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AGA CW Handbook A4_32130-UK
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shielding
gas
and quality contents 5.1 general 5.1.1 weld
quality 5.2 unalloyed and low-alloy steels 5.2.1 mechanical characteristics 5.2.2 visual quality 5.3 stainless steels 5.3.1 mechanical characteristics 5.3.2 corrosion resistance 5.3.3 root protection 5.3.4 visual quality 5.4 aluminium and aluminium alloys 5.5. other metals 5.1 general 5.1.1 weld quality the quality of welding work is the sum of several different factors. if the welded structure is correctly designed, the manufacturing implementa- tion, including the welding process, gap preparation, welding para- meters, filler material and shielding gas, has an essential effect on the quality level reached. for example, mistakes in shielding gas selection may impair the achieved end result with regard to mechanical characte- ristics, corrosion resistance or weld appearance. changes in the mechanical characteristics of a weld may be caused by changes in the metal’s microstructure, poor fusion between the weld and the base material, or disadvantageous penetration profile causing lacks of fusion. corrosion resistance may be reduced, for example, as a result of micro- structure changes and surface oxidation. surface slag and spatter affect the appearance of the weld and often cause problems for the further processing of the workpiece. 5.2 unalloyed and low-alloy steels in the mag welding of unalloyed and low-alloy steels, argon-based shielding gases with 5–25% carbon dioxide or 5–10% oxygen are used. in tig welding, inert shielding gases are used. in both tig and mag welding, the welder’s work environment can be improved by using mison ® shielding gases, which contain a small amount of nitrogen monoxide (no) that reduces the generation of harmful ozone.nitrogen monoxide also stabilises the arc in the tig welding of these steels. 5.2.1 mechanical characteristics the shielding gas used affects the mechanical characteristics of the weld. the lower the carbon dioxide or oxygen content in the shielding gas, the purer (fewer oxide inclusions) the achieved welding material. the granularity of the microstructure also becomes finer, which is bene- ficial for the impact ductility. shielding gas and quality. the effect of the shielding gas on the weld deposit’s manganese and silicon content. a higher co 2 content in a shielding gas increases the burning loss of alloy materials, which reduces the yield strength and tensile strength of the weld. 0 20 40 60 80 100 1.80 1.60 1.40 1.20 1.00 1.00 0.80 0.60 0.40 x % h 2 , x % he arc voltage 0.40 0.60 0.80 1.00 si% mn% 1.00 1.20 1.40 1.60 1.80 the effect of hydrogen and helium added to argon on arc voltage filler material weld deposit si % mn % % co 2 in argon
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