Difference between gas shielding, gas purging & gas trailing?

What is Gas shielding in welding?

During welding operation, air in the welding region is displaced by a shielding gas (either externally supplied or generated by burning of coating from the electrode) in order to avoid contamination of the molten weld pool. This weld contamination is produced mostly by nitrogen, oxygen and water vapor present in the air. For example, nitrogen in solidified steel weld decreases the ductility and impact strength of the weld. In large amounts, nitrogen creates weld porosity. Similarly, oxygen can react with the metal and creates metal oxides or compounds which can creates inclusions. Oxygen can also combine with carbon to form carbon monoxide (CO). Hydrogen from electrode coating moisture or, present in water vapor and oil, combines with either iron or aluminum creates porosity. It can also results in Underbead weld cracking

To avoid all these issues related with contamination of the weld pool, shielding gases are used for shielding.

Click here to know about various shielding gases and their purposes.

gas shielding

What is Trailing gas?

In trailing a gas used to produce a protective atmosphere that extends beyond the weld pool in the direction opposite of travel. 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.

For example, for titanium alloys and stainless steels welding, trailing gas are used to protect the weld from oxidation until it has cooled to a safe temperature.

gas trailing

Trailing Shields (an auxiliary attachment with the tig torch) are connected to the inert-gas supply, diffusing the gas without turbulence through a multi-layer stainless-steel mesh built into the body of the shield. At each side, replaceable silicone rubber side shields provide a barrier to prevent the gas escaping sideways.

As the welding torch is moved forward, the weld remains under an inert gas shield until the welded metal has cooled below its oxidation temperature.

What is Gas Purging?

Weld purging is the process of removing the oxygen, nitrogen, water vapor and any other gases or vapors from the vicinity of the welding joint root,  which may be detrimental to a welding joint as it is being welded and immediately after welding. 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).

Stainless steels, duplex steels, titanium-, nickel- and zirconium- alloys are sensitive to the presence of air, oxygen, hydrogen, water vapor and other vapors and gases that may combine with the hot metal as it is being joined and create detrimental oxides or compounds.

These gases in association with the metal to form undesirable compounds that may reduce corrosion resistance or may be helpful in creating cracks or other structural defects in weld metals.

Weld Purging is generally necessary for the first weld run when joining two separate parts. This sealing weld will be called a “root run” when it takes more than one run or (pass) to fully seal the root area from above.

Once the root run has been completed, it is possible to stop the purging process unless the welding engineer has specified that purging should be continued for the second and third passes for example in case the root weld becomes hot enough to oxidize in the air that will have replaced the purge gas.

Measurement of gas purge quality

There are charts available that show the discoloration of some metals (such as titanium or stainless steel), caused by the presence of oxygen, even at concentrations below 50 parts per million (ppm). It is possible to monitor oxygen levels by an instrument known as an oxygen sensor. Many specific brands of oxygen sensor are available, with varying sensitivity, time of response, and other criteria of interest. 1 part per million sensitivity is more than sufficient for almost all welding purposes.

discoloration of stainless steel

Some of these instruments are hand held and powered by battery for ease of use around construction sites where the materials mentioned are being welded and others are powered by mains electricity due to the power requirements needed to run switching operations, collect data and operate internal pumps to draw the purge gas over the sensor for measurement.

The perfect welding result, without impairment of corrosion resistance and mechanical properties, can only be obtained when using a backing gas with very low oxygen content. For best results, a maximum 20 ppm O2 at the root side can be tolerated. This can be achieved with a purging set-up and can be controlled with a modern oxygen meter.

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