Piet van der Horst

Piet van der Horst

In 1970 Piet made welding his trade en since then he never stopped learning about that trade. By now he is well past his retirement age, but not welding is still not an option for him. It is not just work, it is a passion.

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Gases for welding processes

Inert and active gases

Classification according to the standard In NEN-EN-ISO 14175 standard, gases are divided into main and subgroups. In this article we discuss four main groups of

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Inert and active gases

Classification according to the standard

In NEN-EN-ISO 14175 standard, gases are divided into main and subgroups. In this article we discuss four main groups of gases that are most commonly used in gas arc welding processes.

  • Main group I – Inert gases 
  • Main group M1 to M3 – Oxidizing gases
  • Main group R – Reducing gases
  • Main group N – Low reactive gases

Inert gases

Argon and helium are inert shielding gases and both do not react actively in the puddle. These gases have completely different natural properties. 

  • Helium is 10 times lighter than argon. This means that the flow of the helium shielding gas must be greater than for argon. Otherwise the degree of protection will not be sufficient.  
  • The arc voltage with helium is 1.5 to 2 Volt higher than with argon, so the puddle will also be larger. 
  • Argon is easy to ionize and helium is very difficult to ionize. 
  • The arc stability of helium is poor, while argon has a nice and tight arc. 
  • The thermal conductivity of helium is much larger than that of argon. This means that the penetration depth is also greater when helium is used as a shielding gas. Because the arc stability of 100% helium is so poor, helium is almost exclusively used as a mixed gas with argon.

Helium occurs in a number of places on the earth. Especially in North America and the Rift Valley in East Africa. It is formed by the disintegration of uranium and thorium. In Europe there are almost no helium sources. Natural gas also contains about 1% helium. 

The transport of helium, for example from North America, is expensive. It has to be transported in liquid form and that can only be done at – 269 degrees Celsius. Because of the long distance, the cryogenic tanks will heat up a little, so they sometimes have to be vented. This means that there will never be a 100% delivery. That’s why helium is so expensive and getting more and more expensive.

Oxidizing gases

Gases in the M, R and N group are active. This means that the gas reacts with the puddle and/or the melting drop of the wire. 

The first group of active gases are oxidising shielding gases. These can be subdivided into light, medium and strong-oxidising. As the amount of oxidising gases increases in the mixed gas, the whole will become more active. 

For example, a shielding gas consisting of argon and 0.5% CO2 will hardly react with the puddle or the melting droplet and is therefore slightly oxidising. A shielding gas consisting of argon and 25% to 50% CO2 and 10% to 15% O2 will react very strongly with the puddle and the melting droplet and is therefore highly oxidising. Even more oxidising is a 100% CO2 gas, possibly with 0.5% to 30% O2 added to it.

Very important: When qualified work is carried out, make sure that the wire-gas combination is also qualified. With the increase of the number of available gas mixtures, it is not by default that the selected gas falls within the qualification. The wire supplier can tell you all about this.

Reducing gases

Reducing shielding gases in welding technology are always mixed gases consisting of argon or nitrogen with hydrogen. Argon with hydrogen is used, for example, in TIG welding of stainless steel. Nitrogen and hydrogen are applied as backing gases. Attention: If the proportion of hydrogen is more than 10%, it must be flared because of fire and explosion hazard.

Low reactive gases

Low reactive shielding gases are a mixture of argon and 0.5% to 50% nitrogen. In some mixtures 0.5% to 50% hydrogen is also added. 

These gases are not suitable for the TIG process. However, there is an application for the MAG process; for duplex and super duplex welding, shielding gases containing up to 2% nitrogen and up to 2% CO2 are used. But mostly these gases are used in laser welding and cutting.

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