Lincoln Electric Welder - Manual (Page 35)

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wire is kept centered over the
nonbeveled edge of the joint.
Parameters and procedures for
welding 200 and 300 series stainless
steels by the GMAW spray arc mode
are given in Figure 15. Figure 16
gives parameters and procedures for
welding the 200 and 300 series
stain less steels by the GMAW
shortcircuiting mode.
10.4
WELDING WITH THE
GAS TUNGSTEN
ARC PROCESS
All stainless steel alloys that are
considered weldable can be welded
readily with the gas tungsten arc
process (GTAW).
The preferred electrodes are
thoriated, ceriated, or lanthanated
tungsten as specified in AWS A5.12.
The advantage of these electrodes is
that they have a more stable arc and
can be used with higher currents
than pure tungsten electrodes.
The shielding gas is usually argon,
but helium or mixtures of argon and
helium are used on heavy sections.
The advantages of argon are that
flow rates can be lower, the arc is
more stable and the arc voltage is
somewhat less than with helium.
The lower voltage makes it possible
to weld thin sheet without burn
through.
Filler materials for use with the gas
tungsten arc process are in the form
of solid wire available in coils for
automatic welding or straight lengths
for manual welding. These are
specified in AWS A5.9 which also
applies to filler material for Gas Metal
Arc and Submerged Arc welding.
Consumable inserts, specified in
AWS A5.30, are useful for root
passes with gas tungsten arc.
The DC power source for gas
tungsten arc welding must be a
constant current type, and it is
recommended that a high frequency
voltage be superimposed on the
welding circuit. The high frequency
need be on only to start the arc. As
the electrode is brought close to the
work, the high frequency jumps the
gap from the tungsten to the work
and ignites the welding arc. Since
the tungsten electrode does not
actually touch the work, the
possibility of contaminating the stain -
less steel with tungsten is greatly
reduced. Straight polarity (DC-)
should be used – which produces a
deep, penetrating weld.
A “scratch” start may be used in lieu
of a high frequency start, although
there is some possibility of tungsten
pickup. The arc should not be struck
on a carbon block because of the
likelihood of carbon contamination.
Stainless steels are readily welded
with automatic GTAW. Arc voltage is
proportional to arc length – thus a
reliable signal can be generated to
operate automatic arc voltage control
equipment. Filler metal may be used,
or light gauge material may be joined
by simple fusion of the joint edges.
When “cold” filler metal is used, it is
always added to the front of the
puddle.
The so called “hot wire” method of
welding gives greatly increased
deposition rates and welding speeds.
The wire – which trails the torch, as
illustrated in Figure 17 –
– is resistance
heated by a separate AC power
supply. It is fed through a contact
tube and extends beyond the tube.
The extension is resistance heated so
that it approaches or reaches the
melting point before it contacts the
weld puddle. Thus, the tungsten
electrode furnishes the heat to melt
the base metal and the AC power
supply furnishes a large portion of the
energy needed to resistance melt the
filler wire. The hot wire method is, in
effect, an adaptation of the long
stickout principle used in submerged
arc and self-shielded flux cored arc
welding. The wire used for hot wire
TIG welding is usually 0.045 inch
diameter. Since the wire is melted. or
very nearly melted by its own power
source, the deposition rate can be
controlled almost independently of
the arc.
Using the GTA hot wire method,
deposition rates up to 18 lb/hr can
be achieved when welding at 400 to
500 amp DCEN (Table XVII). Still
greater deposition rates can be
obtained using an automatic oscil -
lated welding technique. Voltage
control is essential to achieve control
of the large puddle when welding at
high deposition rates. For this
reason, TIG hot wire welding requires
the use of voltage control equipment.
By using closely spaced multiple
tungsten electrodes, the welding
speed can also be increased sub -
stantially when GTA welding stainless
steel tubing or sheet. Multiple elec -
trodes practically eliminate the
problem of undercutting at high
speeds.
Procedures and parameters for GTA
welding of stainless steel in thick -
nesses from 1/16 inch to 1/2 inch
(1.6 to 12.7 mm) are given in Figure
18. These include butt, corner, tee
and lap type joints.
Distortion Control in Austenitic,
Precipitation Hardening, and
Duplex Ferritic–Austenitic
Stainless Steels
Austenitic Stainless steels have a
50% greater coefficient of expansion
and 30% lower heat conductivity
than mild steel. Duplex stainless
steels are only slightly better.
Allowance must be made for the
greater expansion and contraction
when designing austenitic stainless
steel structures. More care is
required to control the greater
distortion tendencies. Here are some
specific distortion control hints:
Rigid jigs and fixtures hold parts to
be welded in proper alignment.
Distortion is minimized by allowing
the weld to cool in the fixture.
Copper chill bars placed close to the
weld zone help remove heat and
prevent distortion caused by
expansion. Back-up chill bars under