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10.3 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) 1 1.5 2 3 5 6 8 12
Tungsten electrode (mm)
1.0 1.6 1.6 1.6 – 2.4 2.4 – 3.2 3.2 – 4.0 4.0 4.8 – 6.4
Gas flow (l/min)
3 – 4 3 – 4 4 4 – 5 4 – 6 5 – 6 5 – 6 5 – 7
Current (A)
30
– 60
70
– 100
90
– 110
120
– 150
190
– 250
220
– 340
300
– 360
350
– 450
Consumable size (mm)
1.0 1.6 1.6 – 2.0 2.0 – 3.2 3.2 – 4.0 4.0 – 6.0 4.0 – 6.0 4.0 – 6.0
Polarity: DC –
10.4 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.