Protective gas
As inert gas one is gas or gas mixture called, which has the task of the air of the Earth's atmosphere to displace, especially the oxygen of the air.
Food
Inert gas is widely used in packaging of food used. The protective gas atmosphere consists of natural, odorless and tasteless components of the air, depending on the food to be packaged, e.g. B. carbon dioxide (CO 2 ) or nitrogen (N 2 ) or as with fresh meat from oxygen (O 2 ), the proportions of which vary depending on the product.
Protective gases are not additives within the meaning of the Food Act . They are harmless in terms of food law and do not have to be declared. However, labeling in accordance with Section 9 (7) of the Additive Admissions Ordinance is required: "Packed in a protective atmosphere".
Metal technology
Welding technology
With inert gas welding , the arc and the weld pool are protected from the ingress of atmospheric gases (N 2 , O 2 , H 2 ) by a protective gas . This prevents the metal from reacting with the oxygen in the air ( corrosion , combustion ) or metallurgical or mechanical pores in the melted material. High-quality gas protection is particularly important for high-alloy steels, but also for light metals such as aluminum , magnesium or titanium . If the quality of the gas cover is inadequate, depending on the material and the boundary conditions, tempering colors, soot deposits, increased weld spatter ejection, pores or even structural damage can occur. In addition to the pure protective function, the choice of protective gas can also influence the shape of the seam, the ability to bridge the gap, the ignition behavior, the arc stability or the droplet transfer.
In gas-shielded arc welding according to DIN 1910-100, a distinction is made between gas-shielded metal arc welding (MSG) and gas-shielded tungsten arc welding (WSG) and their sub-processes. The shielding gases used vary depending on the process, material or special process requirements. Shielding gases for gas-shielded metal welding of unalloyed and low-alloy steels are z. B. CO 2 or mixtures of argon and CO 2 . For high-alloy steels, mixed gases that are rich in argon and contain only a few percent O 2 or CO 2 are generally used . Aluminum, magnesium or titanium are usually welded with argon or argon-helium mixtures. If active components such as O 2 or CO 2 or H 2 are contained in the shielding gas, one speaks of metal active gas welding according to DIN EN ISO 14175 and DIN 1910-100. If only argon or helium or their mixtures are used, one speaks of metal inert gas welding. The variety of standardized gas mixtures is now very large. Argon, helium, carbon dioxide, but also oxygen, hydrogen and nitrogen can be used as mixture components. The European standard DIN EN ISO 14175 "Gases and mixed gases for arc welding and related processes" gives a classification of the shielding gases.
The quality of the respective shielding gases is indicated with a code for commercially available gas cylinders: element designation - number of leading nines - point - number of residual impurities. Example: Ag 5.4 means 5 nines and then the number 4, i.e.: Argon with 99.9994 percent argon. The impurity is therefore 0.0006 percent.
A good shielding gas cover depends to a decisive extent on the fluidic design of the welding torch and the correctly selected boundary conditions in welding production. Both diagnostic and numerical methods of flow analysis are used in science and industry to visualize and evaluate the gas flow and the resulting gas coverage on the workpiece.
The methods of the Schlieren technique or Particle Image Velocimetry (PIV) are used for the diagnostic visualization of the gas flow. By measuring the oxygen, the quality of the shielding gas cover can be determined quantitatively in the laboratory, taking into account the arc.
In addition to the methods of diagnostic flow analysis, the shielding gas flow of welding processes can also be analyzed with the help of numerical flow simulation. The advantages of numerical simulation lie in the possibility of visualizing flows in small, hidden areas within the welding torch and of describing complex physical relationships with high resolution in terms of time and location. Cause-and-effect relationships can be recognized very well and traced back to their physical causes.
Hardening technology
Shielding gas is also used in hardening technology, for the atmosphere in the hardening plant, since gaseous nitrogen or hydrogen prevents oxygen from changing the steel to be hardened . The protective gas thus burns off in the furnace inlet. This means that the surface of the hardened workpiece remains shiny and at the same time fewer residues occur that would otherwise have to be laboriously filtered out of the quenching medium.
Continuous casting
In continuous casting, when the liquid metal is transferred between the ladle and the tundish, a shadow tube is usually used to avoid oxidation and nitrogen entry. For this purpose, the shadow tube is filled with a protective gas, usually argon .
Electrical engineering
Protective gas is used in electrical engineering to reduce the conductivity in the vicinity of switching contacts. This serves to quench the spark .
See also
literature
- Rudolf Wolfgang Klingler: Basics of grain technology . Behr's Verlag, Hamburg 1995, ISBN 3-86022-228-7 , p. 245. ( digitized version )
Individual evidence
- ↑ Questions and answers on meat that has been packed in a protective atmosphere with increased oxygen content. Federal Institute for Risk Assessment, accessed on April 25, 2013 .
- ^ TU Dresden - Schlierentechnik
- ↑ TU Dresden - Particle Image Velocimetry (PIV) ( Memento from October 13, 2014 in the Internet Archive )
- ↑ TU Dresden - oxygen measurement
- ^ TU Dresden - Numerical flow simulation
- ↑ M. Dreher, U. Füssel, M. Schnick, S. Rose, M. Hertel: Flow simulation and diagnostics. Modern methods for efficient and innovative MSG welding torch development. In: DVS reports. Volume 267, Düsseldorf 2010, ISBN 978-3-87155-592-3 , pp. 159-165.
- ↑ U. Füssel, M. Dreher, M. Schnick: Fluidic design of torch systems for economical and emission-reduced arc welding. Arc Welding Cluster - Physics and Tools, AiF 15.871 B, duration November 1, 2008 - December 31, 2011.
- ↑ See, for example, the description of continuous cast steel in the product sheet ( Memento of the original from July 14, 2014 in the Internet Archive ) Info: The archive link was automatically inserted and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. a mass flow controller (diagram on double page 4/5).
