Deformation control method for super-large tube plate tailor-welding based on forward-reverse sequence welding

Abstract

The present application belongs to the technical field of tube sheet tailor-welding deformation control, and specifically discloses a super-large tube sheet tailor-welding deformation control method based on forward-reverse sequence welding, which is used to solve the problem of difficult accurate control of super-large tube sheet tailor-welding deformation. The method comprises the following steps: (1) establishing a finite element model of tube sheet tailor-welding, adopting asymmetric double-U-shaped groove design, the depth of the forward groove being different from that of the reverse groove, the depth of the upper groove being 80mm-100mm longer than that of the lower groove, the total number of overturning being 10-50 times, and the number of welding beads in the whole welding process being 10-50; (2) checking the bearing strength; (3) testing the influence of the number of overturning and the number of welding beads on angular deformation respectively; (4) fitting the relationship between post-welding residual angular deformation and the number of overturning and the number of welding beads according to the data results, and forming a super-large tube sheet tailor-welding deformation control process. The present application significantly reduces the maximum post-welding residual angular deformation, and improves the welding quality and pressure-bearing capacity of the tube sheet.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of tube sheet tailor welding deformation control, and particularly relates to a super-large tube sheet tailor welding deformation control method based on forward-reverse sequence welding. BACKGROUND

[0002] A large reactor is a key equipment in a production process, has high design parameters, is difficult to manufacture, and has extremely high requirements on the overall quality and safety of the equipment. A tube sheet, as an important component of the reactor, directly affects the overall performance and operation safety of the equipment. However, in the manufacturing process of the large reactor, the tube sheet tailor welding deformation problem has always been a difficult problem for manufacturing enterprises.

[0003] At present, the control of the tube sheet tailor welding deformation mainly has three aspects of measures: one is a pre-deformation method, which applies appropriate bending to the tube sheet, so that, due to the effects of thermal expansion and contraction in the welding process, the deformation direction is opposite to the pre-deformation, thereby reducing the post-welding deformation. However, due to the difficulty in accurately predicting the super-large tube sheet tailor welding deformation, the pre-deformation amount cannot be accurately matched with the actual welding deformation, correction is difficult, and the assembly precision of the subsequent tube sheet and cylinder is affected; two is a temperature control method, which adjusts parameters such as welding current, welding speed, and welding voltage to reduce heat input, control the local temperature field, and avoid deformation caused by local overheating. However, due to the large thickness and long weld of the super-large tube sheet, heat is not uniformly transmitted along the thickness direction, and the local temperature field is difficult to stably control, resulting in accumulation of asymmetric thermal deformation; three is a fixture fixing method, which designs and uses a special fixture to fix the tube sheet at the welding position according to the structural characteristics of the tube sheet, and uses a pressure tool to apply a certain pressure to the welding position to ensure that the tube sheet does not deform too much during the welding process. However, the fixture cannot completely constrain the complex thermal deformation of the super-large tube sheet, especially when the thickness direction is asymmetric deformation, and a large residual angular deformation still occurs.

[0004] It can be seen that the tube sheet tailor welding deformation is affected by various factors of the welding process, such as welding sequence and number of welding passes. How to develop a scientific and reasonable welding process (number of turns and number of welding passes), reduce the tube sheet tailor welding deformation, and ensure the flatness of the tube sheet after welding is an important problem in the tube sheet manufacturing process. SUMMARY

[0005] The purpose of the present application is to provide a super-large tube sheet tailor welding deformation control method based on forward-reverse sequence welding, which effectively solves the problem of difficult accurate control of super-large tube sheet tailor welding deformation.

[0006] To solve the above technical problems, the technical scheme adopted by the present application is:

[0007] A method for controlling deformation during welding of ultra-large tube sheets based on forward and reverse welding sequence is disclosed. The ultra-large tube sheets have a diameter of 5000mm to 10000mm and a thickness of 60mm to 400mm. The material of the ultra-large tube sheets is steel plate as specified in GB150.2 "Pressure Vessels Part 2: Materials". The method includes the following steps: S1. Establishing a finite element model for tube sheet welding: A two-dimensional finite element model of the ultra-large tube sheet is established, and an asymmetric double U-shaped bevel design is adopted: the bevel radius is 10mm to 12mm, the bevel angle is 3° to 5°, the depths of the forward and reverse bevels are different, the depth of the upper bevel is 80mm to 100mm longer than the depth of the lower bevel, the width of the bevel is 24mm to 28mm, the root distance between the upper and lower bevels is 10mm to 12mm, the total number of flips is 10 to 50, and the number of weld passes in the entire welding process is 10 to 50.

[0008] S2. Bearing strength check: Determine the geometric centroid of the tube sheet and the peak position of the bending moment. Use the four-point bending method to calculate the peak bending moment of the tube sheet. Check the bearing strength required for the tube sheet to flip and determine the minimum thickness of the first weld.

[0009] S3. Effect of the number of flips on angular deformation: Change the number of weld beads in each alternating positive and negative filling weld, keep the total number of weld beads the same, change the number of flips of the tube sheet, and compare the effect of the number of flips on the residual angular deformation after welding.

[0010] S4. Effect of weld pass number on angular deformation: Change the thickness of each forward and reverse filler weld, keep the number of flips the same, change the number of weld passes, and compare the effect of the number of weld passes on the residual angular deformation after welding.

[0011] S5. Based on the data results, fit the relationship between the residual angular deformation after welding and the number of flips and welds. Taking into account the influence of the number of flips and the number of welds on the residual angular deformation after welding, form a process for controlling the welding deformation of ultra-large tube sheets, and achieve optimal control of the maximum residual angular deformation after welding.

[0012] Further, in step S2, the geometric centroid x of the tube sheet is x = 4r / 3π; the peak position of the bending moment is a = rx; the peak bending moment is M. The method for verifying bearing strength is as follows: ≤ Yield strength; where b is the tube sheet diameter, r is the tube sheet radius, G is the tube sheet weight, and h is the minimum thickness of the first weld.

[0013] Furthermore, the welding process used in steps S3 and S4 is as follows: preheating treatment is performed before welding (the preheating temperature is determined according to the material of the steel plate), and CO2 gas shielded welding or argon arc welding is used to ensure root penetration.

[0014] Then, the automatic submerged arc welding is used for the pipe plate to be welded alternately in the positive and negative directions, the thickness of each layer is controlled, and the slag and pores are avoided; during the welding process, the weld penetration is ensured, the incomplete fusion is avoided, the bevels on the reverse side of the weld need to be continuously preheated, and the constant preheating temperature is maintained, so that the deformation caused by the uneven local cooling speed is avoided.

[0015] Finally, one cover welding is performed on the positive and negative surfaces respectively, the weld surface is modified, and the excess height is controlled to be less than or equal to 2 mm.

[0016] After the welding is completed, the pipe plate is placed into a heat treatment furnace for leveling and heat treatment, so that the welding deformation of the pipe plate is further eliminated. Finally, the weld and the base material are polished to be flush, the UT / MT detection is performed every 10 welds during the welding process, and it is ensured that there is no defect.

[0017] Further, the bevels on the positive surface of the pipe plate and the reverse surface of the pipe plate within the range of 200 mm are uniformly preheated before welding.

[0018] Compared with the prior art, the beneficial technical effects of the present application are:

[0019] (1) The present application controls the post-weld residual angular deformation by using the method of sequentially welding the pipe plate in the positive and negative directions, significantly reduces the maximum post-weld residual angular deformation, reduces the risk of inaccurate joint position and assembly error after the pipe plate is welded, and thus improves the welding quality and pressure-bearing capacity of the pipe plate.

[0020] (2) The present application designs different bevel depths on both sides, so that heat is transferred to the thick plate direction during the welding process, and the heat is uniformly distributed along the thickness direction, thereby reducing the post-weld deformation.

[0021] (3) The present application reasonably designs the welding pass and the number of welding turns, explores the influence of the welding process on the post-weld angular deformation, forms the deformation control process of the super-large pipe plate welding, and effectively reduces the post-weld residual angular deformation. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a schematic view of the welding bevel structure of example 1.

[0023] Figure 2 is the post-weld residual angular deformation result corresponding to the case where the number of turns is 10 in example 1, wherein U represents displacement, and U2 represents displacement in the Y direction.

[0024] Figure 3 is the post-weld residual angular deformation result corresponding to the case where the number of turns is 20 in example 1, wherein U represents displacement, and U2 represents displacement in the Y direction.

[0025] Figure 4is the post-weld residual angular distortion result corresponding to the number of 30 passes in Example 1, wherein U represents displacement, and U2 represents displacement in the Y direction.

[0026] Figure 5 is the post-weld residual angular distortion result corresponding to the number of 20 passes in Example 1, wherein U represents displacement, and U2 represents displacement in the Y direction.

[0027] Figure 6 is the two-dimensional cloud diagram of the post-weld residual angular distortion of different pass numbers and overturning times in Example 1.

[0028] Figure 7 is the post-weld residual angular distortion under the optimal welding process in Example 1, wherein U represents displacement, and U2 represents displacement in the Y direction.

[0029] Figure 8 is the schematic diagram of the welding groove structure of Example 2.

[0030] Figure 9 is the two-dimensional cloud diagram of the post-weld residual angular distortion of different pass numbers and overturning times in Example 2.

[0031] Figure 10 is the post-weld residual angular distortion result of Comparative Example 1, wherein U represents displacement, and U2 represents displacement in the Y direction. DETAILED DESCRIPTION

[0032] Example 1: Super-large tube plate butt welding deformation control method based on forward and reverse sequence welding, the tube plate material of the embodiment is SA508 steel plate, the tube plate diameter is 8754mm, the tube plate thickness is 290mm, and the process steps are as follows.

[0033] S1, establish a finite element model of tube plate butt welding: establish a two-dimensional finite element model of super-large tube plate, adopt asymmetric double U-shaped groove design: the forward groove depth is 180mm, the groove angle is 3°, the maximum groove width is 28mm, and the groove radius is 10mm; the reverse groove depth is 100mm, the groove angle is 3°, the maximum groove width is 24mm, and the groove radius is 10mm; the root distance of the upper groove and the lower groove is 10mm. The structure is as shown in Figure 1 .

[0034] S2, bearing strength check: the tube plate diameter b is 8.754m, the tube plate radius r is 4.377m, the tube plate weight is 68473kg, the tube plate gravity G is 671037N, the yield strength of the material is 485MPa, and the four-point bending method is used for calculation.

[0035] The geometric center x of the tube plate: x=4r / 3π; the peak position a of the bending moment: a=r-x=2.52m; the peak bending moment M:

[0036] The carrying strength verification method is: ≤ yield strength; the minimum welding surface thickness h of the first welding is 35 mm.

[0037] S3, the influence of the number of turns on the angular deformation: the number of turns of the pipe plate is changed while the number of welding passes is kept the same, and the influence of the number of turns on the residual angular deformation after welding is compared.

[0038] Before welding, the groove on both sides of the front and back of the steel plate within a range of 200 mm is uniformly preheated by using gas to reduce welding residual stress and welding deformation, the preheating temperature is ≥200℃, and the preheating time is 40-60 min to make the preheating uniform.

[0039] (1) First, 6 mm of the bottom layer is welded on the front and back by using CO2 gas shielded welding to ensure root penetration. (2) Then, the front and back are alternately filled by using automatic submerged arc welding, 3 passes are sequentially welded in the forward direction, each welding pass is 5.8 mm thick, the pipe plate is turned over, 2 passes are sequentially welded in the reverse direction, each welding pass is 4.7 mm thick, the pipe plate is sequentially turned over 20 times, and the total number of welding passes is 50. (3) Finally, 1 pass of the cover welding is performed on the front and back, the thickness is 5 mm, the surface of the weld is modified, the excess height is controlled to be ≤2 mm, and the welding of the entire welded joint is completed.

[0040] The welding process parameters of the automatic submerged arc welding are: welding current 450A-600A, arc voltage 32V-36V, welding speed 50-55 cm / min, and interpass temperature 150-260℃.

[0041] After the welding is completed, heat treatment is performed, the heat treatment process is: furnace temperature ≤350℃, heating rate 50℃ / h-60℃ / h, heat treatment temperature 620℃±20℃, holding time 4 hours, cooling rate 50℃-60℃ / h, and the furnace is discharged when the furnace temperature is ≤200℃ for air cooling. Finally, the weld and the base material are ground to be flush, 100% ultrasonic testing is performed, and the grade I is qualified.

[0042] The total number of welding passes is kept unchanged, the number of turns is reduced, and the number of welding passes of each layer during welding is increased. (1) First, 6 mm of the bottom layer is welded on the front and back by using CO2 gas shielded welding to ensure root penetration. (2) Then, the front and back are alternately filled by using automatic submerged arc welding, 6 passes are sequentially welded in the forward direction, each welding pass is 5.8 mm thick, the pipe plate is turned over, 4 passes are sequentially welded in the reverse direction, each welding pass is 4.7 mm thick, the pipe plate is sequentially turned over 10 times, and the total number of welding passes is 50. (3) Finally, 1 pass of the cover welding is performed on the front and back, the thickness is 5 mm, the surface of the weld is modified, the excess height is controlled to be ≤2 mm, and the welding of the entire welded joint is completed.

[0043] The maximum post-weld residual angular deformation is 53.6 mm when the number of turns is 10, and the maximum post-weld residual angular deformation is 39.3 mm when the number of turns is 20; the maximum post-weld residual angular deformation is reduced by 26.6%, as shown in Figure 2 and Figure 3 Therefore, the increase in the number of turns can reduce the maximum post-weld residual angular deformation.

[0044] S4, Effect of the number of welding passes on angular deformation: Change the thickness of each forward and reverse filling welding, keep the number of turns the same, change the number of welding passes, and compare the effect of the number of welding passes on post-weld residual angular deformation.

[0045] Before welding, uniformly preheat the groove on both sides of the front and back of the steel plate within a range of 200 mm, (1) first use CO2 gas shielded welding to weld 6 mm on the front and back to form a base layer, and ensure root penetration. (2) Then use automatic submerged arc welding to perform alternating filling welding on the front and back, sequentially weld 2 passes in the forward direction, each welding pass is 8.7 mm thick, turn over the tube plate, sequentially weld 1 pass in the reverse direction, each welding pass is 9.4 mm thick; sequentially turn over 20 times, and the total number of welding passes is 30.

[0046] Keep the number of turns unchanged, change the number of welding passes, (1) first use CO2 gas shielded welding to weld 6 mm on the front and back to form a base layer, and ensure root penetration. (2) Then use automatic submerged arc welding to perform alternating filling welding on the front and back, sequentially weld 1 pass in the forward direction, each welding pass is 17.4 mm thick, turn over the tube plate, sequentially weld 1 pass in the reverse direction, each welding pass is 9.4 mm thick; sequentially turn over 20 times, and the total number of welding passes is 20.

[0047] When the number of welding passes is 30, the maximum post-weld residual angular deformation is 22.09 mm; when the number of welding passes is 20, the maximum post-weld residual angular deformation is 12.76 mm, and the maximum post-weld residual angular deformation is reduced by 42.2%, as shown in Figure 4 and Figure 5 Therefore, the reduction in the number of welding passes can reduce the maximum post-weld residual angular deformation.

[0048] S5, Tube plate welding deformation control process: According to the above finite element simulation method of tube plate welding, adjust the number of turns and the number of welding passes for multiple simulations, and obtain the relationship between each factor and angular deformation based on the simulation results.

[0049] The relationship between the maximum post-weld residual angular deformation and the number of turns and the number of welding passes is d = 13.8 + 0.9n - 0.91N, where d is the maximum post-weld angular deformation, and the unit is mm; N is the number of turns, and n is the number of welding passes.

[0050] Taking into account the influence of the number of flips and the number of weld beads on angular deformation, the residual angular deformation after welding for each number of flips and weld beads is represented in a two-dimensional contour plot, such as... Figure 6 As shown.

[0051] pass Figure 6 The results show that the maximum residual angular deformation decreases with decreasing weld bead count and with increasing flip-over count. The number of weld beads and flip-over count are mutually restrictive and influential. For the same number of flip-over counts, fewer weld beads result in a smaller maximum residual angular deformation, while the number of weld beads is constrained by the number of flip-over counts; more flip-over counts lead to more weld beads. The optimal control process for large tube sheet welding is to select a smaller number of weld beads while choosing an appropriate number of flip-over counts. In this embodiment, the optimal residual angular deformation welding control process is determined to be: 30 flip-over counts, 30 weld beads, with a maximum post-weld residual angular deformation of 5.82 mm. Figure 7 As shown.

[0052] Example 2: A method for controlling the welding deformation of ultra-large tube sheet based on forward and reverse welding sequence. The tube sheet material in this example is SA508 steel plate, the tube sheet diameter is 5100mm, and the tube sheet thickness is 200mm. The process steps are different from those in Example 1.

[0053] In step S1, an asymmetric double U-shaped bevel design is adopted: the forward bevel depth is 110mm, the bevel angle is 3°, the maximum bevel width is 25mm, and the bevel radius is 10mm; the reverse bevel depth is 80mm, the bevel angle is 3°, the maximum bevel width is 22mm, and the bevel radius is 10mm; the distance between the roots of the upper and lower bevels is 10mm. The structure is as follows... Figure 8 As shown.

[0054] In steps S3 and S4, (1) first, use CO2 gas shielded welding to weld 5mm thick layers on both sides to ensure root penetration. (2) Then, use automatic submerged arc welding to perform alternating filler welding on both sides. Weld 2 passes in the forward direction, each pass being 10.5mm thick. Flip the tube sheet and weld 2 passes in the reverse direction, each pass being 7.5mm thick. Flip the tube sheet 10 times, for a total of 20 welding passes. (3) Finally, perform one cover pass on each side, 4mm thick, to finish the weld surface and control the reinforcement height to ≤2mm, completing the welding of the entire joint. The residual angular deformation after welding is 21.6mm.

[0055] The automatic submerged arc welding process parameters are: welding current 550A-600A, arc voltage 36V-38V, welding speed 55-58 cm / min, interpass temperature ≤200℃. After welding, heat treatment is carried out, and the heat treatment process is: furnace temperature ≤350℃, heating rate 50℃ / h-60℃ / h, heat treatment temperature 620℃±20℃, holding for 4 hours, cooling rate 50℃-60℃ / h, and when the furnace temperature is ≤200℃, the furnace is discharged and air-cooled. Finally, the weld and the base material are polished to be flush, and 100% ultrasonic testing is carried out, and grade I is qualified.

[0056] The number of times of turning over and the number of welding passes of the filler welding are changed for multiple simulations, and the residual angular deformation corresponding to the number of times of turning over and the number of welding passes is as shown in Figure 9 .

[0057] The relationship between the maximum residual angular deformation after welding and the number of times of turning over and the number of welding passes is: d=2.56+1.46n-1.1N.

[0058] It can be seen from Figure 9 that the residual angular deformation after welding is jointly affected by the number of times of turning over and the number of welding passes, and the two are mutually restricted and balanced. Considering the effects of the number of times of turning over and the number of welding passes on the angular deformation, the optimal residual angular deformation welding control process is: 16 times of turning over, 16 welding passes, and the maximum residual angular deformation after welding is 3.2 mm.

[0059] Comparative Example 1: The pipe plate material of the comparative example is SA508 steel plate, the pipe plate diameter is 8754 mm, and the pipe plate thickness is 290 mm.

[0060] The comparative example also adopts an asymmetric double-U-shaped groove design: the forward groove depth is 180 mm, the groove angle is 3°, the maximum groove width is 28 mm, and the groove radius is 10 mm; the reverse groove depth is 100 mm, the groove angle is 3°, the maximum groove width is 24 mm, and the groove radius is 10 mm; and the root distance of the upper groove and the lower groove is 10 mm. The structure is as shown in Figure 1 .

[0061] The difference between the comparative example and Example 1 is that the comparative example does not adopt the forward and reverse sequence welding deformation control method during the welding of the super-large pipe plate, and therefore, the minimum welding thickness of the first welding pass does not need to be considered.

[0062] The pipe plate welding process of the comparative example is: (1) using automatic submerged arc welding, 30 welding passes are sequentially completed for the welding of the forward groove, and each welding pass has a thickness of 6 mm.

[0063] (2) After the forward welding is completed, the pipe plate is turned over, and reverse welding is performed for 20 passes, each pass having a welding thickness of 5 mm, to finally complete the welding of the entire welded joint.

[0064] After the welding is completed, heat treatment is carried out, and the heat treatment process is as follows: furnace entry temperature ≤ 350℃, heating rate 50℃ / h-60℃ / h, heat treatment temperature 620℃±20℃, holding for 4 hours, cooling rate 50℃-60℃ / h, and furnace temperature ≤ 200℃ when furnace exit, and air cooling.

[0065] Finally, the weld and the base material are polished to be flush, 100% ultrasonic detection is carried out, and grade I is qualified. The maximum residual angular deformation after welding of the comparative example is 72.16mm, as shown in the following table. Figure 10

[0066] The maximum residual angular deformation after welding of the comparative example 1 is larger than the results of any one of the welding processes in the embodiment 1, and by comparing the Figure 10 and Figures 2-5 , the maximum residual angular deformation after welding is reduced by 87% at most. By comparing the Figures 2-7 and Figure 10 , it is concluded that in the process of welding the super-large tube plate, the method of welding the tube plate in the forward and reverse sequence can control the residual angular deformation after welding, significantly reduce the maximum residual angular deformation after welding, reduce the risk of inaccurate joint position and assembly error after the tube plate is welded, and thus improve the welding quality and pressure-bearing capacity of the tube plate.

[0067] Of course, the above description is not a limitation of the present application, and the present application is not limited to the above examples, and the changes, modifications, additions or replacements made by the person skilled in the art within the essential scope of the present application shall also belong to the protection scope of the present application.​

Claims

1. A method for controlling welding deformation of ultra-large tube sheets based on forward and reverse welding sequence, wherein the ultra-large tube sheet has a diameter of 5000mm to 10000mm and a thickness of 60mm to 400mm, characterized in that, The method comprises the following steps: S1, establishing a finite element model of the tube plate butt welding; A two-dimensional finite element model of the super-large tube plate is established, and an asymmetric double-U groove is designed: the groove radius is 10mm-12mm, the groove angle is 3°-5°, the depth of the forward groove is different from that of the reverse groove, the depth of the upper groove is 80mm-100mm longer than that of the lower groove, the width of the groove is 24mm-28mm, the distance between the root of the upper groove and the lower groove is 10mm-12mm, the total number of turns is 10-50, and the number of welding beads in the whole welding process is 10-50; S2, checking the bearing strength; The geometric center of the tube plate is calculated, the peak value position of the bending moment is determined, the peak bending moment of the tube plate is calculated by using the four-point bending method, the bearing strength required for the tube plate turning is checked, and the minimum thickness of the first welding is determined; S3, the influence of the number of turns on the angular deformation; The number of welding beads of each forward and reverse filling welding is changed, the total number of welding beads is kept the same, the number of turns of the tube plate is changed, and the influence of the number of turns on the post-weld residual angular deformation is compared; S4, the influence of the number of welding beads on the angular deformation; The thickness of each forward and reverse filling welding is changed, the number of turns is kept the same, the number of welding passes is changed, and the influence of the number of welding passes on the post-weld residual angular deformation is compared; S5, fitting the relationship between the post-weld residual angular deformation and the number of turns and the number of welding beads according to the data results, comprehensively considering the influence of the number of turns and the number of welding beads on the post-weld residual angular deformation, and forming a super-large tube plate butt welding deformation control process; In step S2, the geometrical center of gravity of the tube sheet : ; Peak position of bending moment : ; peak bending moment : ; The method for verifying the load strength is: ; wherein, is the tube sheet diameter, is the tube sheet radius, is the tube sheet gravity, is the minimum thickness of the first weld; The welding process used in steps S3 and S4 is: Preheating treatment is performed before welding, CO2 gas shielded welding or argon arc welding is used to ensure root penetration; Then, automatic submerged arc welding is used for forward and reverse alternating filling welding of the tube plate, the thickness of each layer is controlled to avoid slag inclusion and porosity; During the welding process, ensure that the welding bead is fully penetrated to avoid incomplete fusion, the groove on the back of the weld needs to be continuously preheated and the preheating temperature needs to be kept constant to avoid deformation caused by uneven local cooling speed; Finally, one pass of cover welding is performed on the front and back surfaces respectively to modify the weld surface and control the excess height to be less than or equal to 2mm; After welding, the tube plate is placed in a heat treatment furnace for leveling and heat treatment, and finally the weld and base material are polished to be flush, and UT / MT detection is performed every 10 passes during welding.

2. The ultra-large tube sheet tailor-welded deformation control method based on positive and negative order welding according to claim 1, characterized by, Uniform preheating is performed within 200mm range on both sides of the groove of the front surface and the back surface of the tube plate before welding.

Images