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AGA CW Handbook A4_32130-UK
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shielding
gas
and quality 5.2.2 visual quality spatter spatter welded onto the base material during welding must usually be removed before painting or other surface
treatment.in addition to welding parameters, shielding gas is another important factor in spatter formation. the lower the carbon dioxide content of the shielding gas, the less spatter is formed. the best result with regard to spatter formation is achieved with the mison ® 8 shielding gas (8% co 2 ). welding is relatively spatter-free also with the mison ® 18 shielding gas (18% co 2 ). surface oxides surface slag is formed of oxides, appearing as brown, glassy areas on the surface of the weld. the slag must be removed before painting or other surface treatment. the more oxidising the shielding gas is (the more it contains carbon dioxide or oxygen), the larger the amount of oxides generated. the least surface slag is formed with the mison ® 8 shielding gas. weld shape different shielding gases provide a different weld shape. the lower the carbon dioxide content of the shielding gas, the better the melt’s flow and spread. the weld bead becomes low with a good fusion with the base material. pure carbon dioxide gives a rather high weld bead with a more abrupt fusion with the base material. when stainless steels are welded, both the type of the welded steel and the filler material must be taken into consideration when choosing the shielding gas. 5.3 stainless steels stainless steels are divided into different types based on their micro- structure (depends on the steel alloys and their quantities). the types are ferritic, martensitic, austenitic, high-alloy austenitic and ferritic-austenitic (duplex and superduplex) stainless steels. when choosing the shielding gas, you must take the type of the welded stainless steel into consideration (see also chapter 7). in the tig welding of stainless steels, argon or argon mixtures with nitrogen or hydrogen are used. in the mag welding of stainless steels with solid and metal-cored wires, shielding gases containing 2–3% carbon dioxide or 1–2% oxygen are used. higher oxygen and carbon dioxide content causes excessive surface oxidation. high-alloy stainless steels are often mig-welded using an inert shielding gas to prevent excessive surface oxidation. when rutile cored wires are used, a more oxidising shielding gas is required. most cored wires designed for welding with shielding gas have been designed for shielding gases containing 15–25% or as much as 100% carbon dioxide. the resulting slag protects the molten metal, so the weld is not carboni- sed regardless of the high co 2 content of the shielding gas. the weld surface oxidation is also minor due to the slag protecting the surface. in both tig and mig/mag welding, the amount of ozone can be reduced by using mison ® shielding gases, which contain a small amount of nitrogen monoxide (no) and improve the welder’s work environment. nitrogen monoxide also stabilises the arc in tig and mig welding.
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