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Managing welding costs by optimising shielding gas mixtures

Home Manufacturing & Processing Petro Chemicals Managing welding costs by optimising shielding gas mixtures

Manufacturing costs have increased significantly in the last few years due to a number of external factors. The current challenge for manufacturers is to manage costs in the welding process without compromising quality or output.

That’s according to Welding Specialist at Air Products, Sean Young, who said one way to manage the costs is to ensure that the correct material and processes are used from to onset to avoid unnecessary costs as a result of unsuccessful welds or rework.

He explained that for a weld to be successful, the source melted and the components to be joined need to be protected from oxidation and atmospheric contamination. This can be achieved by means of a flux or by using a shield gas. In the case of shielding metal arc (SMA) electrodes or submerged arc processes, a flux is used, whereas a gas shield is used with gas metal arc welding (GMAW), gas tungsten arc welding (GTAW) and most flux-cored processes.

He added that selecting optimised shielding gas mixtures for gas metal arc welding (GMAW) of carbon steel is one way in which the costs can be evaluated and minimised.

Young offers specialist services and advice to customers on various components of the process, one of which is the selection of shielding gas. He believes it is important to look at the welding process, the material, its thickness and the metal transfer mode.

“The weld properties are affected by shielding gases, and in order to optimise the choice of shielding gas, it is important to take all the elements into account that can affect the quality of the weld, such as spatter, bead profile, fusion and penetration,” he said.

A number of mixtures and shielding gases are commonly used for welding:

  • CO2 is largely used for GMAW in dip transfer mode of carbon steels
  • Argon is suitable for GMAW of non-ferrous materials and all GTAW applications
  • Argon/CO2, Argon/O2, Argon/CO2/O2 mixes are used for GMAW of carbon steels as well as stainless steels
  • In the case of more advanced GTAW applications, more exotic argon/helium and argon/hydrogen mixes are available
  • T4dcWith more advanced GMAW applications, Argon/He/CO2 and Argon/H2/CO2 mixes are available

Elaborating on Argon and CO2, Young said, “In any mixed shielding gas cylinder, argon is generally the dominant gas. In its pure form, it is an inert gas which is used to keep other gases out and has no chemical effect on the deposited metal weld. On its own, argon is used for all tungsten inert gas welding (GTAW/TIG) and GMAW aluminium and copper and its alloys.”

He said that pure CO2 is perceived as the original shielding gas for GMAW and is widely used for general purpose welding of steels today and it is a cost-effective shielding gas. However, CO2 can destabilise the arc and cause spatter as it violently dissociates into carbon monoxide and oxygen in the arc. This leads to a hotter arc with deep penetration, causing a large droplet formation which

is known to restrict the use of the CO2 to a dip-transfer mode.

Adding minority percentages of active gases such as oxygen and carbon dioxide can make significant improvements to the argon shielding gas for GMAW of carbon steels and stainless steels.

“Adding small percentages of oxygen leads to a shielding gas with improved wetting action and it also decreases the surface tension of the molten metal, producing a flatter weld. Furthermore, the
pinch-off effect is accelerated, and smaller droplet sizes created. The result is a more stable metal transfer, a softer arc and reduced spatter,” said Young.

An improved transfer stability ensures that the GMAW welding process is less sensitive to welding parameters and more tolerant to voltage and current variation. Ultimately, this leads to reduced time for machine set-up which improves the overall productivity. Young said an argon/oxygen mixture of up to 2% oxygen is the ideal for stainless steel applications.

In instances where CO2 is added to Argon in a two-part mix, there is an improvement in the penetration of carbon steel joints. A limit in the percentage CO2 plays a role to obtain a smooth metal transfer in the spray transfer mode and to overcome instability issues.

According to Young, you obtain an improved penetration and welding speed when increasing the CO2 as a result of the increase in temperature in the welding arc. He warns that more than 15% CO2 in argon causes spatter and the instability to re-emerge. According to him, 15% is optimal and in the case of thinner materials where penetration is not required, 3% CO2 is sufficient.

GMAW shielding gas mixtures can be optimised to provide the best weld properties for particular applications by combining the benefits of CO2, O2 and argon.

Elaborating on three-part mixtures, he said, “The mixtures generally consist of argon with CO2 of up to 15% and O2 of up to 3%. It is possible to improve the arc stability, optimise metal transfer characteristics, minimise spatter generation and improve penetration and the bead profiles by using all three the gases.”

He said there is a vast difference between two part and three-part mixtures, in particular when you look at the spatter generated during the welding process. Three-part mixtures generates far less spatter.

“In the long run, welding costs accumulate when you take the time and cost of post-weld activities and cleaning up into consideration. A slightly more expensive gas mixture might make it worthwhile to switch if an accurate cost comparison is done.”

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