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BOC Smootharc TIG 200 DC Operating Manual
3.3
Alloyed steel
TIG welding may be used for welding alloy steels but because deposition
rates are low, it is usually only used for welding sheet and thin sections
for high quality applications, small components, and root passes of
multipass butt joints in plate and pipe.
Standard DC TIG equipment is normally suitable and DCEN polarity is
usually chosen to provide good workpiece heating. Tungsten electrodes
with additions of thorium oxide, cerium oxide, or lanthanum oxide are
used for welding steel and they give good arc stability.
Only inert or reducing gases should be used for TIG welding and pure
argon is normally recommended as the shielding gas for welding
alloy steel.
Filler rods are usually selected to match the chemical composition and
the mechanical properties of the parent plate. The weldability of the
steel may impose restrictions on the choice of filler rod.
Alloy steels with high carbon equivalents are hardenable, and fast
cooling will produce a hard HAZ and this is liable to result in hydrogen
cracking. This form of cracking can be prevented by use of preheat and
suitable welding procedures.
General welding parameters
Plate thickness (mm)
Tungsten electrode (mm)
Gas flow (l/min)
Current (A)
Consumable size (mm)
Polarity: DC –
1
1.5
2
1
1.6
1.6
3 – 4
3 – 4
4
30 – 60
70 – 100
90 – 110
1
1.6
1.6 – 2.0
3.4
Stainless steel
TIG is a high quality process ideally suited for welding of stainless steels,
particularly thin sheet up to about 5 mm thick where weld integrity
and good surface finish are critical. The process has a high degree of
controllability resulting in clean, smooth, high quality welds with good
penetration and strength with very low defect rates.
Standard TIG equipment is suitable and stainless steels are TIG welded
using DCEN polarity.
A thoriated tungsten electrode is normally used but health concerns
have promoted use of ceriated or lanthanated instead. The filler rod
used depends on the type of stainless being welded but, in general, is
matching for austenitic grades, enriched in nickel for duplex grades, and
may be matching or an austenitic type for ferritic and martensitic grades.
Shielding gas is conventionally pure argon, but other gases are available
to provide specific properties and these include argon-hydrogen, argon-
helium mixtures, argon-helium-hydrogen, and argon-nitrogen mixtures.
When welding pipes an inert gas purge is required inside the pipe
to prevent oxidation on the underside of the weld. Gas purging may
also be used to protect the root side of butt welds in plate or sheet
materials too.
3
5
1.6 – 2.4
2.4 – 3.2
4 – 5
4 – 6
120 – 150
190 – 250
2.0 – 3.2
3.2 – 4.0
6
8
12
3.2 – 4.0
4
4.8 – 6.4
5 – 6
5 – 6
5 – 7
220 – 340
300 – 360
350 – 450
4.0 – 6.0
4.0 – 6.0
4.0 – 6.0
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