FIELD OF INVENTION :
The invention relates to a process for longitudinal welding of tubes to flat sheets
or fins, to produce gas tight tube walls for industrial and power station boiler
plant. More particularly, the invention relates to a method to produce a fillet joint
between a tube and sheet (fin) panel by adapting a longitudinal activated TIG
welding.
BACKGROUND OF THE INVENTION :
To produce industrial and power-station boiler plants, thousands of meters of
weld joints have to be made between the boiler wall tubes and flat steel fins
arranged in between. These so-called tube walls form a large part of industrial or
power station boilers. It is known that the conventional GMA (Gas Metal Arc)
Welding process in particular is used to weld such tube walls. The process is
distinguished by high deposition efficiency and a low frequency of defects.
However, so for the welding is being carried out normally in the so called gravity
position, i.e. in the horizontal plane. This means that hitherto welding always had
to be carried out in two passes during welding of the boiler tube walls, since the
quality requirements call for substantial amount of through welding between the
tube and flat fin. In this arrangement, the welding head is fixed and the parts to
be joined are moved. The joints are thus welded firstly on both sides from above
in a first pass. Then the parts to be welded to one another are turned and finish
welded at the remaining two sides in a second pass. But this procedure
necessitates a relatively high proportion of handling and welding time during the
entire fabrication process including non arc time resulting in lower productivity.

Therefore, a process to weld from one side of the assembly with full penetration
and elimate welding from other side could be a solution which is likely to reduce
material handling with lower distortion. Activated TTG welding is the process,
which is hitherto used for making butt weld joints in semi automatic or automatic
mode to achieve full penetration in a single pass upto 12 mm thickness.
However, this procedure has not used for fillet joints so far.
Hence, adaptation of ATIG welding for making a fillet joint by optimizing the
welding parameters shall establish a new and inventive process to make fillet
welds between tube to sheet panel.
U.S. Patent no.5,532,445 discloses a process which adapts combined oscillation
GMAW for welding on the underside with simultaneous SAW being carried out on
the top side, and thereby enabling welding of boiler tube walls in a single
operation, i.e. from above as well as simultaneously from below (overhead
position).
US Patent No.6852945 describes a method and apparatus for welding of boiler
tube panel by adapting laser beam combined with twin arc welding.
US Patent No.3999029 describes a method in which the metal tubing to be
welded is slightly deformed so that it assumes a slightly out of round or ovular
configuration by passing the tubing through at least a pair of ovaling rolls ahead
of the welding station. The portion of the fin or metal plate to be welded to the

tube ahead of the welding station but after the ovaling rolls is continuously fed
into a position laterally adjacent to the flattened side of the ovalled tube. The
ovalled tube and the metal fin or plate are advanced in this configuration at the
same rate of speed towards the welding station. Sliding high frequency electrical
power contacts are brought into contact with both the advancing ovalled tube
and the metal fin or plate slightly in advance of the welding point. Rounding rolls
or drawing dies are provided adjacent to the welding point to bring the ovalled
tube back into round. At the welding point, means are provided to hold the metal
fin or the plate so that the rerounding of the tubes brings the tubes and the fin
or plate into close physical contact at the weld point, thereby providing the
necessary welding force. By means of such a method and apparatus, the V-
shaped gap necessary for high frequency electrical welding is formed and the
weld is made without having to significantly deform the stiff metal strip, fin or
plate.
British patent GB1182927 describes a flanged steel tube adaptable as boiler
tubes is formed in two operations, the latter of which is by cold drawing, wherein
the cross-sectional reduction of the flange portions is greater than that of the
tube portion. A tube is passed three times between a plug and co-operating
roller dies, to form a flanged tube, which is given a further two passes between a
plug and co-operating roller dies, stepped to limit the metal flow. During the final
pass a plug is used having a diameter between 1-5 mm, which is greater than
that of the plug used in the preceding pass.
Japanese patent JP4158921 describes a process in which the tube being heated
partially at high frequency by a heating device, which is formed with a pre-stage
forming roll; a fin part is formed by projecting the part to be worked to the
outside of the tube, and a rough steel tube with fin is made. Next, the fin part is

heated again with a re-heating device, which is subjected to finish-forming for
the constant diameter with a sizing roll on a finishing device and the steel tube
with fin is formed.
GB1204035 describes a fusion welding process to make heat exchangers by SAW
process using twin oscillating wires or strip as filler material.
KOREAN patent KR840001978B discloses a high frequency current welder, in
which, the separated metal strips move continuously forward along their length
to the welding point at comparatively high speed.
US patent 6,364,374 discloses a method and a device for joining two or more
panels together. The method allows joining of panels of material such as glass
fiberboard, and are particularly useful for forming ducts or tubes. Fasteners
provided have two planar members, joined together, and the planar members
have protrusions that are integral to the planar members and are intended to at
least partially protrude the panels joined by the devices.
US patent 6,872,911 describes a method for welding a first component part to a
second component part, the energy to melt the melting regions being supplied
by a laser beam. In this context, the laser beam is directed through the first
component part, which is made of a nonabsorbing material, to the second
component part. The second component part is made of a material which
strongly absorbs the laser beam. At least one stop is provided outside the
melting regions.

US patent 7,028,882 discloses a process and an apparatus for boiler tube panel
welding and straightening. The apparatus for depositing an overlay weld on a
boiler tube panel comprising a plurality of tubes with adjacent tubes joined
together with membranes there between, and then straightening the panel in
response to bowing that occurs as a result of depositing the overlay weld. The
apparatus generally comprises an overlay welding apparatus that deposits an
overlay weld on a surface of a boiler tube panel, and a straightening apparatus
for straightening the panel following deposition of the overlay weld. The
straightening process entails placing the boiler tube panel between two
members, and the pressing a portion of the panel there between.
OBJECT OF THE INVENTION :
It is therefore an object of the invention to propose a method to produce a fillet
joint between a tube and sheet (fin) panel by adapting a longitudinal activated
TIG welding, which enables welding of the boiler tube to fin panel from one side
to have full penetration.
Another object of the invention is to propose a method to produce a fillet joint
between a tube and sheet (fin) panel by adapting a longitudinal activated TIG
welding, which adapts a pulsed technique during the penetration of the ATIG
procedure.
A still another object of the invention is to propose a method to produce a fillet
joint between a tube and sheet (fin) panel by adapting a longitudinal activated
TIG welding, which is many fold productive.

A further object of the invention is to propose a method to produce a fillet joint
between a tube and sheet (fin) panel by adapting a longitudinal activated TIG
welding, which is enabled to implement an welding throughout the thickness (i.e.
both the sides) without distortion and other quality problems.
SUMMARY OF THE INVENTION:
Accordingly, there is provided a method to produce a fillet joint between a tube
and fins by adapting a longitudinal activated TIG Welding procedure, the method
comprising the steps of providing an weldable tube disposed between two fins ,
the fins being tack welded to both sides of the tube ; holding the tack welded
assembly firmly on a holding device; providing a welding torch to carry out a
longitudinal activated TIG (ATTG) welding procedure by adapting a down hand
technique; supplying simultaneously a shielding gas during welding operation to
protect the weldment from environmental contamination; carrying out the
welding by moving the torch in an offset position in relation to the rigidly held
assembly on the holding device; and maintaining the travel speed of the torch in
a pulsed mode of ATIG welding between 50 - 150 m/minute, and achieving a
high penetration in a single/pass to produce a fillet joint. Thus, the present
invention enables the tube, and fin to be fed continuously and joining of the tube
to fin by Activated TIG welding process in the down hand position. The
advantages of the invention can be seen, inter alia, in the fact that the boiler
tube walls can be welded in a single operation by employing this process, i.e.
instead of fusion welding in top and bottom side in two step operation or by
slower single pass welding. According to the invention, a top side welding
between a tube and fins is achieved by Activated TIG welding process for making
a fillet joint, in which welding is carried out in pulsed mode at a speed of 50 to

100 mm/min with argon as shielding gas, and the speed at pulsed mode can be
enhanced between 100 mm to 150 m/min. The welding parameters are selected
so as to achieve the required penetration in a single pass. The position of the
torch are selected to enable the welding to be carried out in forehand technique.
The shielding gas can be selected to be either argon or helium gas. The
invention further allows adaptation of multiple torches for parallel welding of the
assembly. Consequently, an enormous time saving occurs, as the fabrication
process is shortened by about less than 50%. Further, the capital investment
required for the apparatus is minimum compared to other known apparatures.
In this case, either the job will be stationary or the torches move with little offset
to ensure proper penetration, or the torch will be stationary and the job can
move. This process is economical as the weld can be carried out at a speed of
350 mm/min. and the welding is completed in a single pass for the full thickness.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING :
Figure 1 schematically illustrates the inventive method in an arrangement of the
torch on the fins for welding and positioning of the tubes.
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.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION :
For the fabrication of gas-tight boiler-tube walls, a tube (1) has to be welded to
at least one flat fin (2) arranged in between. As shown in figure 1, the fin (2) is
tack welded to both sides of the tube (1) and kept stationary. The straightness of
the weld joint is ensured.
Welding is carried out with the Activated TIG process from the top side i.e. in
down hand position to make the joint. The process welding can be carried out
with multiple torches for welding on a series of tubes and fins assembly.
During the experimentation stage, a tube of dia. 51 x 3.6 mm thick and fins
made of 6 mm thick plates were tack welded. The tack welded tubes (1) were
held on a holding device. Welding was carried out successfully at a speed of 50
to 100 mm/min with argon as the shielding gas and further at a speed of 100
mm to 150 m/min. The welding parameters as adapted, are indicated below.
Semi automated process for the experiment has been used, in which, the torch
was caused to move and the job was kept stationary.
WE CLAIM :
1. A method to produce a fillet joint between a tube and fins by adapting a
longitudinal activated TIG Welding procedure, the method comprising the
steps of:
- providing an weldable tube (1) disposed between two fins (2), the fins
(2) being tack welded to both sides of the tube (1);
holding the tack welded assembly (1,2) firmly on a holding device;
- providing a welding torch to carry out a longitudinal activated TIG
(ATIG) welding procedure by adapting a down hand technique;
supplying simultaneously a shielding gas during welding operation to
protect the weldment from environmental contamination;
- carrying out the welding by moving the torch in an offset position in
relation to the rigidly held assembly on the holding device; and
- maintaining the travel speed of the torch in a pulsed mode of ATIG
welding between 50 - 150 m/minute, and achieving a high penetration
in a single/pass to produce a fillet joint.
2. The method as claimed in claim 1, wherein the shielding gas is selected
from a group consisting of 100% Argon and 100% Helium, and wherein
the gas flow rate is maintained between 16 to 35 LPM.
3. The method as claimed in claim 1 or 2, wherein the voltage and the high
pulse current of the welding machine are respectively selected between 1
to 30v, and 250 to 280 Amp.
4. The method as claimed in any of the preceding claims, wherein a plurality
of welding torches can be adapted for parallel welding of corresponding
numbers of tub-fin assembly.
5. A method to produce a fillet joint between a tube and fins by adapting a
longitudinal activated TIG Welding procedure, as substantially described
and illustrated herein with reference to the accompanying drawings.

The invention relates to an method to produce a fillet joint between a tube
and fins by adapting a longitudinal activated TIG Welding procedure, the
method comprising the steps of providing an weldable tube (1) disposed
between two fins (2), the fins (2) being tack welded to both sides of the tube
(1); holding the tack welded assembly (1,2) firmly on a holding device;
providing a welding torch to carry out a longitudinal activated TIG (ATIG)
welding procedure by adapting a down hand technique; supplying
simultaneously as shielding gas during welding operation to protect the
weldment from environmental contamination; carrying out the welding by
moving the torch in an offset position in respect of the rigidly held assembly
on the holding device; and maintaining the travel speed of the torch in a
pulsed mode of ATIG welding between 50 - 150 m/minute, and achieving a
high penetration in a single/pass to produce a fillet joint.