First, Types of shielding gases..
There are two types of gas that is applicable to welding:
- Inert gases. Inert refers to “non-reactive gases which don’t have valance electron cell thus not taking part into any chemical reactions”. This includes but is not limited to the noble gases (which are generally nonreactive in most conditions). For example, argon as inert gas will not react with any oxygen or metal in the welding arc and puddle, thus we will not see any slag or silicate or oxides on the surface of TIG welding with argon or helium.
- Reactive gases. As the name suggests these gases are chosen for their ability to react with other elements or compounds. They can create changes in the state of the weld or the welding conditions. For example, CO2 as shielding gas, break into ‘CO’ and ‘O’ in the welding arc. The oxygen reacts then with silicon/ manganese presents in the weld puddle to creates oxides.
Purpose of Gases Used for In Welding?
There is a range of different uses for gas in welding. This can include keep the arc clear of impurities (such as dust, other gases, dirt, etc.),
Also used for assisting arc stability and ensuring proper metal transfer for many welding processes. making sure that the welding pool stays clean below the seam (this is known as purging), for blanketing and for heating too.
The main purposes of gases in welding are:
- Shielding: Air that gets into the arc will form air bubbles in the molten metal. This makes the weld weak and unattractive. When you MIG or TIG weld, you have to use a shielding gas except in the case where the filler material is “flux-coated” or “flux-cored”.
- Purging: A purging gas does the same job as a shielding gas but on the underside of the weld. This is typically done during stainless steel welding and it’s done by sealing the bottom of the joint and blowing gas over it (you can use the same or a different gas as that being used on the other side of the joint).
- Trailing: Trailing isn’t common, but it’s done when you want to ensure that a weld won’t be stained or contaminated after the weld is finished. The space around the weld is filled with gas to drive out any airborne contaminants. Trailing is extensively used for Titanium welding where the solidified weld can still react with atmospheric oxygen to create titanium oxide.
Importance of Shielding gases in Welding
Shielding gases are mandatory requirements for all conventional welding processes.
They serve various functions although they are required primarily to shield the weld pool from atmospheric contamination and to provide an ionized path that can allow the flow of electricity from an electrode to a workpiece. Even processes that do not have an external gas supply such as Shielded Metal Arc Welding (SMAW or MMAW) and Gasless Flux-cored Arc Welding (FCAW) all have a shielding gas which is generated by the decomposition of the flux by the welding arc heat.
The shielding gas can also have an effect on arc stability, weld shape, and depth of penetration as well as the mechanical properties and metallurgy of stainless steel weldments.
SHIELDING GASES & THEIR EFFECTS ON WELDING PROPERTIES
Effect of Argon (Ar) on Welding
Argon (Inert gas) accounts for 1 percent of air and is a by-product of the air-reduction processes used to produce oxygen. This gas is good for shielding welds in the flat position and in deep groves.
Argon is suitable for easier starts and alternating current applications, as well as for longer arcs at lower voltages. In pure form, argon is often used with aluminum and nonferrous metals. The addition of helium improves argon’s heat transfer properties and combining argon with carbon dioxide or oxygen can help stabilize the arc.
Effect of Helium (He) on Welding
Helium (Inert gas) is effective for mechanized applications but less forgiving for manual welding. Since pure helium creates an erratic arc, it can result in spatter when working with steel. Even so, pure helium is ideal for magnesium, copper, and aluminum. When mixed with argon, helium can provide cathode cleaning. Other blends can be used on aluminum and stainless steel.
Effect of Hydrogen (H) on Welding
Hydrogen is often used in combination with other gases. When added to argon, it can deepen penetration and increase welding speeds. On grades of stainless steel that are sensitive to oxygen, it can result in cleaner weld surfaces and better bead profiles.
The mixture of argon, carbon dioxide, and hydrogen can raise arc temperature, narrow the arc, and improve weld penetration. Hydrogen isn’t perfect, though. If used incorrectly, it can cause weld porosity, a phenomenon brought on by too much-trapped gas, resulting in the formation of round holes. Cracking can also occur under the bead in carbon and low-alloy steels.
Effect of Nitrogen (N) on Welding
Nitrogen increases weld penetration and arc stability. Gas blends containing nitrogen can increase the mechanical properties of alloys containing nitrogen and prevent pitting corrosion and nitrogen loss from the metal. Nitrogen promotes austenitic phases in austenitic and duplex stainless steel welding. It is used in purging for Duplex stainless steel welding.
Effect of Oxygen (O) on Welding
Similar to hydrogen, oxygen is usually used with other gases to shield the weld.
For example, oxygen is usually used in combination with argon during the welding process for these benefits:
- Arc stabilization
- Spatter minimization
- Metal transfer improvement
This gas can cause oxidation, however, so it can’t be used with copper, aluminum, or magnesium. And be conservative with using it: An abundance of oxygen can result in brittleness.
Effect of Carbon-dioxide (CO2) on Welding
Carbon dioxide is best suited for steel and is especially useful in metal inert gas (MIG) welding because it increases weld speed, penetration, and mechanical properties.
While inexpensive, carbon dioxide is not without its faults when used in welding. It causes a shakier arc and spatter loses, and working with it can produce a lot of smoke fumes in a workplace. Mixing carbon dioxide with argon, however, can minimize spatter.
Carbon dioxide should also not to be used for thin metals especially aluminum & copper which are having high thermal conductivity. It’s usually too hot for thin metal to sustain.
OVERVIEW OF THE SHIELDING GAS GROUPS BASED ON ISO 14175
Group R contains argon/hydrogen mixtures which have a reducing effect. In addition to argon and helium, the gases in Group R1 are used for TIG welding and plasma welding, while gases in subgroup 2, which have a higher hydrogen content (H) are used for plasma cutting and backing (forming gases).
Group I combines the inert gases. It includes argon (Ar), helium (He) and argon/helium mixtures. They are used for TIG, MIG and plasma welding, and for backing.
The large M group, which is subdivided into M1, M2 and M3, combines mixed gases for MAG welding. There are 3 or 4 subgroups in each group. The gases are classified from M1.1 to M3.3 according to their oxidisation behaviour, i.e. M1.1 is the least oxidising, and M3.3 is the strongest oxidising agent. The main component of these gases is argon. Oxygen (O) or carbon dioxide (CO2) or oxygen and carbon dioxide (three-component gases) are mixed with active components.
In the range of gases for MAG welding, Group C includes pure carbon dioxide and a carbon dioxide/oxygen mixture. The latter is not important in Germany. The gases in Group C are the most strongly oxidizing, because the CO2 decomposes at the high temperature of the arc, producing large amounts of oxygen in addition to carbon monoxide.
Finally, group F includes nitrogen (N) and a nitrogen/hydrogen mixture. Both gases can be used for plasma cutting and forming.
OVERVIEW OF THE SHIELDING GAS GROUPS BASED ON SFA 5.32
AWS A5.32 “Specification for Welding Shielding Gases”, prescribes the requirements for the classification of shielding gases, similar to the way AWS 5.18 “Specification for Carbon Steel Electrodes and Rods for Gas Metal Arc Welding”, prescribes a classification system for identifying carbon steel electrodes and rods. The classification system outlined in the AWS A5.32 is a system that clearly identifies the chemical composition of the shielding gas in question, similar to the way in which welding wire is identified. To make an analogy: If you order E70S-6 welding wire you should feel confident that you will receive a welding wire that contains a certain percentage of silicon, manganese, and so on. Similarly, when you order a shielding gas you should be confident that when you order SG-AC-10, that product should be 10% carbon dioxide, 90% argon, and that the product is consistent from cylinder to cylinder. AWS A5.32 not only establishes an identification system for shielding gases, but it also specifies purity and dew point levels that are required for individual gases which are shown in table X. Requirements for dew point, purity, and mix accuracy of gas mixtures is also covered in the specification.
In order to comply with this specification, a shielding gas supplier is required to test individually filled cylinders or one cylinder from each filling manifold to verify mix accuracy, purity, and dew point.
To ensure that your gases comply with AWS specification SFA A5.32, have a look at the cylinder if your gases are meeting this specification. The below diagram highlights the main requirements, their classification, and types of shielding gases to be used in welding as per SFA 5.32.