welding-shielding-gases-3.html
AGA CW Handbook A4_32130-UK
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4 purpose of
shielding
gas
the effect of shielding gas on mig/mag weld
ing material transfer the shielding gas has a large effect on material transfer, as well as the size of drops and the forces affecting the drops in the arc. shielding effect the shielding gas shrouds the molten pool and the hot metal from the effects of the surrounding air. the work environment the shielding gas affects the formation of fumes and smoke. arc stability the shielding gas affects the stability and ignition of the arc. weld appearance the shielding gas has a significant effect on the amount of spatter and slag. metallurgy and mechanical properties the shielding gas has an effect on the scale loss of alloy materials and the dissolving of oxygen, nitrogen, hydrogen and carbon in the molten pool. this affects the mechanical and corrosion proper- ties of the weld. weld profile shape the shielding gas affects the height of the weld bead, the weld penetration and its fusion with the base material. welding speed the selection of a shielding gas affects the welding speed and through this the overall welding costs. contents 1.1 what are the effects of shielding gas 1.2 effects of the different shielding gas components 1.2.1 argon 1.2.2 carbon dioxide and oxygen 1.2.3 carbon dioxide or oxygen? 1.2.4 helium 1.2.5 hydrogen 1.2.6 nitrogen 1.2.7 nitrogen monoxide 1.1 what are the effects of shielding gas the basic task of a shielding gas in gas arc welding is to shield the heated and molten metal from the effects of the surrounding air and provide advantageous ignition conditions for the arc. if the surrounding air gets into contact with the hot metal and the molten pool, the oxygen in the air will oxidise the molten metal and its surroundings, while the nitrogen and humidity in the air cause porosity in the weld. the composition of the shielding gas affects the way in which material is transferred from the melting filler wire to the molten pool, which in turn affects the number and size of weld spatter. the shielding gas also affects the appearance and shape of the weld, welding speed, scale loss of alloy materials (which affects the strength of the weld), corrosion properties and the formation of oxides (slag formation) on the surface of the weld bead. 1.2 effects of the different shielding gas components 1.2.1 argon argon (ar) is an inert (nonreactive) gas. it does not cause oxidation and has no other effects on the chemical composition of the weld. for this reason, argon is the main component in most shielding gases for tig and mig/mag welding. 1.2.2 carbon dioxide and oxygen pure argon is not suited to mag welding steel, as the arc becomes too unstable. an oxidising component is required in the shielding gas to increase the stability of the arc and ensure even material transfer du- ring welding. carbon monoxide (co 2 ), oxygen (o 2 ) or a mixture of them acts as such an oxidiser. the amount of the oxidising component in the shielding gas depends on the steel grade and the welded structure purpose of shielding gas.
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