3.0. FINER ELEMENTS OF PRACTICAL TRAINING.
3.1
ELEMENT
1 : PREPARATION AND SETTING READY GTAW
EQUIPMENTS & ACCESSORIES IN THE FOLLOWING ORDER.
a) Selection and setting of
proper diameter of Tungsten electrode and
gas cup size
b)
Grinding the tungsten electrodes with proper tip angle
c)
Electrode extension
d)
Selection of electrode polarity
e) Connection of water hoses with water line, gas hoses with gas line and
check for water/gas
leakages.
f) Setting Argon gas pressure 1.5 to 2.5 kg / Sq. cm and gas flow rate 6-10
LPM. Purge out
the Argon gas between flow meter and torch nozzle.
g) Ensure a proper connection of HF unit/Foot pedal switch/current
regulator (remote)
and ear thing with good clamph) Turn on the
main power first and then turn on the
power
source.
i) Setting of proper current
j) Wearing of all the needed safety equipments (VIZ: helmet with filter
glass,
gloves, apron,
safety
shoes, dark blue cotton clothing
etc)
3.2 ELEMENT 2 :
ARC STARTING AND BEAD ON PLATE
WELDING, MAINTAINING THE CORRECT TORCH ANGLE AND
ARC LENGTH, ARC STOPPING. (REF FIG 2.1 - 2.7 )
3.2.1: ARC STARTING:
Arc can be initiated in a “non-touch” ignition method with the
help of
high frequency (HF) unit by keeping the ceramic nozzle only touching job
(tungsten rod not touching the job) and lifting the arc after initiation. The
arc may also be initiated by “scratch” method in which the tungsten is
scratched along (touched to) the base material and lifted away quickly
without blunting the tungsten tip. (Similar to striking a match box stick)
3.2.2: BEAD ON PLATE WELDING:
With proper maintenance of torch angle, arc length is practiced on parallel
lines drawn on a plate. The sequences to be practiced are as below. After
arc initiation as above,keep the torch vertical, maintain proper arc
length(approximately 1 1/2 times the electrode dia),create
a molten
puddle at the plate end on the line drawn. Move the torch a little back,
slowly tilt the torch and position it at an angle of 60 0 - 75
0
to the work
surface, move it in the direction of welding with addition of filler rod held
at 15 0 - 20 0 angle to the work surface. Care is taken always to maintain
the required arc length and electrode not touching the puddle to avoid
tungsten inclusion in weld metal.
Filler rod addition to be done to the
front/leading edge of the weld puddle keeping it well with in the gas
shielding. Filler rod is added slowly by dipping in and out in a dab without
touching tungsten rod.
3.2.3 : TO STOP WELDING
To stop welding, filler metal is first withdrawn from the weld pool, (but
momentarily kept under the gas shield to avoid oxidation and
contamination of filler rod hot tip), then the torch is moved a little ahead to
melt the lastly added filler metal dab, move then the torch to the leading
edge of weld puddle, lift it just enough to extinguish the arc. This is done
to protect the weld crater and filler rod from contamination.
3.3. ELEMENT 3 : RESTARTING AND MERGING TECHNIQUE
Following the techniques practiced in element 2, beads are laid on the
parallel lines on the plate,. At 50mm intervals between “ start - stops ”of
each
bead. (Ref fig . 3.1, 3.2. ) The restart and merging techniques are
practiced as follows.
3.3.1 RESTART : Arc restarting is done on the existing bead at a point 2
to 3 mm ahead of the spot where the filler rod was withdrawn earlier.
Again filler rod addition to start just after crossing the earlier spot of filler
rod withdrawal as above and bead laying done as per element-2.
3.3.2. MERGING ON THE STARTING EDGE OF EARLIER BEAD:
During merging, the filler rod addition should continue and stopped till the
spot is reached where the filler rod addition started on the earlier bead.
3.4 ELEMENT 4 : ROOT WELDING OF PLATE BUTT JOINTS IN
DOWN HAND, HORIZONTAL, VERTICAL AND OVERHEAD
POSITIONS. (Ref fig 4.1 to 4.4)
3.4.1: During butt-welding of plate in any position, the tungsten electrode
should be pointed towards both the edges of plate at root and not one edge
alone. The torch
oscillation is done slightly sideways to melt the two
edges of plate at the root with addition of filler metal. This oscillation is
required to be followed both for “key hole” and “walk on groove”
technique.
3.4.2 : For narrow root gap “walk-on-groove” technique adopted and for
wider / larger root gap the “key-hole” technique adopted.
3.4.3 :
During horizontal, vertical and overhead position welding larger
size weld pool should not be created. This is achieved by proper control of
welding parameters and filler addition in small volume.
3.4.4 : In horizontal welding , the filler rod is fed at the upper edge of the
weld puddle and the electrode is pointed slightly upward for melting and
immediately slightly downward to allow the molten metal to solidify and
prevent sagging of weld pool. The regular rhythm of dipping the filler rod
in and out of the pool without touching tungsten should continue.
3.4.5:
In vertical welding the filler is added at the leading edge of pool
for “ upward ” progression and at the bottom edge of pool for “down”
progression.
3.4.6 : In overhead welding the filler rod is dipped in out frequently
to
add filler metal in small amounts. Simultaneously the electrode is slightly
moved up and down to allow the melting and solidify the
progressively to prevent sagging.
molten pool
3.5 ELEMENT 5 : WELDING ROOT OF PIPE OR TUBE IN 2G/5G
POSITION. (fig 5.1 to 5.6 )
3.5.1 :
Select a pipe/tube of 400mm length with V grooves of 3mm
depth, spaced at 25mm interval machined on lathe on the
surface of
pipe/tube. ( fig 5.1 )
3.5.2 : The grooved pipe/tube is fixed in 5G position
3.5.3 : In the groove the arc is started at 6 0 clock position, fill the groove
with filler rod, in antilock wise direction, stopping the weld at 12
0
clock
position, holding the torch in right hand. Similarly holding the torch in left
hand, filling of groove is done with filler addition in the clockwise
direction with proper restart and merging technique explained in element3. (fig 5.2)
3.5.4 :
In both clockwise and anti clockwise direction the vertical up
progression is followed.
3.5.5 : After the above training, the pipe/tube butt welds are practiced in
5G and 2G position to achieve a good controlled root penetration
throughout. ( 5.3 to 5.5)
3.5.6
:
The fit up for butt welds should be done carefully maintaining
uniform root gap for 2G position, and slightly wider gap (1-1.5mm more)
at top than bottom for 5 G position. This is done to avoid the tendency of
closing of root gap at the top due to heat of welding as the welding
progress from bottom to top.
3.5.7 :
The techniques for stopping and restarting will be practiced
as explained in fig 5.6.a and 5.6.b for best results
3.5.8 : All the techniques explained in elements 2,3 & 4 must be
followed meticulously to have satisfactory founds.