9 Effect of shielding gas: ozone9.1 Mild steel weldingThe choice of shielding gas can be a significant factor inozone emission. The reduced ozone emissions ofAir Products Maxx® shielding gases are achieved byadopting two formulation strategies:• Changing the wavelength or lowering its intensity reducesozone producing radiation.This is typically achieved by adding carbon dioxide orhelium to argon-based shielding gases.• Adding chemical species that reacts with ozone to the gasmixture. For argon, two popular choices are hydrogen andnitric oxide. Hydrogen reacts with ozone to form watervapour and oxygen (and also changes the UV spectrum ofthe arc). Nitric oxide converts ozone to nitrogen dioxide.MAGMAG welding of mild steel is usually performed with argonbased shielding gases containing carbon dioxide and/oroxygen. Helium may also be added to the mixture to improveoperability and increase productivity.Figure 16 demonstrates the beneficial effect of largerconcentrations of carbon dioxide in the shielding gas onthe ozone level in the welder's breathing zone, duringarcing, when MAG welding mild steel with dip transfer.This reduction is believed to arise from a change in the UVspectrum due to the composition of the shielding gases.26
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