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AGA CW Handbook A4_32130-UK
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shielding
gas
and quality argon and formier ® 10 (n 2 + 10% h 2 ) are the most common root shiel- ding gases for austenitic stainless steels. due to the hydrogen added to the root shielding gas the gas is reducing, which reduces the oxidation of the root surface and improves its fusion. root shielding gases with hydrogen content are not recommended for the root protection of ferritic, martensitic or ferritic-austenitic (duplex, superduplex) steels. high-purity nitrogen can be used as root protection in the welding of duplex steels. nitrogen improves spot corrosion resistance by forming a thin austenitic layer on the root surface. mison
® shielding gases are not recommended to be used for root protection of stainless steels, as they tend to cause discolouration in the root surface. 5.3.4 visual quality the oxidation of the weld can be reduced in the tig welding of austeni- tic (not ferritic or martensitic) stainless steels by using a shielding gas containing hydrogen, such as mison ® h2, which contains 2% hydrogen. the result is not only reduced weld oxidation, but also higher penetrati- on and a more even fusion between the weld and the base material. 5.4 aluminium and aluminium alloys only inert gases are used in the gas arc welding of aluminium and aluminium alloys. mison ® ar is recommended due to its ozone-reducing and work environment improving effect. the nitrogen monoxide (no) added into mison ® shielding gases does not affect the mechanical characteristics or corrosion resistance of the weld. penetration can be improved by adding helium into the shielding gas (mison ® he30, varigon ® he50, varigon ® he70). helium enables better penetration and reduces the danger of lack of fusion. this is especially important when welding thick materials, redu- cing the need for preheating. the higher heat transfer can also be utilised by increasing welding speed. aluminium and its alloys react easily with hydrogen and moi- sture, forming pores. for this reason, it is essential that the purity of the shielding gas is retained all the way to the arc when welding alumi- nium. some necessary measures for guaranteeing the purity from the cylinder to the gun are presented in chapter 11. 5.5 other metals only inert shielding gases are used in the gas arc welding of copper and its alloys. when thick materials are welded, argon-helium mixtures pro- vide more heat and increased penetration. the need for preheating is also reduced (preheating is often required due to copper’s high thermal conductivity. the nitrogen monoxide (no) added into mison ® shielding gases (mison ® ar, mison ® he30) does not affect the mechanical characteri- stics or corrosion resistance of the weld. titanium and its alloys react easily with hydrogen, oxygen and nitrogen, which results in brittleness. high hydrogen concentrations also cause porosity. only inert gases should be used in the welding of these metals. the nitrogen monoxide (no) added into the mison ® ar shielding gas does not affect the mechanical characteristics or corrosion resistance of the weld. however, there may be some discolouration in the weld. because titanium and titanium alloys react easily with hydrogen, oxy- gen and nitrogen, it is important that the purity of the shielding gas is retained all the way to the arc. for demanding applications, we recommend using high-purity argon (over 99.996%), argon 4.6.
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