FIELD OF THE INVENTION
The invention relates to a process for longitudinal welding of tubes to flat sheets
or fins which serve the purpose of gas tight tube walls for industrial and power
station boiler plant More particularly, the invention relates to a process for high
frequency longitudinal welding of tube to sheet (fin) panels and subsequently
welding flat fin to fin by either laser hybrid welding process or MIG welding.
BACKGROUND OF THE INVENTION
To produce industrial and power-station boiler plants, thousands of meters of
welded joints have to be produced between the boiler wall tubes and the flat
steel fins arranged in between These so-called tube walls form a large part of
Industrial or power-station barters, It is known that the submerged-are welding
(SAW) process in particular is used to weld such tube walls. The SAW process
has the advantage that the arc burns in a stabilized manner and the arc is free of
spatter under a powder layer, which at the same time assumes the function of
providing protection against harmful atmosphere and deoxidation including
forming of a slag for the satisfactory metallurgical course of the process, The
process is distinguished by high deposition efficiency and a low frequency of
defects. However, it has the disadvantage that welding can normally be earned
out only in the so-called gravity position, i.e. in the honzontal plane. This means
that hitherto welding always had to be carried out In two passes during the
welding of the boiler tube walls, because the quality requirements call for
substantiaf through-welding between the tube and the flat fin. In this
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arrangement, the welding heart is fixed and the parts to be joined are moved.
The joints are this welded first of all on both sides from above in a first pass,
Then the weldable members are turned and finish-welded at the remaining two
sides in a second pass, But this process necessitates a relatively high proportion
of handling and welding time in the entire fabrication process Since the
possibilities of increasing the welding speed have in the meantime been
exhausted, only the use of another welding process is suitable for accelerating
the entire process.
U.S. Patent No. 5,532,445 discloses a process in which combined oscillation
GMAW is simultaneously carried out on the underside anile SAW is being carried
out in known manner on the top side and there by boiler tube waits can be
welded in a single operation, i.e. from above and simultaneously from below
(overhead position),
US patent No.6852945 discloses a process for welding of boiler tube panel by
making use of laser combined with twin are welding and further discloses and
apparatus and process of manufacturing of boiler tube to fin welding.
U.K. patent No, GB11S2927 describes a process, which produces flanged
steel pipe for use as boiler tubes being formed in two operations The latter
operation adapts cold drawing process, which, emphasizes cross-sectional
reduction of the flange portions
US patent no,3999029 discloses a process 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 ovaling rolls ahead of the welding
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station. Also, ahead of the welding station but after the ovaling rolls, the fin or
metal plate to be welded to the tube is continuously fed into a position laterally
adjacent to the flattened side of the ovaled tube. The ovatad tube and the metal
fin or plate are advanced in this configuration at the same rate of speed towards
the welding station. High frequency sliding electrical power contacts are brought
into contact with both the advancing ovaled 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 ovaled tube back into round.
Also at the welding point, means are provided to hold the metal fin or the plate
so that the refunding 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
GBH82927 relates to a flanged steel tube for use as boiler tubes being formed
in two operatvons, the latter of which is by cold drewing, wherin 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 1-5 mm, greater than that of the plug used in the
preceding pass.
Japanese JP4158921 relates to a method in which the tube being heated
partially with the high frequency heating of a heating device. It is formed with
the pre-stage forming roll, a fin part is formed by projecting the part to be
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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 reheating device, it ts subjected finesh
forming for the constant diameter with a sizing roll on a finishing device and the
steel tube with fin is formed.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process for welding of
boiler tube panel in which the bolters fuse walls are welded in a single operation.
Another Object of the invention is to propose a process for welding of boiler tube
panels, which is highly productive
A further object to the invention is to propose a process for welding of bailer
tube panels which is semi-automatic and carried-out throughout the thickness
(i.e both the sides) without the problem of distortion and other quality
problems,
A still further object of the invention is to propose a process for welding of boiler
tube to fin which employs high-frequency welding.
An yet another object of the invention is to propose a process for welding of
boiler tube to fin which eliminates the disadvantages of the prior art, in particular
the problem of employing multi-step welding procedures.
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A still another object of the invention is to propose a process for welding of
boiler tube to fin in which boiler tubes and the fins can be continuously fed for
joining in a single step
Another further object of the invention is to propose a process for welding of
boiler tube to fin which is economical and consumes less time
SUMMARY OF THE INVENTION
Accordingly, there is provided a process for welding of boiler tube panel which
enables the tube and fin to be fed continuously and joining of tube to fin iS
achieved by high frequency welding The advantages of the invention can be
seen, inter alia, in the fact that the boiler tube walls can the welded in a single
operation by employing HF welding, i.e. instead of fusion welding in top end
bottom side in two step operation or by slower single pass welding by laser
hybrid welding Consequently, an enormous time saving occurs, as the
fabrication process is shortened by about less than 80%, Further, the capital
investment required for the apparatus of HF welding plant is minimum compared
to laser hybrid welding and other fusion welding. The existing underutilized spiral
fin welding plant can also be modified with minimum investment on mechanical
feeding of the fin alone. The process runs largely automatically and is therefore
especially economical ft is advantageous if the magnitude of the feed rate is
selected as a function of the tube wall thickness and it is tn the range of 6m/mn
to 10m/min, preferably 8m/min Further, fin to fin butt can be carried out with
either conventional MIG welding or laser hybrid welding
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This of welding system uses a continuous HF electric resistance welding process
At high frequency, the current characteristically flows in the surface of the
material to a depth of approximately 0.8 mm in case of carbon steels and low
alloy steels at their welding temperature. This enables a very high power density
concentration at the welding surfaces (tube to fin) and enables the process to
weld at a high speeds (about 8m/mm.) with low heat input The joining is also
enhanced by the forging action which occurs as the heated surface are brought
together Consequently the resulting joint is characterized by a distinct lack of
the grain coarsening which FS frequently seen in fusion type welds
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - Shows an embodiment of the apparatus according to the Invention.
Figure 2 - Shows a schematic diagram of the tube to fin welding process.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Referring now to the drawings. Wherein like reference numerals designate
identical or corresponding parts throughout the several views. The welded joint
shown in FIG 1 can be made with the feast possible outlay by the process
according to the invention For the fabrication of gas-tight boiler-tube walls, the
tubes (l) have to be welded to flat steel fins (2) arranged in between.
As shown in fig-1, to produce boiler membrane panel of tube to fin, one tube (1)
and one fin (2) are fed from the modified spiral fin welding machine. The tube
(1) is fed in the usual way through a plurality set of rollers (8) and the fin (2) is
fed from the conventional uncoiling equipments
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As shown in fig-2, a first copper contact (3) is placed over the tube through
pneumatic pressure and a second copper contact (4) is placed over the fin (2)
The HF current source is connected between the contacts (3,4) to enable Seating
between the fin and the tube, the pressure between the fin and the tube being
maintained continuously by a roller guide (7) having slot for locating the fin (2)
By continuously feeding the tube (l) and the fin (2), the longitudinal fin (2) to
the tube (I) welding is achieved Further, the fin to fin butt welding shall be
carried out by laser hybrid welding.
As shown jn figure l, the fin (2) is fed via the first set of rollers (8) and the tube
(1) iS red through the second set of rollers (8) and allowing the second set of
rollers (8) to come in contact. The tube (1) is fed by the first set of rollers (8) as
shown in fig 1 and the fin (2) is fed by another set of rollers parallel to the tube
(1), while not touching the tube upon the point of welding zone from the
conventional uncoiling equipments (not shown). A welding station is provided at
which the fins (2) are welded to the tube (1) at welding point. The prior art
techniques of high frequency electrical resistance heating are utilized to provide
the heat necessary to cause the weld at welding point, Sliding contacts or
electrodes are slidingly contacted with the advancing tube (1) and the fins (2) in
advance, of the welding point so that the high frequency oscillating current is
passed along the surface of the materials and through the welding point. The
first and second copper contacts (3,4), including a third and fourth copper
contacts (4,6) are connected to a high frequency oscillating current source (not
shown)
For carrying out the welding and for the proper control of the flow of the high
frequency current in advance of the weld point, it is necessary to bring the tube
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(1) and the fin (2) together at a slight angle known as V angle In order to
assure that a good weld is formed at welding point, the fins (2) are held end
guided by rollers (7,8) providing a firm contact with the tube (1) at the weld
point. As best seen in FIG 2, fins (2) are moved into a line parallel to the axis of
the tube (1) by guiding the rollers (8) A suitable forging pressure is applied at
the point of welding through guide rolls. The third and the fourth copper
contacts (5,6) are provided to facilitate preheating of the fin (2) and to ensure
better deformation and formation of V angle prior to welding point.
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WE CLAIM
1. A process for high-frequency longitudinal welding of a. tube to a flat steel fin
comprising,
- feeding of atleast one tubes through a set of rollers; feeding a steel flat fin in
the longitudinal direction of the tube; pressing the tube and the fiat steel fin
to be joined against one another; and welding between the tube and the fin
by HF welding procedure.
2. The process as claimed in claim 1, wherein the welding is earned out at a
feed rate in the range of 6 to 10 m/min.
3. The process as claimed in damn 2, wherein the magnitude of the feed rate is
selected as a function of a wall thickness of the tube.
4 The process as claimed in claim 1, wherein the welding parameters are
selected in such a way that a non-through welded thickness of the flat steel
fin is al most 0.1 times of the thickness of the fiat steel bar, wherein a sum of
weld thicknesses on a top side and an underside of the joints is at least 0 90
times of the thickness of the flat steel bar, and wherein a thickness of molten
portion of the tube material is rstricted to 1 mm.
5. A process for high-frequency longitudinal welding of a tube to a flat steel fin
comprising
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- positioning of a plurality of HF capper contacts over a tube and a fin;
- connecting atleast two of said plurality of contacts to atleast one HF electric
oscillator;
- pressing the tube and the flat steel fin against one another,
- passing the tube and the flat steel bar towards the HF copper contacts;
welding the tube and the flat steel bar together by HF welding throughout the
thickness, and moving the tube and the flat steel bar in a longitudinal
direction during welding by a transport device.
6. The process as claimed in claim 5, where in the welding and bending of the
fin is achieved simultaneously,
7. A process for high-frequency longitudinal welding of a tube to a flat steel fin
as substantially described herein with reference to the accompanying
drawings
Dated this 16th day of FEBRUARY 2006

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