Claims:WE CLAIM:
1. A method for controlling the welding induced distortion in welding of triangular shaped panel (10) incorporating a weld sequencing technique wherein
points A, B, C, D, E, F and G are marked, on a fin (2) connecting last tube (1) and penultimate tube (1) of said panel (10), in a way that AG is the length of the fillet weld required to be made to connect the longest fin (2) to the longest tube (1) of said panel (10);
said points are marked such that D is the midpoint of line segment AG and CE;
BC = EF = one-tenth the length of AG;
CE is equal to one-fifth the length of AG;
said markings are made only in the top side of the panel;
distortion restraining stiffeners (5 & 6) are tack welded to the panel at appropriate locations to deter both the transverse and longitudinal bending caused due to welding;
two welding personnel (W1 and W2) are employed simultaneously for carrying out the welding on either side of said panel;
said tube (1) to fin fillet weld that is closest to the Centre of Gravity (CG) of the panel is chosen as the first weld for beginning welding;
segments BC and EF are welded simultaneously by said welding personnel W1 and W2;
The start point of welding of said segments BC and EF is near the median OD and the end point of welding is away from the median OD;
The tube (1) to fin filletweld that is next nearer to the CG is welded next using the method described above;
after welding of all BC and EF segments, said panel (10) is inverted to bring the bottom side up;
all the fillet welds are welded to their full lengths by W1 and W2 simultaneously;
each of the fillet welds is further divided into two equal segments by the OD into one right half and one left half;
The welding of right and left halves of each of the fillet welds is started from the median OD and completed at the farther end from the median OD by said welding personnel W1 and W2 concurrently;
After completion of welding in the bottom side, said panel is turned upside down again to bring said panel to its original position;
Remaining segments (AB, CD, DE & FG) are welded by said welding personnel W1 and W2 simultaneously;
the fillet weld which is nearest to the Center of Gravity is welded first;
In each of the fillet weld, segment CE is welded first starting from the median OD by said welding personnel W1 and W2 in such a way that W1 welds segment DC (starting from D proceeding towards C) and W2 welds segment DE (starting from D proceeding towards E);
the welding in the segment BA (starting from B proceeding towards A) is done by W1 and the welding in the segment FG (starting from F proceeding towards G) is done by W2;
After the completion of welding, said panel is sent for Post Weld Heat Treatment (PWHT) process with the distortion restraining stiffeners (5) and (6) still intact;
After PWHT, said distortion restraining stiffeners (5 & 6) are removed;
Said panel is checked for flatness in all locations.

2. The method as claimed in claim 1, wherein said panel (10) is turned upside down at least once during the course of welding.

3. The method as claimed in claim 1 wherein welding starts fromfrom near or at the point of intersection of mid-length of the weld line with the median line of the triangular shape of the panel and proceeding towards both the right and left extremities of the panel by using a plurality of welders welding simultaneously.

4. The method as claimed in claim 1, wherein said sequencing method employs any known welding process which could either be a manual or a semiautomatic or an automatic welding process.

5. The method as claimed in claim 1, wherein said distortion restraining stiffeners (5) & (6) are attached to the panel by means of tack welds at least 20 mm with the spacing between the tacks of at least 100 mm.
, Description:FIELD OF THE INVENTION
[001] The present invention generally relates to a method of weld sequencing. The present invention more particularity relates to a weld sequencing method for controlling distortion in welded, triangular shaped tubular panels used in supercritical boiler applications.
BACKGROUND OF THE INVENTION
[002] Distortion is a common problem encountered in components fabricated by welding.The distortion phenomenon is caused by the localized heating and cooling effects associated with the weld and the consequential shrinkage forces of the weld material. This shrinkage force acts with respect to the neutral axis of the component and produces a bending moment which in turn causes distortion of the welded component. There are several varieties of distortion based on the geometrical configuration of the component and the positioning of the welds.

[003] Distortion in welded components lead to numerousundesirable effects like loss of aesthetics, problem in matching components in an assembly and reduction of service life etc. Although welding induced distortion cannot be made zero, it can be minimized to an acceptable level by following various distortion control practices which include techniques like sequential welding, back to back welding, addition of restraints, presetting etc. The problem of welding distortion is more prominent in components where the length of weld is high. Correction of distortion is a rather cumbersome process which adds up to the both cycle time and cost of manufacture on account of the additional rework. Therefore, the ideal solution is to go for an in-process control of distortion like weld sequencing etc.

[004] 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, this patent does not disclose any details regarding the weld sequencing aspects to be followed for fabrication of such tubular panels so that the welding induced distortion is minimized at the time of fabrication itself. But, the present invention discloses a method of controlling distortion during welding of triangular shaped panels by employing a weld sequencing technique and therefore different from the US patent US 7028882 mentioned above.

[005] 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 welding of triangular panels used in supercritical boiler applications with an ad-hoc weld sequencing technique and is not related to hard-facing operation that is addressed in the US patent 7703660 B2 mentioned above.

[006] 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 weld sequencing for the purpose of controlling welding induced distortion in welding of triangular shaped panels used in supercritical boilers is disclosed.

[007] 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 sequencing weld deposition in welding of triangular panels used in supercritical boiler application.

[008] To mitigate the problem of welding induced distortion, a method of weld sequencing has been disclosed in the present invention. The present invention relates to a method of controlling weld distortion in fabricating triangular shaped tubular panels used for supercritical boilers which are used in thermal power plants for the conversion of water to superheated steam which turns the turbine thereby producing electricity. These boilers are made of numerous tubular panels which are composed of tubes and fins welded continually. The triangular panel is located in all four corners of the boiler both on top and bottom of burner panels.

[009] The triangular panel consists of tubes and fins welded alternately with the tubes bent in two planes. From one end to the other end of the panel, it consists of tubes and fins of gradually decreasing lengths leading to formation of a triangular configuration.

[0010] On account of the high cumulative weld length, the panel is highly prone to welding induced distortion. The method of weld sequencing that would control the welding induced distortion in such triangular panels has been disclosed in this invention.
OBJECTS OF THE INVENTION
[0011] It is therefore an object of the invention to provide a method incorporating an appropriate weld sequencing technique for the purpose of controlling the welding induced distortion during welding of triangular shaped tubular panels which are a part of the water wall panels being used in the supercritical boilers.

SUMMARY OF THE INVENTION
[0012] According to the present invention, a distortion control method for controlling the welding induced distortion of triangular tubular panel of supercritical boilers is disclosed. The triangular panel is a tubular panel of alternately welded tubes with the tubes and the fins bent in two planes. The tubes and fins will be of gradually decreasing lengths as one proceeds from one end of the panel to the other end. The panel will be contained within a rectangular space of 500 mm x 500 mm. There will be multiple tubes and fins of gradually decreasing lengths and there are fillet welds between the adjacently placed fins and tubes. Each fin will be welded to two other tubes containing the fin, by way of four fillet welds. Therefore, the panel has to be welded both on the top and underside of the fins.

[0013] To mitigate the problem of welding induced distortion, a method of weld sequencing has been disclosed in the present invention. According to this method, 2 welding personnel are employed to perform the welding of the triangular panel simultaneously. Out of the two welders, one is available to the left of the median line of the triangular panel and the other is present on the right of the median line of the triangular panel. The welding is stated in the top side of the panel in stage 1. The two welders start welding the fillet weld connecting the tube and fin that are present closest to the Centre of Gravity (CG) of the triangular panel in top side. Instead of welding the full length of the fillet weld, each of the two welders weld a segment measuring one-tenth the total fillet weld length and the distance between the two segments welded by these two welders is one-fifth the total fillet weld length. Next, the fillet weld that is next nearest to the CG of the panel is chosen and the welding is completed in a similar pattern as mentioned above. This is repeated until all the fillet welds are covered in top side of the panel. Next, the bottom side of the panel is welded in a similar manner using two welders one each for the left and right sides of the panel with the only difference being that instead of welding one-fifth the total weld lengths, the full welding is carried out in each of the fillet welds i.e. each welder welds one-half the weld length and the total length of fillet welds are completed one by one based on the increasing order of their location from the CG of the panel. The panel is then inverted and the leftover fillet weld segments are completed again by using two welders and order of selection of fillet welds will be based on increasing distance from CG of the component.

[0014] This procedure can effectively control the welding induced distortion in triangular panels used in supercritical boiler applications.
BRIEF DESCRIPTIONS OF THE ACCOMPANYING DRAWINGS
[0015] The proposed invention will be better understood by the following description with reference to the accompanying drawings:

Figure 1 shows the sketch of triangular panel with tubes and fins placed alternately
Figure 2 shows the sectional view of the panel at the section A-A.
Figure 3 shows the Schematic sketch of triangular panel with weld sequence and representation of welders and direction of progression of welding
Figure 4 shows the sketch of triangular panel with stiffeners placed at multiple locations to avoid distortion
DETAIL DESCRIPTION OF THE INVENTION
[0016] The present invention describes the method of controlling welding induced distortion in welding of triangular panels of supercritical boilers.
Figure 1 shows the sketch of a typical triangular panel used in supercritical boiler application.
[0017] The cross sectional view of the panel shown in figure 1 taken along the section A-A indicated in figure 1 is shown in figure 2. The triangular panel as seen from figure 2 is composed of tubes (1) welded with fins (2) in an alternate manner. It is also seen from figure 1 that the tubes (1) are bent to a small angle in their ends. The fins between two adjacent tubes are welded to them by means of four fillet welds namely one group of two fillet welds represented as (3) in the top side of the triangular panel and one more group of two fillet welds represented as (4) in the bottom side of the triangular panel (figure 2).

[0018] There can be any number of tubes (1) and fins (2) in the panel depending upon the on the design of the panel. But, as observed from figure 1, the tubes and fins are not of equal length as one proceeds from one endto the other end of the panel. The tubes and fins are gradually decreasing in length from one end (Line segment AG) to the other end (vertex O) which results in a triangular configuration of the panel (figure 3).

[0019] The fit-up between the bent tubes (1) and the fins (2) is first carried out and the panel is fully assembled. Thereafter, tack welds of not less than 30 mm in length are made at regular spacing of not less than 200 mm in each fillet welds connecting the tubes (1) and the fins (2). The tack welds are started from the fillet weld that is closest to the CG of the panel and welds that are in increasing order of the distance of separation from the CG are chosen subsequently. Within a fillet weld, the tack welds are laid starting from the point of intersection of the weld line with the median OD of the triangular panel and proceeding towards the left and right extremities of the panel respectively by using two welding personnel W1 and W2 (figure 3) concurrently.

[0020] Then, the distortion restraining stiffeners (5) & (6) are attached to the panel by means of tack welds of size not less than 20 mm with the spacing between the tacks not lesser than 100 mm (figure 4). These can be typically ‘C’, ‘L’ or other varieties of channel sections or other stiffened members capable of restraining the distortion induced during the course of welding of the panel. Out of (5) and (6), (5) is meant for deterring transverse bending distortion (bending happening transverse to the length of the welds) and (6) is meant for deterring longitudinal bending distortion.

[0021] The entire welding of the panel is carried out in three stages as detailed below
Stage 1:
(1) In the fin (2) connecting the last tube (1) and the penultimate tube of the triangular panel, the points A, B, C, D, E, F and G are marked in such a way that AG is the length of the fillet weld required to be made to connect the longest fin with the longest tube of the panel, D is the midpoint of line segment AG and CE, BC = EF = one-tenth the length of AG and CE is equal to one-fifth the length of AG. These markings are made only in the top side of the panel.
(2) All the remaining fillet welds in the top side of the panel are also divided into two one-tenth segments with a spacing of one-fifth segment using a similar method stated in the previous step.
(3) Two welders – W1 and W2 are employed simultaneously for carrying out the welding.
(4) The tube to fin fillet weld that is closest to the Centre of Gravity (CG) of the panel is chosen as the first weld for beginning welding. The two one-tenth segments available on both the right and left side of the median OD of the panel are welded simultaneously by the two welders W1 and W2. The start point of welding of both the one-tenth segments is near the median OD and the end point of welding is away from the median OD. In other words, the welders simultaneously start the welding at the start points as described above and proceed towards the end point of the one-tenth segments in a direction parallel to CB and EF on both the sides
(5) The tube to fin fillet weld that is next nearer to the CG is chosen next. The scheme of welding is same as described in the previous step.
(6) Likewise, the other fillet welds are chosen on the increasing order of their distance of separation from the CG of the panel. The fillet weld closest to the CG is welded first as stated in point (4) and the fillet welds are carried out one by one following the scheme of weld sequence stated in step (4).

Stage 2:
(1) After completion of two one-tenth segments in each of the fillet welds all along the panel, the panel is inverted to bring the bottom side up.
(2) In this position, all the fillet welds are welded to their full lengths by employing two welders simultaneously. Each of the fillet welds is divided into two equal segments by the OD into one right half and one left half. The right and left halves of each of the fillet welds is started from the median OD and completed at the farther end from the median OD and this is done by two welders welding concurrently.
Stage 3:
(1) After completion of welding in the bottom side, the panel is then turned up again to bring the top side back on top.
(2) Already, two one-tenth segments in each of the fillet welds are completed in stage1. The left-over weld segments in each of the fillet weld is completed in this stage.
(3) The fillet weld that is nearest to the CG is taken up for welding first and the welding is carried out as mentioned in the next step (4) below
(4) In each of the fillet weld, the one-fifth segment present in the centre of the length is welded first starting from the median OD and by using two welders simultaneously and the remaining 3/10th segments on the either sides of the median are again welded by using two welders simultaneously. For example, in case of segment AG, the welding in the segment DC (starting from D proceeding towards C) is carried out by welder W1 and the welding in the segment DE (starting from D proceeding towards E) is carried out by welder W2. Then, the welding in the segment BA (starting from B proceeding towards A) is carried out by welder W1 and the welding in the segment FG (starting from F proceeding towards G) is carried out by welder W2 (figure 3).
(5) After completion of the fillet weld as stated in the previous step (3) above, the fillet weld that is next nearest to the CG is chosen and the welding is carried out in the same weld sequence scheme as stated in step (3). Likewise, each of the filet welds are welded in the order of increasing distance of positioning from the CG of the panel.
(6) In this manner, all the fillet welds of the panel are carried out and the total welding of the panel is completed. After the completion of welding, the panel is sent for Post Weld Heat Treatment (PWHT) process with the distortion restraining stiffeners (5) and (6) still intact. These are to be removed only after PWHT.
(7) After removal of distortion restraining stiffeners, the panel is checked for flatness in all locations. If the flatness achieved in the panel is within the permissible limits, then no further corrections are necessary.
(8) If there are some portions where the flatness of the panel is lost due to weld distortion, then the panel is subjected to local flame correction on places where the magnitude of the distortion is more than the permissible limits.
[0022] 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.