Shielding Gases and their Influence in Welding

Sricharan Sunder, 09MT3008Piyush Verma, 09MT3018

Department of Metallurgical and Materials EngineeringIIT Kharagpur

April 2, 2013

Abstract

During a welding process, oxygen and other atmospheric gases can react withmolten metal causing defects that weaken the weld. The primary function ofa shielding gas is to protect the molten weld from atmospheric contamina-tion and resulting imperfections. In addition to its shielding function, eachgas or gas blend has unique properties, weld appearance and shape, fumegeneration, 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 gascan and should be tailored to meet the process, material, and applicationrequirements.

Introduction

For all practical purposes, a welding arc can be thought of as a conversiondevice that changes electrical energy into heat.

Arc temperatures are very high, producing more than enough heat to meltany known matearial. The characteristic of an arc depends on the shieldinggas 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 anelectron 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 toionize the lighter is harder to ionize.

Since heat in the arc is roughly measured by the product of current andvoltage (arc power), the use of helium yields a much higher available heatthan does argon. Conversely you can understand that since argon ionizes ata lower voltage it will initiate the arc easier than does helium.

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Influences and Effects of Various Shielding Gases

on Different Properties

Dissociation and Recombination

When two or more atoms combine they form a molecule. Shielding gasessuch as hydrogen and oxygen are molecules. When gases such as hydrogenare heated to the temperatures present in the arc plasma, these gases breakdown, or dissociate into their separate atoms. They are then at least par-tially ionized, producing free electrons and current flow. As the dissociatedgas comes in contact with the relatively cooled work surface, the atom s re-combine, and in the process generate additional heat. This process does notoccur 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 beconsiderably greater with gases such as hydrogen and oxygen.

Reactivity

Reactivity, as it applies to shielding gases, is a comparative measurementof how readily a given sh ielding gas will react with the molten weld metal.Nitrogen is sometimes considered an inert gas. However, at the temperaturesassociated with weldings, it may react and have an effect on weld chemistry.

Hydrogen also reacts with the weld metal but as a reducing gas. Hydrogenwill (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 belowthe surface upon those at the surface. An inward pull, or internal pressureis thus created, which tends to restrain the liquid from flowing. Its strengthvaries with the chemical nature of the liquid.

In welding, the surface tension between molten metals and the surroundingatmosphere has a pronounced influence on bead shape. If the energy value ishigh, 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 attributedto the high surface tension of liquid iron in a inert atmosphere.

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Gas Purity

Depending on the metal being welded and the welding process used, evenvery minute gas impurities can have a significant effect on welding speed,weld surface appearance, weld bead coalescence, weld color, and porositylevels. The effects of any given impurity, either by itself or in combinationwith others, on the many metals and processes available create endless pos-sibilities.

There is always the possibility of the gas being contaminated either asdelivered, or more likely, somewhere between the supply and the end usepoint. Your shielding gas supplier is equipped with the analytical equipmentto determine purity levels anywhere in the system, and in most cases willassist in identifying the cause and solution.

Flow of Shielding Gas

The proper flow of shielding gas is a very important factor for high qualitywelding. Insufficient or excessive amounts of shielding gas can have negativeaffects on welding quality. Excessive gas can cause turbulence to occur andair contamination to be introduced; this causes oxidation and discolorationof the weld deposit. Insufficient gas flow can also result in poorly protectedwelds that is, oxidation from the air can occur, and once again oxidizedand discolored weld deposits can be produced. While discolored depositsdepending on application, may only be cosmetically displeasing the presenceof oxide can reduce mechanical properties (strength and fatigue) and reducetoughness.

Conclusion

So it can be seen that as a matter of fact, the shielding gases are verymuch responsible for the surface finish and the properties of the weld as theshielding gases effect a lot of other factors which determine the weld directlyor indirectly. It also effects the drop radius as follows.

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Thus we can see that the shielding gas also effects the droplet size thatis released from the electrode and hence controls the volume of filler metalgoing per unit time into the weld.

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