1. Shielding Gases and their Influence in Welding
Sricharan Sunder, 09MT3008
Piyush Verma, 09MT3018
Department of Metallurgical and Materials Engineering
IIT Kharagpur
April 2, 2013

2. Abstract
During a welding process, oxygen and other atmospheric gases can react with
molten metal causing defects that weaken the weld. The primary function of
a shielding gas is to protect the molten weld from atmospheric contamina-
tion and resulting imperfections. In addition to its shielding function, each
gas or gas blend has unique properties, weld appearance and shape, fume
generation, weld color, and arc stability.
The primary gases used for electric welding and cutting are argon, helium,
hydrogen, nitrogen, oxygen, and carbon dioxide. The composition of the gas
can and should be tailored to meet the process, material, and application
requirements.

3. Introduction
For all practical purposes, a welding arc can be thought of as a conversion
device that changes electrical energy into heat.
Arc temperatures are very high, producing more than enough heat to melt
any known matearial. The characteristic of an arc depends on the shielding
gas that is used in the arc gap because the gas affects the arc constituent i.e.
the anode, cathode, and plasma regions of the arc.
To form the arc plasma the shielding gas must be forced to remove an
electron from a gas atom, making it an ion, or electrically charged gas atom.
This is referred to as ionization. The heavier the gas atom the easier it is to
ionize the lighter is harder to ionize.
Since heat in the arc is roughly measured by the product of current and
voltage (arc power), the use of helium yields a much higher available heat
than does argon. Conversely you can understand that since argon ionizes at
a lower voltage it will initiate the arc easier than does helium.
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4. Influences and Effects of Various Shielding Gases
on Different Properties
Dissociation and Recombination
When two or more atoms combine they form a molecule. Shielding gases
such as hydrogen and oxygen are molecules. When gases such as hydrogen
are heated to the temperatures present in the arc plasma, these gases break
down, or dissociate into their separate atoms. They are then at least par-
tially ionized, producing free electrons and current flow. As the dissociated
gas comes in contact with the relatively cooled work surface, the atom s re-
combine, and in the process generate additional heat. This process does not
occur with gases such as argon, which cons ists of a single atom. Therefore,
at the same arc temperature, the heat generated at the work surface can be
considerably greater with gases such as hydrogen and oxygen.
Reactivity
Reactivity, as it applies to shielding gases, is a comparative measurement
of how readily a given sh ielding gas will react with the molten weld metal.
Nitrogen is sometimes considered an inert gas. However, at the temperatures
associated with weldings, it may react and have an effect on weld chemistry.
Hydrogen also reacts with the weld metal but as a reducing gas. Hydrogen
will (preferentially) react with an oxidizing agent over the molten weld metal,
thereby helping to prevent the formation of oxides in the molten weld metal.
Surface Tension
In any liquid there is an attractive force exerted by the molecules below
the surface upon those at the surface. An inward pull, or internal pressure
is thus created, which tends to restrain the liquid from flowing. Its strength
varies with the chemical nature of the liquid.
In welding, the surface tension between molten metals and the surrounding
atmosphere has a pronounced influence on bead shape. If the energy value is
high, a convex, irregular bead will result. Low values promote flatter beads.
Pure argon shielding is usually associated with high interfacial energy, pro-
ducing sluggish puddle and a high crowned bead. This is partially attributed
to the high surface tension of liquid iron in a inert atmosphere.
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5. Gas Purity
Depending on the metal being welded and the welding process used, even
very minute gas impurities can have a significant effect on welding speed,
weld surface appearance, weld bead coalescence, weld color, and porosity
levels. The effects of any given impurity, either by itself or in combination
with others, on the many metals and processes available create endless pos-
sibilities.
There is always the possibility of the gas being contaminated either as
delivered, or more likely, somewhere between the supply and the end use
point. Your shielding gas supplier is equipped with the analytical equipment
to determine purity levels anywhere in the system, and in most cases will
assist in identifying the cause and solution.
Flow of Shielding Gas
The proper flow of shielding gas is a very important factor for high quality
welding. Insufficient or excessive amounts of shielding gas can have negative
affects on welding quality. Excessive gas can cause turbulence to occur and
air contamination to be introduced; this causes oxidation and discoloration
of the weld deposit. Insufficient gas flow can also result in poorly protected
welds that is, oxidation from the air can occur, and once again oxidized
and discolored weld deposits can be produced. While discolored deposits
depending on application, may only be cosmetically displeasing the presence
of oxide can reduce mechanical properties (strength and fatigue) and reduce
toughness.
Conclusion
So it can be seen that as a matter of fact, the shielding gases are very
much responsible for the surface finish and the properties of the weld as the
shielding gases effect a lot of other factors which determine the weld directly
or indirectly. It also effects the drop radius as follows.
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6. Thus we can see that the shielding gas also effects the droplet size that
is released from the electrode and hence controls the volume of filler metal
going per unit time into the weld.
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