate needs to be proportional to another term called PSI or “pound per square inch”.
It means you need to up the PSI with a higher CFH. PSI basically informs you of how much pressure a particular thing is under. It doesn’t essentially tell you the amount of gas being used. So, I won’t go into further details.
Let’s get back to our original question. This time, I’ll provide a list that shows the relationship between the flow rate and the thickness of the metal being used.
- 15 CFH to 20 CFH for 1/4-inch or thinner metal
- 18 CFH to 22 CFH for 3/8-inch metal
- 20 CFH to 27 CFH for 1/2-inch metal
- 25 CFH to 35 CFH for 5/8-inch metal
- 30 CFH to 40 CFH for 3/4-inch metal
Speaking of the duration that you can go on welding for, I can give you an estimate. Suppose, stainless steel is your chosen metal, and the thickness is 1/4 inch. Setting the flow rate at 20 CFH for a tank that holds 80 CFH, you can expect a maximum of 4 hours of operation. Note that this number is not constant due to the reasons I mentioned above.
One CFH Variable to Consider
Apart from the environment within which you’ll work, the specific shielding gas you’re using has a little, if not much, to do with the flow rate. Not all gases last up to the same duration, but the change is very minimal whether you use a single gas or mixture.
For instance, carbon dioxide usually lasts longer than argon in most cases given that your MIG gun releases gas only while welding. This makes CO2 the most economical choice for both DIY and commercial projects. At this point, I have one more thing to inform which brings us close to the end of this discussion.
Have you ever checked the regulator? If you have not, I think it is time you took a look as it includes two dial gauges, one of which reads the amount of gas in your tank and another tells the flow rate you use while welding. This component helps you know when to get a refill.
To wrap up, I would like you to focus on the metal and the gas you’ll use. Argon and CO2 work well for mild steel and add helium to the mixture for stainless steel. Choose 100% argon or mix helium with it for aluminum. Why is this paragraph important?
In a mixture that uses more CO2 than argon or helium, you’ll have to get a larger CO2 tank. The same applies to other shielding gases. I think you got the point. More questions? Feel free to ask me. Until then stay safe and weld properly.
Last Updated on March 24, 2021 by Gary Hargrave