How to Choose the Right Shielding Gas: A Guide for MIG, MAG, and TIG Welding
Shielding gases play a crucial role in almost all arc welding processes. They protect the weld pool and the arc from the influence of the ambient atmosphere, while also affecting arc stability, penetration profile, welding speed, mechanical properties, and the final weld quality.
Choosing the wrong shielding gas can lead to porosity, poor mechanical properties, oxidation, spatter, or even cracking. Shielding gases are more than just a consumable; they are an essential process parameter for every welding process.
This comprehensive blog explains how to select the right shielding gases, what factors to consider, practical best practices, and what to avoid. Additionally, backing gases will be discussed. The international standard ISO 14175:2008 is used as the primary guideline throughout this guide.
A shielding gas performs multiple functions:
- Protecting the weld pool from oxygen (O₂), nitrogen (N₂), and hydrogen (H₂) in the ambient air.
- Stabilizing the electrical arc.
- Influencing heat input and weld penetration.
- Reducing spatter.
- Influencing the mechanical properties of the weld.
- Improving the weld appearance.
Selecting the right gas is, therefore, not a minor detail, but an essential part of the welding process.
The ISO 14175:2008 standard classifies shielding gases and gas mixtures for arc welding and allied processes. This standard ensures a standardized designation; as a result, the type of gas is always clearly defined, regardless of the supplier or trade name.
Inert gases do not react with the weld pool. Argon and helium fall into this category, as do their mixtures. These gases are primarily used in TIG, MIG, and plasma welding of non-ferrous metals and high-alloy materials. Key characteristics include a highly stable arc, little to no spatter formation, and no metallurgical influence on the weld.
These consist of argon or helium with controlled amounts of active components, such as carbon dioxide (CO₂) or oxygen (O₂). These mixtures are primarily used in MAG welding of carbon steels and low-alloy steels.
Pure carbon dioxide is used as a shielding gas in robust steel applications where deep penetration is more critical than spatter-free welding. Characteristics of this shielding gas include deep penetration, an unstable arc, and significant spatter.
These gases contain hydrogen and are used to improve the fluidity of the weld pool, particularly in austenitic stainless steels and nickel alloys. This gas is not suitable for welding carbon steel due to the risk of hydrogen-induced cracking (HIC).
These are primarily used as backing gases for duplex and super duplex stainless steels to maintain the desired microstructure.
worden niet gebruikt als beschermgassen bij het vlamboog lassen.
zijn gasmengsels die componenten bevatten die niet in de lijst zijn opgenomen of mengsels die buiten de vermelde samenstellingen vallen.
The full group classification in the table below according to ISO 14175:2008
|
Symbol |
Components in nominal percentage of volume |
||||||
|
Main |
Sub |
Oxidizing |
Inert |
Reducing |
Low reactivity |
||
|
CO2 |
O2 |
Ar |
He |
H2 |
N2 |
||
|
I |
1 |
|
|
100 |
|
|
|
|
|
2 |
|
|
|
100 |
|
|
|
|
3 |
|
|
balance |
0,5 ≤ He ≤ 95 |
|
|
|
M1 |
1 |
0,5 ≤ CO2 ≤ 5 |
|
balancea |
|
0,5 ≤ H2 ≤ 5 |
|
|
|
2 |
0,5 ≤ CO2 ≤ 5 |
|
balancea |
|
|
|
|
|
3 |
|
0,5 ≤ O2 ≤ 3 |
balancea |
|
|
|
|
|
4 |
0,5 ≤ CO2 ≤ 5 |
0,5 ≤ O2 ≤ 3 |
balancea |
|
|
|
|
M2 |
0 |
5 < CO2 ≤ 15 |
|
balancea |
|
|
|
|
|
1 |
15 < CO2 ≤ 25 |
|
balancea |
|
|
|
|
|
2 |
|
3 < O2 ≤ 10 |
balancea |
|
|
|
|
|
3 |
0,5 ≤ CO2 ≤ 5 |
3 < O2 ≤ 10 |
|
|
|
|
|
|
4 |
5 < CO2 ≤ 15 |
0,5 ≤ O2 ≤ 3 |
|
|
|
|
|
|
5 |
5 < CO2 ≤ 15 |
3 < O2 ≤ 10 |
|
|
|
|
|
|
6 |
15 < CO2 ≤ 25 |
0,5 ≤ O2 ≤ 3 |
|
|
|
|
|
|
7 |
15 < CO2 ≤ 25 |
3 < O2 ≤ 10 |
|
|
|
|
|
M3 |
1 |
25 < CO2 ≤ 50 |
|
|
|
|
|
|
|
2 |
|
10 < O2 ≤ 15 |
|
|
|
|
|
|
3 |
25 < CO2 ≤ 50 |
2 < O2 ≤ 10 |
|
|
|
|
|
|
4 |
5 < CO2 ≤ 25 |
10 < O2 ≤ 15 |
|
|
|
|
|
|
5 |
25 < CO2 ≤ 50 |
10 < O2 ≤ 15 |
|
|
|
|
|
C |
1 |
100 |
|
|
|
|
|
|
|
2 |
balance |
0,5 < O2 ≤ 30 |
|
|
|
|
|
R |
1 |
|
|
|
|
0,5 ≤ H2 ≤ 15 |
|
|
|
2 |
|
|
|
|
15 ≤ H2 ≤ 50 |
|
|
N |
1 |
|
|
|
|
|
100 |
|
|
2 |
|
|
|
|
|
0,5 ≤ N2 ≤ 5 |
|
|
3 |
|
|
|
|
|
5 ≤ H2 ≤ 50 |
|
|
4 |
|
|
|
|
0,5 ≤ H2 ≤ 10 |
0,5 ≤ N2 ≤ 5 |
|
|
5 |
|
|
|
|
0,5 ≤ H2 ≤ 50 |
Balance |
|
O |
1 |
|
100 |
|
|
|
|
|
Z |
Gas mixtures containing components not listed, or mixtures outside the composition ranges listed.b |
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| a For the purpose of this classification, argon may be substituted partially or completely by helium. | |||||||
| b Two gas mixtures with the same Z-classification may not be interchangeable. | |||||||
Meest gebruikte gassen: Argon (I1) - Argon/Helium mengsels (I3)
Bij TIG-lassen wordt vrijwel altijd met inerte gassen gewerkt. Argon is hierbij de standaardkeuze vanwege de stabiele boog en goede ontsteking. Voor dikkere materialen of hogere lassnelheden kan helium worden toegevoegd om de warmte-inbreng te verhogen.
Meest gebruikte gassen: Argon (I1) en Argon/Helium mengsels (I3)
Bij MIG-lassen van aluminium, koper en nikkel wordt eveneens gebruikgemaakt van inerte gassen. Argon is geschikt voor dun materiaal, terwijl Argon/Helium-mengsels voordelen bieden bij grotere materiaaldiktes.
Meest gebruikte gassen: Argon/Kooldioxide mengsels
MAG-lassen onderscheidt zich juist door het gebruik van actieve gasmengsels. Voor on- en laaggelegeerd staal worden argon/CO₂ en argon/O₂ mengsels toegepast. De hoeveelheid actief gas moet zorgvuldig worden gekozen: te veel actief gas verhoogt oxidatie en spatten, terwijl te veel inert gas kan leiden tot een onstabiele vlamboog. Hoe hoger het actieve gas aandeel, hoe hoger de kans op oxidatie en slechtere kerftaaiheid
To prevent the back of the weld (the weld root) from oxidizing due to contact with atmospheric oxygen, a shielding gas must be applied to the root side when welding certain types of steel. This is known as backing gas. If the weld or base material oxidizes at the root, the corrosion resistance and mechanical properties of the joint will decrease significantly. In addition to oxidation protection, backing gas ensures a smooth, high-quality root penetration.
Commonly Used Backing Gases
- Argon (most widely used)
- Argon/Hydrogen mixtures
- Nitrogen/Hydrogen mixtures (limited applicability)
Nitrogen/Hydrogen mixtures are frequently used because they are relatively cost-effective. The hydrogen content typically ranges between 5% and 20%. However, for hydrogen-sensitive stainless steels, such as duplex and martensitic stainless steel, hydrogen-containing backing gases must not be used. When the hydrogen content exceeds 10%, the exiting gas must be flared off due to the risk of explosion.
To ensure proper protection of the root, sufficient purging time is necessary to reduce the oxygen content as much as possible. The oxygen level must be sufficiently low before welding and remain stable during the process (<50 ppm, often <20 ppm for stainless steel) to prevent oxidation or the burning off of alloying elements. Additionally, a controlled gas outlet must always be present to prevent overpressure and gas entrapment.
Oxidation on the root side can be identified by heat tint (discoloration), ranging from light brown to blue. This discoloration occurs even with very trace amounts of oxygen.
When using argon or argon/hydrogen, it must be noted that these gases are heavier than air. When welding pipes horizontally, air may remain trapped at the top (12 o'clock position). Conversely, nitrogen/hydrogen mixtures are lighter than air, which can lead to the opposite effect.
The image below illustrates the degree of discoloration (heat tint) on the inside of austenitic stainless steel tubes, categorized according to the AWS D18.2 standard. These levels indicate the quality of the purge and the resulting corrosion resistance of the weld.
For MAG welding of unalloyed and low-alloy steels, argon/CO₂ mixtures are most commonly used. Mixtures containing approximately 15% to 18% CO₂ offer an ideal balance between penetration and productivity. For higher quality requirements and improved toughness, mixtures with lower CO₂ or O₂ concentrations are utilized. For TIG welding, argon is the standard choice, potentially supplemented by argon as a backing gas.
| welding process |
ISO Group |
Typical composition |
Application / Points of interest |
| MAG | M21 | Ar + 18 - 20% CO₂ | Standard construction, good penetration |
| MAG | M20 | Ar + 8 - 15% CO₂ | Less spatter, better weld appearance |
| MAG | M12 | Ar + ≤2% CO₂ | Thin sheet metal, higher weld quality |
| MAG | M13 | Ar + 1 - 3% O₂ | Very stable arc |
| MAG | C1 | 100% CO₂ | Deep penetration, lots of spattering, rough appearance |
| TIG | I1 | 100% Ar | Root and pipe welding |
| Backing | I1 | 100% Ar | Protection of penetration during TIG/MAG |
Not recommended: Hydrogen-containing gases, high O₂ contents (>3%) for impact toughness requirements
Austenitic stainless steel requires careful gas selection. Low levels of active gas are permitted in MIG/MAG welding, whereas TIG welding often utilizes argon or argon with a small hydrogen addition to ensure a smooth root penetration. Backing gases are almost always mandatory in these applications.
| Welding process |
ISO Group |
Typical composition |
Application / Points of interest |
| TIG | I1 | 100% Ar | Standard TIG welding |
| TIG | R1 | Ar + 2 - 5% H₂ | Improved penetration and smooth weld penetration |
| MIG/MAG | M12 | Ar + 1 - 2% CO₂ | Low oxidation, good mechanical properties |
| MIG/MAG | M13 | Ar + 1 - 2% O₂ | Very stable arc |
| Backing | I1 | 100% Ar | Best quality base layers |
| Backing | N5 | N₂ + 5 - 10% H₂ | Root and pipe welding |
Not recommended: - 100% CO₂ - High active gas percentages
For duplex and super duplex stainless steels, the microstructure plays a decisive role. Nitrogen-containing shielding and backing gases help maintain the correct austenite/ferrite balance. Hydrogen-containing mixtures may only be used when explicitly qualified and approved.
| Welding process |
ISO Group |
Typical composition | Application / Points of interest |
| TIG | I1 / N2 | 100% Ar | Stable austenite/ ferrite balance |
| MIG/MAG | M12 + N₂ | Ar of Ar + 1 - 3% N₂ | Maintain microstructure |
| Backing | N1 | Ar + 1 - 2% CO₂ + N₂ | Promotes austenite formation |
| Backing | N5 | N₂ + max. 5% H₂ | Only if allowed in WPS |
Not recommended: - Hydrogen-rich mixtures without procedure qualification
Aluminum and aluminum alloys are welded exclusively with inert gases. The use of active gases or nitrogen will invariably cause weld defects and is therefore strictly prohibited.
| Welding process |
ISO Group |
Typical composition | Application / Points of interest |
| TIG | I1 | 100% Ar | Thin and medium- thick material |
| TIG | R1 | Ar + He | Thicker materials, higher heat input |
| MIG | I1 / I3 | 100% Ar | Standard MIG aluminum |
| MIG | I1 | Ar + 30 - 70% He | Higher welding speed, deep penetration |
Not recommended: - Active gases (CO₂, O₂) - Nitrogen
Nickel and nickel alloys are typically welded using argon or argon/helium mixtures. Small amounts of hydrogen can be added to improve fluidity, provided that the base material is compatible.
| Welding process |
ISO Group |
Typical composition | Application / Points of interest |
| TIG | I1 | 100% Ar | Standard applications |
| TIG | R1 | Ar + 2 - 5% H₂ | Improved fluidity |
| MIG | I1 / I3 | Ar of Ar/He | Thicker materials |
| Backing | I1 | 100% Ar | Base layer protection |
For steel grades with high impact toughness requirements, especially at low temperatures, low-active gas mixtures are preferred. This helps to avoid oxygen uptake and the formation of brittle weld structures.
| Welding process |
ISO Group |
Typical composition | Application / Points of interest |
| MAG | M12 | Ar + ≤2% CO₂ | Low oxygen uptake |
| TIG | M13 | Ar + 1% O₂ | Good toughness |
| MIG | I1 | 100% Ar | Base layers |
Not recommended: M21 and higher for strict impact toughness requirements
The set gas flow rate deserves extra attention. A flow rate that is too low provides insufficient protection, while a flow rate that is too high causes turbulence, which can actually draw in ambient air. As a rule of thumb, flow rates typically range between 6 and 12 l/min for TIG welding and 12 to 20 l/min for MIG/MAG welding, depending on the gas cup diameter, environment, and welding position.
Environmental factors such as drafts and wind significantly impact the effectiveness of the shielding gas. In these situations, shielding by means of a welding tent or adjustments to the setup is necessary.
The quality of the gas delivery system also plays a crucial role. Leaking couplings, contaminated hoses, or unsuitable pressure regulators can severely disrupt the gas protection.