FIELD OF THE INVENTION
The present invention generally relates to a process for ATIG (Activated TIG)
welding of boiler tubes of boiler grade material using a deep flux, the welded
boiler tubes for pressure acting as pressure parts of the boiler and allowing
effective heat transfer in the equipment. More particularly, the invention relates
to an improved ATIG welding process for joining high wall thickness tube to tube
joints.
BACKGROUND OF THE INVENTION
A manufacturing process of thermal power boilers involves thousands of tube to
tube welded joints with the thickness range from 4.0 mm to 12.0 mm and
diameter range from 40 mm to 80 mm. As the welded components act as
pressure parts, TIG welding process is considered as a more reliable and defect
free welding procedure.
For welding of Carbon Steel tube to tube joint of above said diameter and
thickness range with conventional TIG welding process with or without the
addition of filler wire, the penetration is limited up to 2.5 to 3 mm. Further, the
conventional TIG welding process entails low productivity.
The TIG process has the advantage of controlled heat input and clean weld due
to the absence of flux. The weld is free of spatter under the protection of a
shielding gas, which at the same time provides protection against harmful
atmosphere contamination of the weld. The process is distinguished by high
quality weld and a low frequency of defects. However, it has the limitation of low
deposition rate. This means that prior art welding is carried out in at least five
passes during the welding of the boiler tube walls, because, the quality of
requirement call for a substantial through welding between tube to tube joint.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a process for ATIG
welding of boiler tubes of high thickness using deep penetration flux which
eliminates the disadvantages of prior art.
Another object of the invention is to propose a process for ATIG welding of boiler
tubes of high thickness using deep penetration flux which achieves full
penetration between 4.0 to 12.00 mm or in single pass without addition of filler
wire.
A further object of the present invention is to propose a process for ATIG
welding of boiler tubes of high thickness using deep penetration flux which
improves the production of TIG welding process by reducing the number of
passes.
These and other objects and advantages of the invention will be apparent from
the ensuing description. At the outset of the description, which follows, it is to be
understood that the ensuing description only illustrates a particular form of this
invention. However, such a particular form is only an exemplary embodiment and
the teachings of the invention are not intended to be taken restrictively.
SUMMARY OF THE INVENTION
Accordingly, there is provided a process for ATIG welding of boiler tubes of high
thickness using deep penetration flux which eliminates the disadvantages of prior
art. The process allows the tube to be rotated and welded in a single pass by
ATIG welding using deep penetration flux.
According to the invention, a penetration enhancing flux used progressively
achieves higher penetration and improve the productivity compared to
conventional TIG welding process.
Advantageously, the present invention reduces the requirement of filler material
and eliminates the necessity of edge preparation to minimize the weld cycle
time, including the number of weld passes.
The penetration enhancing flux further ensures radiographic-quality welds with
higher productivity including reduction in welding costs without sacrificing
mechanical property and desired microstructure of the weld zone.
The advantages of the invention can be seen, inter alia, the carbon steel tube to
tube join can be welded in a single operation instead of five passes by
convention TIG. The carbon steel tube to tube joint can be either welded without
edge preparation or by using square edge preparation instead of either J-groove
or V-groove by conventional TIG. The inventive process is suitable for ATIG
welding of fixed position of torch and by rotating the tubes in horizontal axis.
The process parameters for achieving full penetration welds in a single pass for
the wall thickness range of 4 to 12 mm and diameter range of 40 to 80 mm, are
established.
The head rotation speed, delay time, pulse current and wire feed speed having
significant influence on the weld penetration including the other optimum
process parameters providing best results in terms of penetration, bead shape
and mechanical properties, have been established successfully.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 illustrates the application of a paste material with a brush on prepared
weld surface in accordance with the invention.
Figure 2 illustrates a joint ready with paste applied on it for subsequent TIG
welding in accordance with the invention.
Figure 3 illustrates a device used for holding and rotating the tubes in
accordance with the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
As shown in figure 1, two tubes (1, 2) with prepared square edge is provided
with a flux by a paint brush (3) over the weld surface. The paste of the flux (4) is
uniformly applied all around the center line with a width of 7 to 8 mm on either
side. A pair of holding chuck (5,6) used to clamp and rotate the job during
welding.
According to the prior art, J-grooved joint preparation is used in TIG-welding,
and it is filled in around four to five passes depending on the wall thickness,
whereas in ATIG welding process of the invention, the joint can be completed in
a single pass without any edge preparation.
The ATIG Flux are available in the form of powder mixed with acetone and form
a paste. According to the invention, this paste form of the flux is applied over the
weld surface prior to welding to enhance the penetration. Basically, ATIG flux is
a combination of certain oxides such as Silicon oxide, Sodium manganese silicon
oxide. Sodium Titanium Oxide, Aluminum oxide Potassium Chromium oxide and
Potassium sodium titanium oxide and also some halides as a minor element.
At first the tubes are fitted together without any root gap and tack welded, then
the ATIG flux is applied uniformly by using a paint brush on the area surface to
be jointed. Then the welding is started by rotating the tubes horizontally and
keeping the torch stationary. ATIG welding can be carried out without any edge
preparation.
Since the known ATIG welding requires a single pass to complete the joint, a
relatively high proportion of handling and welding time in the entire fabrication
process is needed.
This affects the productivity and production in the shop floor. The arc time per
weldments is high, besides the filler wire and power consumption are also low.
According to the invention, the parameters for welding are set in a system as
shown in figure 3, and the tube is rotated and welded in 1G position without
feeding a filler wire in the weld pool and the weld is completed in a single pass.
According to the invention, once the alignment is over, the joint is tack welded at
two locations in opposite direction. Then the tube (1) is rotated and the flux (4)
is applied manually using the brush (3) as shown in figures 1 and 2. The flux
deposition should be uniform around the circumference of the tube to get
uniform penetration. A non-uniformity in the flux thickness may alter the weld
quality.
Figure 3 shows a welding system comprising a welding power source, a TIG
torch fixed on the X-Y direction and a cross slider which allows fine movement
of the torch to set the weld axis.
Figure 3 further shows that the holding device comprises a head stock and tail
stock for holding and rotating the tube, wherein the TIG torch is held to allow
X-Y linear motion. The deep penetration flux when applied on the weld joint
surface of the base metal, the weld penetration significantly increases.
In this regard, the "A-T1G welding of the boiler tubes of boiler grade material
using a deep flux" overcomes the limitations of the conventional TIG welding
method.
WE CLAIM:
1. An improved process for ATIG welding of carbon steel grade high wall
thickness tube to tube to produce pressure part components in boiler
system, the process comprising the steps of :-
- tack welding of the tubes after fitting the tubes without any root gap,
- providing ATIG flux in paste form using a brush on the surface area to
be joined;
- holding the tubes in a welding system having a power source, head,
stock, tail, stock, and welding torch;
- welding parameters are established and stored in the system; and
- welding the tubes in a single pass in 1G position while rotating the
tubes without feeding any filler wire.
2. The process as claimed in claim 1, wherein the tube is rotated in
horizontal axis with the head stock and tail stock positioner.
3. The process as claimed in claim 1, wherein the TIG torch is held
stationary.
4. The process as claimed in claim 1, wherein the welding is carried out with
a wide range of welding speed corresponding to the wall thickness of the
tube.
5. The process as claimed in claim 1, wherein a square butt edge
preparation is made.

ABSTRACT

The invention relates to an improved process for ATIG welding of carbon steel
grade high wall thickness tube to tube to produce part components in boiler
system, the process comprising the steps of tack welding of the tubes after
fitting the tubes without any root gap, providing ATIG flux in paste form using a
brush on the surface area to be joined; holding the tubes in a welding system
having a power source, head stock, tail stock, and welding torch; welding
parameters are established and stored in the system; and welding the tubes in a
single pass in IG position while rotating the tubes without feeding any filler wire.