A METHOD WITH WELD SEQUENCING TECHNIQUE FOR CONTROL OF LONGITUDINAL AND SIDE BOW DISTORTIONS OF TUBULAR PANELS
FIELD OF THE INVENTION
The present invention discloses a method and a technique involving weld sequencing for enabling effective control of distortion in fabrication of flat fin welded tubular panels, used in the boiler industry.
BACKGROUND OF THE INVENTION
Distortion is a major bottleneck in achieving enhanced levels of productivity in components fabricated by welding. Owing to the localized application of heat during welding, the zones of material which melted and fused by the heat of welding tries to shrink, but the adjoining zones partially prevent this shrinkage and partially undergo plastic deformation by way of yielding. The contraction of the weld during the cooling phase leads to the formation of a shrinkage force in the weld. This shrinkage force and the plastically deformed zones together constitute a shape change of the component alongside the shrinkage in the length of the weld. This change of shape of welded component together with contraction of weld length is referred to as weld distortion.
The presence of distortion leads to many unwelcome effects like loss of aesthetics, problem of pairing in an assembly of components, reduced load carrying capacity etc. Any amount of welding whether high or low will always lead to some residual distortion which cannot be eliminated completely but can be brought down significantly and can be kept within permissible limits.
Prevention is better than cure and hence control is better than correction of distortion as the latter process results in an inordinate delay in completion of fabrication leading to increase of ‘takt’ time. Control of distortion can be effected by following certain techniques like weld sequencing, disposing components

back-to-back and simultaneous welding, introduction of high restraints and clamps etc. However, it is to be noted that there is no one universal solution for control of distortion that is applicable to all welded components. The most suitable control method for a given case has to be tailor-made and ad-hoc in nature so that the distortion is kept within the stipulated limits.
US Patent Document US 7028882 describes a method for depositing an overlay weld on a boiler tube panel comprising a plurality of tubes with adjacent tubes joined together and then straightening the bow that occurs due to overlaying. However, there are techniques revealed for welding the tubular panel with control of distortion. But, the method disclosed herein, in this invention relates to a method of controlling both longitudinal and side bow distortion by employing a suitable weld sequencing technique in fabricating a flat fin welded tubular panel which is not a correction method.
US patent 7703660 B2 refers to a method of determination by numerical modeling and application of an optimum welding sequence that reduces the welding induced distortion and residual stress formation while depositing hard-facing layers on a boiler water wall panel. Our invention is different in the sense that it is concerned with a method of application of weld sequencing aimed at controlling bowing distortion while welding finned tubular panels and there is no hard-facing operation involved.
US patent US 6023044 discusses a control method in a multi-layer welding where the weld line and a gap width of work pieces to be welded are detected by a laser sensor mounted on a robot, during a welding for a first layer and welding conditions are adjusted in accordance with the detected gap width for a second

and subsequent layer and performed by using the stored data in such a manner that the welding torch is made to follow the weld line, and the welding conditions are adjusted in accordance with the gap width. But, in our invention, a method of employing a technique with specialized weld sequence scheme, without any need of sensors or robot, for control of welding induced bowing distortion encountered while welding finned tubular panels has been described.
US patent 5591363 describes a process of depositing layers of weld metal onto a ferrous NiMoV low alloy steel turbine component, where during the deposition of a first layer of weld metal, low levels of amperage are used to prevent a dramatic increase in a level of hardness of the HAZ and during the deposition of a second layer of weld metal, higher levels of amperage are used to temper the heat affected zone. But, our invention relates to a method of welding finned tubular panels used in the thermal power boilers by employing a technique consisting of an ad-hoc weld sequencing scheme aimed to controlling the welding induced bowing distortion happening in such panels.
Indian patent application number 818/KOL/2015 describes a method of carrying out attachment welds in thin panel type of structures but does not cover the method of welding the panel starting from tubes and fins. The scope of Indian patent application number 818/KOL/2015 is restricted only to the method of welding of the attachments to the panel in a specified manner so as to control the welding induced distortion and there are no disclosures regarding how the tubular panel is to be welded. But, our invention provides a method incorporating a special welding sequence to control the different types of distortions that are seen in fabricating tubular panel type of structures and welding of attachments is not the subject matter of our invention.

Indian patent application number 201631017624reveals a method of controlling distortion while welding of burner panels of boilers. This is pertaining to carrying out welding in sequential manner and in multi-planes by simultaneously deploying a plurality of welders. But, our invention talks about controlling distortion while welding simple tubular panels. Neither multi-plane welding nor simultaneous deployment of a multiple welders are being talked about in our invention.
Indian patent application number 201731026115 discloses a special welding sequence that can effectively mitigate the distortion produced, on account of fabricating a triangular shaped panel by way of welding. The triangular shape of the panel requires a special welding sequence that would control the bending of panel along a line connecting any of the two vertices among the available three vertices of the triangular shaped panel. But, our invention is different from this Indian patent application because in this present invention, we have defined a special welding sequence for controlling various types of distortions that could occur in fabricating tubular panels that are not triangularly shaped. Furthermore, the sequence of welding described in our invention is totally different from that detailed in the above mentioned Indian patent application which is exclusively devised for welding triangular shaped panels and not for other shapes or forms of panels.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to provide a technique involving a weld sequencing scheme for the control of distortion in flat fin welded tubular panels. Another object of the invention is to provide simultaneous control on the longitudinal bowing and side bowing types of distortion experienced by such flat fin welded tubular panels, caused because of welding.

SUMMARY OF THE INVENTION
According to the invention, a distortion control method incorporating a weld sequencing technique for controlling both the longitudinal and side bow types of distortion of flat fin welded tubular panel is disclosed. The tubular panel consists of alternately placed tubes and fins for a designed length. Fillet welds are placed between each tube and an adjoining fin on both the top side and hind side of the fin. These panels are typically fabricated as long welded structures. Owing to the heavy deposition of welds on both the top side and bottom side of the tube fin assembly, huge warping or distortion of the panel is likely to be noticed after welding, if proper weld sequence is not followed.
This invention discloses a method in order to avoid the welding distortion experienced by such flat fin welded tubular panels. In the first stage, fit-up of the panel is made by placing the tubes and fins alternately to each other in a scallop bar arrangement. Tack welds are firstly placed between each tube and the adjoining fin in the center line of the length of the panel. The next set of tack welds are placed at a line that is parallel to the center line of the length of the panel and at a distance of 175 mm from the latter, moving towards longitudinal extremity of the panel. The next set of tack welds are placed at another line parallel to center line of the length of the panel and located at a distance of 175 mm from the latter, moving towards the other longitudinal extremity of the panel. Similarly, series of tack welds are placed at equal distances form the center line of the length of the panel moving alternately towards both the longitudinal extremities of the panel.
After tack weld, in the second stage, the panel is subjected to full welding of all the fillet welds in a sequence as described here. In one side of the panel, (25 – 30) % of the overall length of the fillet welds is placed starting from the

centre most tube and alternately moving towards both the lateral extremities of the panel. This 30% weld length is placed in such a way that the center line of the length of this weld segment coincides with the center line of the length of the panel. In the third stage, the panel is turned upside down and the fillet welds are welded for the full length starting from the centermost tube of the panel and proceeding towards both the lateral extremities of the panel. In the fourth and final stage, the panel is again turned upside down to bring it to the same position as it was in stage two. The leftover welds connecting each tube to fin are welded starting from the centermost tube of the panel and proceeding towards both the lateral extremities of the panel. This completes all the welds in the panel.
This procedure can effectively control welding induced distortions in finned tubular panels.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The proposed invention will be better understood by the following description with reference to the accompanying drawings:
Figure 1 shows the general arrangement and constructional details of flat fin welded tubular panel (illustrative only)
Figure 2 shows the illustrative sketch of the longitudinal bowing distortion of flat fin welded tubular panel
Figure 3 shows the illustrative sketch of the side bow type of distortion of flat fin welded tubular panel
Figure 4 shows the illustrative sketch of the fit-up of panels using scallop bar arrangement to be followed in stage 1 of panel fabrication (panel in top side)

Figure 5 shows schematic sketch of tack welding sequence to be followed in stage 1 of panel fabrication (panel in top side)
Figure 6 shows the schematic sketch of weld sequence and direction of welding to be followed in stage 2 of panel fabrication (panel in top side)
Figure 7 shows the schematic sketch of weld sequence and direction of welding to be followed in stage 3 of panel fabrication (panel in hind side)
Figure 8 shows the schematic sketch of weld sequence and direction of welding to be followed in stage 4 of panel fabrication (panel in top side)
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
This invention describes a method of controlling welding induced distortions in fabrication of flat fin welded tubular panels, the lifeline of the boiler industry. Figure 1 shows the typical construction of a flat fin welded tubular panel. It consists of alternately placed tubes (1) and fins (2) so as to form a membrane panel wall that will be used for heating fluids in thermal power plants. Each tube is connected to the adjacent fins by way of fillet welds (3). These fillet welds are present both in the top side as well as in the bottom side of the panel in order to make a pressure tight membrane water wall panel. For a panel consisting of ‘n’ tubes with fins placed in between each pair of tubes and with edge bars on both the extreme tubes, there will be a set consisting of 2n number of fillet welds in the top of the panel and an equal number of fillet welds in the hind side as well, leading to a total of 4n numbers of fillet welds, each of which runs all along the length of the panel. All these fillet welds shall have to be completed using a carefully planned weld sequence. If these fillet welds are welded in a haphazard

manner, then it may result in the formation of a longitudinal bow of the panel as depicted in Figure 2 or it may be result in the formation of a side bow of the panel as depicted in Figure 3 or it may also result in the simultaneous formation of both these types of distortion. The method of controlling the occurrence of these two types of distortion is detailed in this invention.
The fabrication of the panel is carried out in multiple stages as described hereinafter. All the welds that are referred in the below mentioned fabrication procedure are fillet welds connecting tubes and fins. The size of the fillet welds depends on the design of the panel which varies from one case to another.
Stage 1
a) The required number of tubes and fins are placed in a scallop bar (4) as seen in figure 4. The scallop bar (4) consist of a suitable profile that will enable proper seating of the tubes and fins in the required numbers and with the required pitch spacing. This position of the panel shall hereinafter be referred to as the top side of the panel.
b) After placement of the required numbers of tubes and fins in the scallop bar (4), tack welds are to be placed at various locations spread across the length and width of the panel. The length of each tack weld shall be in the range of 30 – 60 mm.
c) The sequence of placement of tack welds are shown illustratively in Figure 5. The first set of tack welds are placed along the line ‘A’ that coincides with the centre of length of the panel. The tack welding sequence shall be 1-2-3-4-5-6-7-8-9-10 etc. along the line ‘A’. In other words, along the line ‘A’, the centre most tube is first tack welded and then subsequent tack welds are placed alternately with respect to the centre most tube and in the increasing distance of separation from the centre most tube and proceeding towards both the lateral extremities of the panel.

d) The next set of tack welds shall be placed in the sequence A-B-C-D-E etc. (Figure 5). In other words, after completing the first set of tack welds along the centre line (line ‘A’) of the length of the panel, the next set of tack welds are placed at a distance of at least 175 mm away and to the top of the line ‘A’. The next set of tack welds are placed along the line ‘B’ which is also placed at a distance of at least 175 mm away and to the bottom of the line ‘A’. In this manner, subsequent sets of tack welds are placed alternately to the top and bottom of the centre line ‘A’ of the panel following the sequence A-B-C-D-E etc. In each of these sets of tack welds, the welding sequence shall be followed as explained in point © above
e) Following the methodology prescribed in points © and (d) above, the whole of the panel is tack welded and taken to next stage of fabrication as described below
Stage 2
a) In the centremost tube of the panel, the first weld is laid for a length of about 25% to 30% of the length of the weld required to be made between one tube and an adjacent fin (figure 6). This weld segment shall be deposited in such a way that the centreline of this weld segment shall coincide with that of the centreline of the length of the panel (figure 6).
b) Following this weld, the subsequent welds are made as per the sequence 1-2-3-4-5-6-7-8-9-10 etc. as shown in Figure 6. In other words, after depositing the first weld segment as detailed in point (a) above, the next weld to the immediate right of the already laid first weld is to be completed.
c) This is followed by the completion of the weld segment that is present to the immediate left of the firstly laid weld segment. Likewise, weld segments are completed by placing them alternately to the right and left of the centremost tube and in the increasing order of distance of separation of the weld segments from the centremost tube in the sequence 1-2-3-4-5-6-7-8-9-10 etc.

Each of these weld segments are of length 25% to 30% of the overall length of the weld required to be made between the corresponding tubes and fins.
Stage 3
a) After completion of stage 2 of fabrication of panel, it is turned upside down. This position of the panel shall hereinafter be referred as hind side of the panel.
b) Welding is commenced in the centremost tube. Starting from the centreline of the length of the panel, the welding is completed by proceeding towards one longitudinal end (top end) of the panel. This is shown as weld segment ‘1a’ in figure 7.
c) Following this, the welding is again commenced in the mid-length of the centremost tube and proceeded towards the other longitudinal extremity (bottom end) of the panel. This is shown as weld segment ‘1b’ in figure 7. This completes the weld between the centremost tube and one of the adjacent fin in the hind side of the panel. It is to be noted that the top side of the panel refers to the position in which it existed in stage 1 & stage 2.
d) Next, the subsequent fillet welds are chosen based on the sequence 1a-1b-2a-2b-3a-3b-4a-4b-5a-5b-6a-6b-7a-7b-8a-8b-9a-9b-10a-10b etc. as shown schematically in Figure 7. In other words, the subsequent fillet welds between the remaining tubes and fins are completed by placing these welds alternately to the right and left of the centremost tube and in the increasing order of distance of separation of the welds from the centremost tube in the sequence 1a-1b-2a-2b-3a-3b-4a-4b-5a-5b-6a-6b-7a-7b-8a-8b-9a-9b-10a-10b etc. as depicted in figure 7.

Stage 4
a) After completion of all the fillet welds in the hind side of the panel in stage 3, the panel is again inverted to bring the top side of the panel facing upwards.
b) Already, a portion of each of the fillet welds between the various tubes and fins shall have been completed in stage 2. The remaining unwelded portions shall be welded following the sequence 1a-1b-2a-2b-3a-3b-4a-4b-5a-5b-6a-6b-7a-7b-8a-8b-9a-9b-10a-10b etc. as shown in figure 8. In other words, the segment of weld (‘1a’ in figure 8) to the top of the already laid weld (laid in stage 1) in the centremost tube is first completed followed by the segment of weld (‘1b’ in figure 8) that lies to the bottom of the already laid weld (laid in stage 1) which is completed next.
c) Subsequent to the completion of the welds ‘1a’ and ‘1b’ (figure 8) in the centremost tube of the panel, the next tube that is present to the immediate right of the centremost tube is chosen and the segment of weld (‘2a’ in figure 8) to the top of the already laid weld (laid in stage 1) in this tube is first completed and the segment of weld (‘2b’ in figure 8) that lies to the bottom of the already laid weld (laid in stage 1) is completed thereafter.
d) Then, the next tube that is present to the immediate left of the centremost tube is chosen and the segment of weld (‘3a’ in figure 8) to the top of the already laid weld (laid in stage 1) in this tube is first and the segment of weld (‘3b’ in figure 8) that lies to the bottom of the already laid weld (laid in stage 1) is completed thereafter.

e) Similarly, the other tubes present alternately to the centremost tube are selected based on their increasing distance of separation from the centremost tube and the remaining weld segments shall be completed in accordance with weld sequence 1a-1b-2a-2b-3a-3b-4a-4b-5a-5b-6a-6b-7a-7b-8a-8b-9a-9b-10a-10b etc. shown in figure 8.
f) After completion of all the fillet welds, any minor residual distortion can be corrected by flame straightening. With this step, the fabrication of the flat fin welded panel is completed.
The proposed invention as narrated herein above should not be read and construed in a restrictive manner, as some modifications, adaptations and alterations are possible within the scope and limit of the invention, as defined in the encompassed appended claims.

WE CLAIM
1. A method for weld sequencing technique for control of longitudinal and side bow distortion of tubular panel comprising:
placing the required number of tubes (1) and fins (2) alternately in between a pair of scallop bar (4) wherein the said scallop bars (4) are so profiled that tubes (1) are placed in the semicircular grooves and the fins (2) are placed in between every pair of tubes;
performing the tack welding operation along the length of the tubes (1) in the following sequence,
1(a) performing the tack welding operation partially along the line ‘A’ that coincides with the centre of length of the panel in the weld sequence 1-2-3-4-5-6-7-8-9-10 as shown in fig 5 in a manner that the central most tube is first tack welded and then subsequent tack welds are placed alternately with the increasing distance of separation from central most tube and proceeding towards both lateral extremities of the panel;
1(b) tack welding the panels in top side in the sequence A-B-C-D-E etc (Fig 5) in a manner wherein after completing the tack welding along the line “A’, the next set of tack welding is done along the line ‘B’ which is 175mm away and to the top of line ‘A’ in a sequence same as 1(a) and then in the same manner for the tack welds along the line C, D, E;
1(c) after tack welding, continuing regular welding in the top side of the tubular panel as per the sequence wherein the centermost tube is welded first for a length of 25% to 30% of the total weld length and this is followed by welding (for a length of 25% to 30% of the total weld length) of every alternately placed left and right tube welds in the increasing order of distance of separation of the welds from the centermost tube of the panel as shown in figure 6.

1(d) after completion of partial welding in top side of the panel, turning the panel upside down to bring the hind side of panel facing up and commencing the welding with the centermost tube and at the centre line of the length of the panel proceeding towards the longitudinal (top end) of the panel as shown in figure-7 shown as weld segment 1(a);
1(e) commencing again the welding from the mid length of the centre most tube and proceeding towards the other longitudinal extremity (bottom end) of the panel towards the weld segment ‘1b’ in Fig.7.
1(f) performing the subsequent fillet welds in the sequence 1a-1b-2a-2b-3a-3b-4a-4b-5a-5b-6a-6b-7a-7b-8a-8b-9a-9b-10a-10b etc for the remaining tubes as shown in figure 7.
1(g) after completing the fillet welds in the hind side of the panel at stage 1(e) and 1(f), the top side is placed up for completion of remaining unwelded portion between the various tubes and fins in the welding sequence of 1a-1b-2a-2b-3a-3b-4a-4b-5a-5b-6a-6b-7a-7b-8a-8b-9a-9b-10a-10b as shown in figure 8 which complete the remaining welding that was initially done at stage 1(a) starting from the centre of tubes and for the remaining tubes left and right and lying in the mid length of the panel.
2. The method of weld sequencing as claimed in claim 1, wherein the length of each tack weld in 30-60 mm.

3. The method of weld sequencing as claimed in claim 1, wherein the initial welding is carried out in the mid-length of panel at line ‘A’ on the top side for a partial length of 25% to 30% of length and the remaining length of welds are completed at later stage after completing the full welding in the hind side of the panel in a sequence 1a-1b-2a-2b-3a-3b-4a-4b-5a-5b-6a-6b-7a-7b-8a-8b-9a-9b-10a-10b as shown in figure 7.