shielding-gas-surface-23.html
AGA CW Handbook A4_32130-UK
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24 5.3.1 mechanical characteristics with the condition that the
shielding
gas is suited to the welded steel and the filler material type used, it does not affect the mechanical characteristics of the weld. 5.3.2. corrosion resistance one of the basic issues with stainless steels is to understand the effect of the welding process on corrosion resistance. if the
carbon
dioxide
content
of the
shielding
gas in mag welding with solid and metal-cored wires exceeds 3%, the result may be the harmful
carbon
isation of the weld deposit.
carbon
reacts with the chromium in the steel, forming chromium carbides at grain boundaries. correspondingly, the chromium
content
in areas near the grain boundaries decreases and corrosion resistance is reduced (grain boundary corrosion). however, most stainless steel today have very low
carbon
content
or they are stabilised, so the above will not usually be a problem when stainless steels are welded. nitrogen is added as an alloy in some stainless steels to improve corro- sion resistance but also to increase their strength. examples of this are high-alloy austenitic and superduplex steels. nitrogen loss occurring during the welding of these steels can impair their corrosion properties. in mag welding, and to some extent, in tig welding with filler materi- al, this can be rather easily compensated using a filler material with a suitable composition. in tig welding without filler material, the nitrogen loss must be compensated by using a
shielding
gas containing nitrogen (mison ® n2). the nitrogen monoxide (no) added into mison ®
shielding
gases does not affect the corrosion resistance of stainless steels. 5.3.3 root protection in some applications, the root side of the weld must be protected. otherwise, an oxide layer is formed, containing chromium originating from the metal underneath the layer. the chromium
content
near the root surface is reduced, increasing the risk of corrosion. argon, nitrogen-hydrogen mixtures and argon-hydro- gen mixtures are used for root protection.
shielding
gas
carbon
dioxide
content
(% co2) weld
carbon
content
(% c) 0.040 0.030 0.020 0.010 0.000 0 2 4 6 8 10 mag spray arc welding, solid wire weld sheet filler material weld
carbon
content
(% c)
shielding
gas
carbon
dioxide
content
(% co 2 ) on the left, a root surface protected with the formier ® 10 root
shielding
gas. on the right, a root surface unprotected during welding. the effect of the
carbon
dioxide
content
in a
shielding
gas on the
carbon
isation of austenitic stainless steel. when the
carbon
dioxide
content
rises above three per cent, weld
carbon
content
of 0.03% is approached, above which the danger of grain boundary corrosion is considered to increase.
shielding
gas and quality
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