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AGA CW Handbook A4_32130-UK
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6 purpose of
shielding
gas
helium and argon-helium mixtures can be used for root shielding when it is necessary to get the gas to rise up in order to achieve root shiel- ding. as helium is lighter than air, it rises upwards and is safe to use as a non-flammable gas. 1.2.5 hydrogen hydrogen (h 2 ) can be used as a shielding gas component in the tig welding of austenitic stainless steels. added hydrogen provides a hotter and narrower arc, enabling higher welding speed and better penetration. it also makes the fusion of the weld beam and the base material smoother and reduces the oxidisation of the weld. in root shielding, added hydrogen is beneficial due to its oxide-reducing effect. formier ® 10, 10% of hydrogen in nitrogen
, is an often used root shielding gas. however, it is not recommended for use as the root shiel- ding gas for ferritic-austenitic (duplex) steels. argon or pure nitrogen is better suited to that purpose. 1.2.6 nitrogen nitrogen (n 2 ) is used as a shielding gas component in the tig welding of stainless austenitic steels with nitrogen additions and superduplex steels. in these steels, nitrogen is used as an additive with up to 0.5% content to improve the strength of the steel and prevent spot corrosion. adding a few per cent of nitrogen in the shielding gas helps prevent the nitrogen loss otherwise occurring in the weld during welding. formier ® 10, 10% of hydrogen in nitrogen, is an often used reducing root shielding gas. it improves the root-side spot corrosion resistance on austenitic steels. the same effect can be achieved in the welding of superduplex steels by using pure nitrogen for root shielding. 1.2.7 nitrogen monoxide nitrogen monoxide (no) added to mison ® shielding gases reduces the amount of ozone generated during welding. this improves the welder’s work environment and reduces the irritation of mucous membranes caused by ozone. improvements in the work environment also improve the welder’s ability to concentrate, productivity and welding quality. the nitrogen monoxide in the mison ® shielding gases also have a sta- bilising effect on the arc in the mig welding and soldering of stainless steels and aluminium. the higher the helium or hydrogen content in the shielding gas, the higher the arc voltage. heat transfer to the weld increases, which can be utilised as better penetration and higher welding speed. the effect of hydrogen and helium added to argon on arc voltage argon + x% hydrogen argon + x% helium 0 20 40 60 80 100 24 20 16 12 8 x % h 2 , x % he arc voltage arc voltage x% h 2 , x% he
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