Principles and Use of Shielding Gases for Arc Welding *
SELECTING SHIELDING GAS MIXTURES
To select the best shielding gas mixture for an application, consider the following factors:
- Type of base material
- Welding position
- Transfer mode(s)
- Weld appearance, acceptable spatter level
- Versatility requirements, e.g. one gas mixture to accommodate various thicknesses and transfer modes
APPLICATIONS AND ADVANTAGES OF GAS MIXTURES
1) Gas Mixtures for GMAW of Carbon Steels
Argon / Oxygen (Ar + 2% O2; Ar + 5% O2)
The advantages of argon I oxygen mixtures are:
- The oxygen content creates lower molten weld pool surface tension and greater heat transfer, resulting in better wetting and less chance of undercut than with pure argon. It also improves arc stability.
- Due to the high argon content, the mixtures produce spray transfer, little spatter and low alloy losses. Note that the alloy losses increase with increased oxygen content.
- They can be used for pulsed current transfer.
Argon / Carbon dioxide (Ar + 10% CO2; Ar +15% CO2; Ar + 25% CO2)
The advantages of argon I carbon dioxide mixtures are:
- Free oxygen produced from the dissociation of the carbon dioxide results in improved wetting over pure argon.
- Mixtures with over 20% carbon dioxide are well suited to short-circuit transfer. Those containing 10% or less carbon dioxide are best for spray transfer.
- The extra heat transfer provided by the carbon dioxide improves penetration in spray transfer over pure argon.
- The high argon content reduces spatter and the loss of alloying elements. The carburizing effect is limited and less deoxidizers are required in the filler metal than when pure carbon dioxide is used.
Argon I Carbon dioxide I Oxygen (Ar + 5% CO2 + 4% O2; Ar + 8% CO2 + 2% O2)
The advantages or argon I carbon dioxide / oxygen mixtures are:
- These tri-mixes have all the advantages of argon I oxygen and argon I carbon dioxide mixtures and more.
- They offer good performance in both short-circuit and spray transfer modes, and are well suited for pulsed current welding.
- The mixtures allow spray transfer at low amperages which is crucial for pulsed current welding.
- Spatter and fumes are typically less than argon / CO2 mixtures.
- They have a smaller globular transfer zone than mixtures rich in carbon dioxide.
- Possible faster welding speeds than with argon / carbon dioxide mixtures.
2) Gas Mixtures for GMAW of Stainless Steels
The gas mixtures commonly used for gas metal arc welding of stainless steel are:
Argon I Oxygen (Ar + 2% O2)
The advantages of argon I oxygen mixtures for welding stainless steel are:
- They offer improved wetting and greater heat transfer over pure argon. - The high argon content allows use of the spray transfer mode. - Chromium oxidation is limited by keeping the oxygen content low. - There is no carbon enrichment of the weld from the shielding gas mixture. - They can be used on ferritic or martensitic stainless steels without concern of hydrogen embrittlement.
Helium I Argon I Carbon dioxide (90% He + 7.5% Ar + 2.5 % CO2)
The advantages of helium / argon I carbon dioxide mixtures for welding stainless steel are:
- They allow the use of short-circuit transfer for joining thin materials and welding out-of-position.
- The high heat input of helium produces deep penetration.
- They can be used on ferritic or martensitic stainless steels without concern of hydrogen embrittlement.
- Wetting and bead appearance are good.
Argon I Carbon dioxide I Hydrogen (96.25% Ar + 2.75% CO2 + 1% H2)
The advantages of argon / carbon dioxide I hydrogen mixtures are:
- In addition to good performance in short-circuit and spray transfer modes, they are well suited for pulsed current welding.
- They are suitable for all-position welding on all material thicknesses.
- The hydrogen reduces surface oxidation resulting in a shiny bead appearance.
- Faster travel speeds are possible than with (Ar + 10% CO2) or (90% He + 7.5% Ar + 2.5% CO2).
- The low carbon dioxide content minimizes carbon enrichment.
NOTE: Shielding gas mixtures containing hydrogen are recommended only for welding austenitic (300 series) stainless steels. Where hydrogen embrittlement is not a problem.
A gas mixture which is occasionally used with special restrictions for welding stainless steel is:
Argon I Carbon dioxide (Ar + 10% CO2)
Its advantages are:
- It offers improved wetting and greater heat transfer over pure argon.
- Either short-circuit or spray transfer may be used.
- Its use is restricted to applications where carbon enrichment is not a concern. Carbon enrichment can lead to carbide precipitation and result in reduced corrosion resistance. It has been established that if the carbon dioxide content of a shielding gas mixture is greater than 3%, sufficient quantities of carbon released from the breakdown of carbon monoxide can be absorbed by the stainless steel to cause carbon enrichment and subsequently carbide precipitation. (Carbon monoxide is one of the products from the ionization of carbon dioxide in the plasma column). The carbon combines with chromium and forms chromium carbides that precipitate along the grain boundaries. This results in the depletion of chromium in the region surrounding the grain boundaries which in turn causes a significant reduction in corrosion resistance.
The oxidizing power of the carbon dioxide results in some chromium oxidation which manifests itself as a difficult-to-remove grey oxide layer on the surface of the bead.
3) Gas Mixture for GMAW of Non-Ferrous Metals
The gas mixtures commonly used for gas metal arc welding of non-ferrous metals are:
Argon I Helium (Ar + 25% He; Ar + 50% He; Ar + 75% He)
The important concern to address when selecting a shielding gas mixture for welding non-ferrous metals is that the mixture must be inert. Non-ferrous metals such as aluminum, magnesium or copper oxidize very readily and the oxides of these materials melt at higher temperatures than the metal itself. If a mixture with an oxidizing potential is used, large amounts of solidified oxides form on the molten weld pool surface and are dispersed throughout the weld by the kinetic action of the metal droplets. The result is serious weakening of the weld. Mixtures with hydrogen can’t be used because the hydrogen is readily soluble in these metals during the molten state, but solubility decreases dramatically as they cool. The hydrogen is forced out of solution and this causes porosity, rupturing of grain boundaries and cracking.
The advantages of argon / helium mixtures are:
- They combine the individual advantages of both helium and argon. By varying the ratios of the two gases, a mixture can be tailored to offer the best performance for a specific application. For example, the helium content should be increased for thicker base materials; this effectively increases the heat input into the metal, allowing faster welding speeds. Conversely, argon rich mixtures should be used on thin materials for greater control during welding.
4) Gas Mixtures for GTAW on Non-Ferrous Metals
The gas mixtures commonly used for gas tungsten arc welding of non-ferrous metals are:
Argon I Helium (Ar + 25% He; Ar + 50% He; Ar + 75% He)
As more heat input is desired, the percentage of helium in the mixture is increased.
5) Gas Mixtures for GTAW of Stainless Steels
The gas mixture commonly used for gas tungsten arc welding of ferritic or martensitic stainless steels and carbon steel is:
Argon I Helium (Ar + 25% He)
Its advantages are:
One advantage of this mixture over pure argon is increased heat input for faster travel on thicker materials. Another advantage is improved wetting.
The gas mixtures commonly used for gas tungsten arc welding of austenitic (300 series) stainless steels are:
Argon / Hydrogen (Ar + 2.5% H2, Ar + 5% H2)
2.5% H2 is used primarily for manual GTAW applications while 5% H2 mix, with its higher heat input, is used primarily for automated GTAW applications.
The advantages of the argon I hydrogen mixtures are:
- Improved wetting over pure argon - Faster travel speeds possible due to higher heat input - A very shiny bead appearance can be achieved due to reducing action of the hydrogen
6) Gas Mixtures for FCAW
Presently, the gas mixture commonly used for flux cored arc welding applications of steel and stainless steel is:
Argon / Carbon dioxide (Ar + 25% CO2)
Its primary advantage over pure carbon dioxide for this application is a very significant reduction in spatter.
* Reference: Mr. Viwek Vaidya Seminars (Air Liquide)
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