FIELD OF INVENTION
The present invention relates to a method to control welding induced distortion/deflection in thin panel type of structures with discretely located attachment welds placed only on single side. More particularly, the invention pertains to welding of discretely placed attachments to a thin tubular panel type of structure for controlling distortion within the stipulated limits.
BACKGROUND OF INVENTION
Distortion is a perennial problem in any welded structure. The occurrence of distortion is attributable to the shrinkage forces of the weld material. This shrinkage force acts with respect to the neutral axis of the component and causes distortion. The presence of distortion leads to many undesirable effects like loss of aesthetics, problem of matching in case of assembly and reduction of service life etc. Although welding induced distortion cannot be eliminated completely, it can be minimized to an acceptable level by following various distortion control methods which includes special techniques like sequential welding, back to back welding, addition of restraints, presetting etc. The problem is more pronounced especially when welds are made only in one side of the structure.

Once distortion has occurred, it is a common practice that one has to go for correction of distortion either mechanically or thermally. This adds up to the manufacturing cycle time by way of rework. Therefore the ideal solution is to employ case specific methods for mitigation of distortion so that it is well controlled 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 is no device and process available to control distortion during the course of the overlaying process, so that no correction of distortion becomes necessary after completion of the welding. But, the method disclosed herein in this invention relates to a method of controlling distortion by employing a localized pre-bending method coupled with appropriate weld sequencing in tubular panels with one side welding of attachments and it 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 localized pre-bending with a weld sequencing method

for welding of attachment to a thin tubular panel without the use of any numerical simulation.
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 localized pre-bending along with a weld sequencing technique for one side welding of attachments present in a tubular panel used for boilers.
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 the method of introducing a localized pre-bending and weld sequencing technique for minimizing occurrence of distortion during welding of attachments to a thin tubular panel in one side only.

OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a method to control welding induced distortion/deflection in thin panel type of structures with discretely located attachment welds placed only on single side, which is capable of providing a method of arriving at the required extent of local pre-bending for controlling the welded induced distortion during welding of attachments to a thin tubular panel used in boiler industries.
Another objection of the invention is to propose a method to control welding induced distortion/deflection in thin panel type of structures with discretely located attachment welds placed only on single side, which is capable of providing a method for pre-bending the said panel locally in the area where the attachments are welded.
A still another object of the invention is to propose a method to control welding induced distortion/deflection in thin panel type of structures with discretely located attachment welds placed only on single side, which can provide an appropriate weld sequence to be followed while welding an array of attachment placed at one side of the thin tubular panel.

A further objection of the invention is to propose a method to control welding induced distortion/deflection in thin panel type of structures with discretely located attachment welds placed only on single side, which is able to provide a method of applying dead weights to ensure a proper pre-bending of the panel arresting its lift up due to shrinkage forces when the weld pads are present near the ends of the panel.
SUMMARY OF THE INVENTION
According to the invention, a method for controlling the welding induced distortion/deflection of a thin tubular panel structure is disclosed. The tubular panel consists of alternately placed tubes and fins for a designed length. Attachments are required to be welded to the tubular panel at discrete locations over the length of the panel and on one side. The attachment consists of an array of weld pads that are to be welded at designated locations onto the panel. This invention discloses a method in order to avoid the welding distortion caused due to the welding of the attachments in one side of the panel. Initially, the extent of distortion due to the welds at discrete locations is calculated based on a special analytical method. Thereafter the panel is pre-bent locally to the extent, the distortion is expected in the location where the attachment welds are present. In case of weld pads present near the ends of the

panel, the dead weights are added to ensure a proper pre-bending of the panel and that the panel does not lift up due to shrinkage forces. Then the welding is carried out in this pre-bent condition by an appropriate procedure. After completion of welding, the dead weights are released. This procedure can effectively control the welding induced distortion in thin tubular panels containing discretely located one side welds.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The proposed invention will be better understood by the following description
with reference to the accompanying drawings:
Fig.1 - Shows the schematic view of a tubular panel with sample attachment
welds.
Fig.2 - Shows the cross-sectional view along section A-A of the panel shown in
Fig.l.
Fig.3 - Shows the schematic view of the Panel under pre-bent condition with
addition of dead weight for providing restraint.
Fig.4 - Shows the weld sequence for attachment welding in one location.
Fig.5 - Shows a photograph of tubular panel.
Fig.6 - Shows a photographic end view of a typical tubular Panel.
Fig.7 - Shows the photographic view of panel with dead weights.

Fig.8 - Shows the photographic view of panel end lifted up due to distortion
without pre-bending when the dead weight is not placed in the required area. Distortion 'D' is shown.
Fig.9 - Shows the close-up view of weld pads in the Panel. Length 'L' is shown.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
This invention describes the method of controlling welding induced distortion/deflection in welding of attachments to thin panel type of structure where the said attachments are welded in only one side of the panel. Figure 1 shows the schematic sketch of the thin panel type of structure. The cross sectional view of the panel (1) is shown in Figure 2. The panel (1) as seen from Figure 2 is composed of tubes (5) welded with fins (6) in an alternate manner. There can be any number of tubes and fins in the cross section of the panel depending upon the design of the panel. The panel (1) is welded separately and the attachments called as weld pads (2) are placed at the necessary locations as per the design of the panel. The attachments (2) may be present at any discrete locations along the length of the panel (1). However, these attachments (2) are present only on one side of the panel and there are no such attachments in the reverse side of the said panel (1). The

detailed procedure of controlling distortion in welding of these attachments (2) to the panel (1) is disclosed herein.
The Moment of Inertia of the panel (1) is calculated using the standard formulas or can be calculated from 3D modelling computer aided drafting software. The number of attachment welds (2) and the length of each of the welds present at discrete locations as prescribed by the design of the panel (1) are calculated. Assuming the attachments as full length welds, the expected distortion of the panel
is calculated using opens resource formula for prediction of longitudinal bending type
of distortion/deflection, which is given below

Where,
D represents the longitudinal bending distortion/deflection in mm
n is the efficiency of welding process (the values for any welding process may be
referred from literatures)
q, Heat rate added by welding, in Watts (Taken as product of voltage and current
used for the welding process multiplied by the total number of welds.)
v, speed of welding, in mm/sec
a, Coefficient of thermal expansion, in /°C

C, Specific Heat Capacity of panel material, in j/ Kg °C
p, Density of panel material, in Kg/mm3
Z, Distance of the weld axis line from Neutral axis of the cross section of the
component, in mm
I, Moment of Inertia of the cross section of the panel, in mm4
L, Length of individual attachment welds, in mm
The panel is now treated as a simply supported beam with a point load in its centre and which produces a deflection of 'D' as calculated from the above mentioned formula. Let this force be termed as 'F1' (in KN). This force F1 is divided

by the length of the panel, L1 and we obtain Now two cases
arise. There can be only one set of attachment welds or there can be multiple set of attachment welds in the panel (1). The procedure that is to be followed is disclosed in a case specific basis hereon.
One set of attachment weld placed any whehre on the panel
Now the panel is assumed as a cantilever beam. The fixed end of the cantilever is at
the mid-length of the panel (1). The length of the cantilever beam is taken as equal
to half the length of the panel (1). Now the cantilever is assumed as having a
uniformly distributed load 'F2' acting over the span equal to the exact length as that
of the attachment weld, L (in mm). Here 'F2' represents the fictive force equivalent

to the force of shrinkage of the attachment welds. The formula for such a case (cantilever beam with uniformly distributed load over a partial span) is normally available in open resources. Using this formula, the deflection at the mid-point of the span containing the attachment weld, 'D' (in mm) is calculated. The Moment of Inertia of the cross section of the cantilever beam is taken as equal to the Moment Inertia of the Panel (1). In case there are two sets of attachment welds one placed at right portion of the panel and the other at left portion of the panel, the deflections at each of the locations can be calculated treating the case as two separate cantilever beams one beam representing the left half, length of the panel and the other beam representing right half length of the panel and the deflections are calculated separately.
Multiple sets of attachment weld placed at discrete locations along the length of panel
In case there are two attachment welds on either left half of the panel or the right half of the panel, then the fictive force equivalent to the force of shrinkage of the first set of attachment welds is calculated as where N1 represents
the length of first set of attachment welds. Now F' is obtained in KN. The fictive force equivalent to the force of shrinkage of the second set of attachment welds is calculated as where N2 represents the length of second set of

attachment welds. Similarly the fictive force equivalent to the force of shrinkage of the nth set of attachment welds is calculated as where Nn represents
the length of nth set of attachment welds. All these fictive shrinkage forces are summed now to obtain the cumulative fictive force equivalent to all the sets of attachment welds is calculated as The length of
attachment welds are summed as
is calculated. This Fs is considered as a fictional uniformly distributed load acting on a cantilever beam of length equal to the half the length of the panel (1) and the fixed end of the cantilever being at the mid-length of the panel (1) over the span containing all the attachment welds. The Moment of Inertia of the cross section of the cantilever beam is taken as equal to the Moment Inertia of the Panel (1). The deflection *D' (in mm) at the mid-point of the span containing all the attachment welds is calculated. This calculation is done using the established formulas for the case of cantilever beam loaded with a uniformly distributed load acting over a partial span of the beam.
Once the deflection calculations are made, the panel is pre-bent locally to the extent of calculations, exactly at the location of the attachment weld as shown schematically in Figure 3. The method of pre-bending is carried out at exactly beneath the mid-point of the span of the panel where the attachments are welded. This pre-bending is done by placing a tube or a pipe (3) with outside diameter equal

to the deflection 'D' as calculated earlier just beneath the panel exactly at the location containing the attachment welds. The length of the pipe (3) is at least equal to the width of the panel. The tree end of the panel is restrained by placing dead weights (4) that are sufficiently heavy to resist the upward movement of the panel at the free end during welding.
In this pre-bent condition, the attachments are welded by using a suitable welding process. The sequence of the welding is shown in figure 4. The weld pad attachments are welded starting from the centre and alternate weld pads are welded subsequently. This is shown by the numbering in figure 4. The weld pad present in the centre is welded first. Then the closest weld pad to the top of the centre weld pad is welded. Then the closest weld pad to the bottom of the centre weld pad is welded. Thereafter the weld pads are welded alternately using the sequence as shown in figure 4.
After completion of welding, the dead weights (4) are removed and the tube/pipe (3) placed beneath the panel is removed. The panel is checked for the flatness throughout the length and in case of overly distorted portions being found, those areas are subjected to a hot correction in order to maintain flatness of the panel.

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 to control welding induced distortion/deflection in thin panel type of structures with discretely located attachment welds placed only on single side, the said method comprising;
preparing the Panel (1) composed of tubes (5) welded with fins (6) in an
alternate manner;
placing of a plurality of attachments (2) to be welded at designated locations
over the length only on one side of the panel (1);
calculating the deflection (D) at the mid-point of a span containing all the attachment weld (2), the said span of a Panel (1) being equal to the exact length (L) as that of the attachment weld;
pre-bending the panel (1) locally to the extent of said deflection (D) exactly at the location of the attachment weld (2), by placing a tube or pipe (3) having length at least equal to the width of the panel (1) and outside diameter equal to the deflection (D) just beneath the panel exactly at the location containing the
attachment welds (2);
restraining the free end of the panel (1) by placing dead weights (4) to resist the upward movement of the Panel (1) at the free end during welding; wherein in

the pre-bent condition the attachments are welded with a suitable welding process when the weld pad present in the centre (1') is welded first and then closest weld pad (2') to the top of the centre weld pad (1') is welded, then closest weld pad (30 to the bottom of the centre weld pad (1') is welded and then the weld pads are welded in sequence of 4' and 5', when after completion of welding, the dead weights (4) and pipe/tube (3) are removed and the panel (1) is checked for flatness throughout the length.