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AGA CW Handbook A4_32130-UK
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7 purpose of
shielding
gas
contents 2.1 background 2.2 ozone is generated during gas arc welding
2.3 ozone – good and bad 2.4 the mison ® shielding gas programme 2.5 science in the background 2.1 background the importance of shielding gas in the protection of the electrode, molten pool and hot metal during gas arc welding was examined in the previous chapter. the composition of shielding gases is based on wide- scale research and development work, the goal of which is to develop shielding gases for the optimisation of the welding process and meet the development challenges of new materials and techniques. 2.2 ozone is generated during gas arc welding! the main focus of the development work has usually been only on the role of the shielding gas in the protection of the weld. aga has develo- ped a shielding gas programme which also takes into consideration the reduction of problematic air impurities generated during welding. this programme is called mison ® – the shielding gas product pro- gramme for the protection of the welder and the weld. welding fumes and gases – are generated in the air as impurities during all welding. the fumes mainly consist of metal oxides, while the gases consist of ozone, nitrogen oxides and carbon monoxide. the risk of exposure to these impurities is reduced by various means, such as fresh air helmets and respirators, general ventilation, local exhaust, etc. these means, which are naturally always required, share the purpose of protecting the welder from impurities. however, the most effective protection is achieved when the genera- tion of impurities can be prevented entirely, or the generated amount limited. this is the fundamental idea of the mison ® shielding gases, which effectively reduce the amount of ozone generated during wel- ding. ozone is one of the most harmful impurities generated during welding. its health limit (highest acceptable average concentration during a workday htp 8h , 2009) is only 0.05 ppm, which is 600 times lower than that of carbon monoxide, for example. a majority of the measures taken to improve the productivity and quality of welding (such as lowering the carbon dioxide content of the shielding gas or changes in welding parameters) have increased the amount of ozone generated. this would appear to be the price we have to pay for development. however, mison ® shielding gases help keep the amount of generated ozone low while allowing the optimisation of the productivity and quality of welding. the mison ® shielding gas programme. during his or her career, a welder is exposed numerous times to ozone concentrations exceeding the htp limits, unless proper protective measures are taken. when mison ® shielding gases are used, most of the generated ozone is eliminated in the vicinity of the arc without ever getting to the welder’s breathing zone.
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