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Shielding gas blends for carbon steel GMAW
Select the right argon/Co2 blend for your weld requirements
- By Samantha Noland
- September 28, 2015
- Article
- Consumables
One of the most frequently asked questions pertaining to shielding gases is which gas blend should be used. Unfortunately, there is no definitive answer because different metals, thicknesses, weld requirements, and processes can use a number of possible gas blends.
Shielding gas is important to the welding process for a couple of reasons. First, it prevents atmospheric air from entering into the weld pool and causing porosity, cracking, and nitriding that could destroy the strength of the weld joint. Second, shielding gas acts as a medium to help the flow of electrons in the arc, allowing the electrons to continue to flow and giving the weld different properties based on which gas is selected.
Two of the most commonly used gases for gas metal arc welding (GMAW) carbon steel are carbon dioxide (CO2) and argon (Ar). Let’s look at the various qualities each individual gas brings to the weld and how they work together when combined.
CO2
Using CO2 as the sole shielding gas yields a weld bead that is wide and fairly deep. It is most commonly used in gas-shielded flux-cored arc welding (FCAW) and must be run at a relatively high voltage compared with other gases because of its atomic structure and the release of electrons, which can lead to problems when welding thin materials. The best attribute of CO2 is that it is a deep-penetrating gas, which is why it’s included in the mixes most of the time.
CO2 is a reactive gas, which means it produces oxygen and carbon monoxide during the welding process, leading to oxidizing of the weld metal. Using a welding wire with high manganese or silicon content will prevent this from happening, but both will be deposited in the weld pool and will appear on the surface of the metal after welding. Remember to perform the necessary cleanup before you paint or galvanize your component.
Generally speaking, using pure CO2 produces a lot of spatter because of its interaction with the high voltage, the flow of electrons, and the material you are using. That, paired with the fact that it tends to generate more fume, often leads people to seek other options.
Argon
Welding with pure argon yields a concentrated and focused arc, resulting in a narrow profile and deep penetration. The bead profile generally has a higher peak in the center of the weld with better wet-in at the toes compared with CO2.
Unlike CO2, argon is nonreactive, eliminating the potential for oxidation in the weld pool. This means you can use a wire with low silicon and manganese content if the metal itself does not require the additional cleaning elements. It also requires little energy (a lower voltage setting) to continue the electron flow across the arc, which results in a cleaner arc start and more stable flow of electrons across the gas than CO2.
Argon is the standard for gas tungsten arc welding (GTAW). In GMAW, however, argon is used in conjunction with other gases.
Ar/CO2 Mixtures
Separately, argon and CO2 possess desirable and undesirable qualities. But together these gases can complement each other’s strengths while equalizing each other’s weaknesses, so to speak.
For example, pure argon is not recommended for GMAW on ferrous materials because of the penetration profile (that is, the shape of the fusion zone within the base metal). Pure argon produces a narrow and fingerlike weld, so it is easy to miss joints and fail to penetrate as deep as needed in the right direction. However, when CO2 is added to the mix, the characteristics of the two gases combine to provide excellent weld quality.
Argon and CO22 cannot be mixed and properly delivered via cylinders in mixtures with much more than 25 percent CO2. Co2 is pumped as a liquid and, because of the way it and the compressed argon flows out of the cylinder, only partial cylinder use is possible and the quality of the mix cannot be guaranteed. These mixes, therefore, are usually 75 to 95 percent argon and 5 to 25 percent CO2.
75 Percent Argon/25 Percent CO2.This is the most commonly used gas blend for short-circuit GMAW. The CO2 provides a wider-penetration bead than argon can on its own. On the flip side, the argon lends stability to the CO2 that helps reduce spatter and control the weld profile. It is a short-arc process, so some spatter, even at optimal settings, is still present. This mixture also can be used effectively on flux-core wires made to operate with a mixed gas.
85 Percent Argon/15 Percent CO2. If your application requires a faster and cleaner weld, using a minimum of 85 percent argon with 15 percent CO2 combined with higher voltages allows for spray transfer. The amount of argon must be at this 85 percent or more level for the spray action to occur correctly.
Spray transfer propels small droplets of molten metal into the weld puddle as opposed to contacting the metal and shorting (short-arc) or transferring a large, difficult-to-control droplet (globular). Because of the more fluid puddle and higher deposition rates in spray transfer, this process is not recommended for out-of-position welding. If a part requires out-of-position welding with high deposition, pulse or globular transfer modes are recommended. Globular transfer modes should be used only for out-of-position welding. For both pulse and globular transfer modes, gases comprising 85 percent or more argon are recommended.
90 Percent Argon/10 Percent CO2. This is a very common mix for spray transfer often used in robotics because it provides the fastest travel speed. It is also commonly used with metal-core wire.
Asking the Right Questions
Selecting the right argon/CO2 mix requires you to ask certain questions regarding the desired weld specifications and production requirements. What bead profile is required? Where is the joint located? What are our cleanup requirements?
Getting answers to these questions is critical to selecting the appropriate gas for your application. This is where it is important to ask an expert and run some tests. You may find that less cleanup or faster travel speeds result from a simple bump in argon content. With the current shortage of welders, any productivity gains using the existing workforce are useful.
About the Author
Samantha Noland
nexAir
363 Walnut St.,
Memphis, TN 38126
888-639-2474
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The Welder, formerly known as Practical Welding Today, is a showcase of the real people who make the products we use and work with every day. This magazine has served the welding community in North America well for more than 20 years.
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