A low-stress welding method for tube-tubesheet joints

Abstract

The present invention relates to the technical field of petrochemical equipment manufacturing, and particularly relates to a low-stress welding method for tube-tubesheet joints, including a pre-positioning step, a zoning step, a heating step, a stress measurement and control step, a welding step, and a heat removal step. By dividing the joint to be welded into multiple groups of welding zones, the multiple heat exchange tubes in each group of welding zones are temperature-controlled simultaneously, with fewer circuit paths, not easily confused, and convenient for operation. By using a laser flatness measuring instrument arranged beside the second tubesheet to measure the flatness of the outer side surface of the second tubesheet to indirectly feedback the thermal elongation of the heat exchange tubes in each welding group, there is no need to install a linear displacement sensor on the side wall of the heat exchange tube as in the prior art, avoiding the problem of high installation difficulty of the elongation measurement device, and the disassembly and assembly are relatively simple. The electronic components such as the laser flatness measuring instrument are separated from the heat exchange tubes, avoiding the influence of the high temperature of the heat exchange tubes on the measurement accuracy of the electronic components, prolonging their service life, and saving costs.

Description

Technical Field

[0001] The present invention relates to the technical field of petrochemical equipment manufacturing, and specifically relates to a low-stress welding method for tube-to-tubesheet joints. Background Art

[0002] In chemical equipment, shell-and-tube heat exchangers are an important type of equipment, and the welded joints between the tubesheet and the heat exchange tubes are the most important and most prone to failure in this equipment. Their quality directly affects the service life of the equipment and the stability of the device. Therefore, the welding between the heat exchange tubes and the tubesheet is particularly important.

[0003] Chinese patent document with publication number CN102151958A discloses a welding method for the welded joints between the tubesheet and the heat exchange tubes of a heat exchanger. In the assembly of the heat exchange tubes and the tubesheet, a structure that recesses 0.5 - 1 mm from the surface of the tubesheet is adopted. In the welding process, a bottom-up welding sequence is used, and a "3"-shaped or "S"-shaped welding path is adopted. During welding, each heat exchange tube is welded for half a circle. After welding 8 - 10 heat exchange tubes without extinguishing the arc at one time, the arc is extinguished on the tubesheet; then, in the same way, the other half circle is welded from bottom to top. This can reduce the number of arc starting and stopping times, improve the welding efficiency, and reduce the number of arc starting and stopping points, thereby reducing the probability of welding defects. In this way, the tungsten electrode can easily penetrate into the root of the angle between the heat exchange tube and the tubesheet during welding. During autogenous welding, the arc can reach the sharp corner position, and the root can be completely penetrated without leaving any cavities, preventing the retention of shell-side corrosion media, avoiding crevice corrosion, and improving the service life of the weld.

[0004] The quench heat exchanger can rapidly cool the pyrolysis feedstock after high-temperature pyrolysis, thereby preventing the occurrence of secondary reactions. The quench heat exchanger is one of the key equipment of the pyrolysis furnace. On the pyrolysis furnace, the quench heat exchanger is usually arranged with a single-stage quench heat exchanger or a multi-stage quench heat exchanger. Single-stage means only one stage of heat exchange, and multi-stage means two or even three stages of heat exchange. The single-stage quench heat exchanger has the characteristics of low pressure drop and simple layout; the multi-stage quench heat exchanger has the characteristics of high ethylene yield, rapid termination of secondary reactions, and more ultra-high pressure steam recovery. Among them, the two-stage quench heat exchanger is generally a plate-type quench heat exchanger, which is mainly composed of a pyrolysis gas inlet header, a quench heat exchanger shell, a pyrolysis gas outlet header, heat exchange tubes, a front tube sheet, a rear tube sheet, baffle plates, riser pipe joints, downcomer pipe joints, etc. There are multiple heat exchange tubes in the quench heat exchanger shell. The quench heat exchanger shell and the heat exchange tubes are connected by the front and rear tube sheets. During operation, high-temperature pyrolysis gas is introduced into the heat exchange tubes, and a cooling medium is introduced between the heat exchange tubes and the quench heat exchanger shell. Under the action of the cooling medium, the rapid cooling of the high-temperature pyrolysis gas is achieved. At high temperatures, the heat exchange tubes and the quench heat exchanger shell will expand and elongate. However, due to the large temperature difference between the media in the heat exchange tubes and the quench heat exchanger shell, the elongation of the heat exchange tubes will be greater than that of the quench heat exchanger shell. On the other hand, there is also a temperature difference between multiple heat exchange tubes, which is likely to cause a large temperature stress at the connection position between the heat exchange tubes and the front and rear tube sheets, resulting in cracks in the heat exchange tubes and even fractures of the heat exchange tubes.

[0005] To avoid cracks or fractures in the heat exchange tubes, it is necessary to prestretch the heat exchange tubes to reduce the elongation difference between the heat exchange tubes and the quench heat exchanger shell. Currently, the main method for prestretching the heat exchange tubes is to blow hot air into all the heat exchange tubes, heat all the heat exchange tubes simultaneously to achieve pre-stretching of the heat exchange tubes, and then weld the heat exchange tubes. This method is simple to implement, but the temperature control accuracy is not high, and the temperature between each heat exchange tube is uneven. Another method is to insert a heating tooling into the inner hole of each heat exchange tube. However, this method is only used when the number of heat exchange tubes is small. For example, the quench heat exchanger prestretching heating device disclosed in the Chinese invention patent with the patent number CN102489908A is a prestretching heating device applied to a single heat exchange tube. If the number of heat exchange tubes that require simultaneous heating and elongation reaches a certain number, such as 60, the temperature between the heat exchange tubes will interfere with each other. Especially the heat of the heat exchange tubes located below will be transferred upward, affecting the temperature of the heat exchange tubes above. Therefore, if the above-mentioned quench heat exchanger prestretching heating device is used to simultaneously prestretch multiple heat exchange tubes, the prestretching elongation will deviate.

[0006] Subsequently, the applicant developed a cluster-type prestressed elongation heating system for a quench heat exchanger, as disclosed in the Chinese patent document with the publication number CN109338077A, which includes an intelligent centralized control module, multiple control modules, multiple heating tools, and multiple elongation measuring devices. The intelligent centralized control module is respectively connected to each control module, and each control module is respectively connected to a heating tool and an elongation measuring device. The intelligent centralized control module commands each control module to respectively control the heating of the heating tool connected thereto, so that each heat exchange tube into which the heating tool is inserted is heated and elongated, and commands each control module to precisely control the heat generation amount of each heating tool according to the elongation data feedback by the elongation measuring device, so as to precisely control the elongation amount of each heat exchange tube into which the heating tool is inserted, so that the elongation amount of each heat exchange tube into which the heating tool is inserted can be precisely controlled, thereby avoiding deviation of the prestressed elongation amount of the heat exchange tube.

[0007] Furthermore, it is found in the application that the above-mentioned prior art controls the heat generation amount of the heating tool by measuring the elongation amount of each heated heat exchange tube, and each heating tool is respectively inserted into the interior of each heat exchange tube, that is, each heat exchange tube is plugged with a heating tool. Thus, there are areas for improvement as follows:

[0008] I. When the number of heat exchange tubes corresponding to the same tube sheet exceeds sixty, or even hundreds, each heat exchange tube is connected with a control module, a heating tool, and an elongation measuring device, and the number of circuit paths is relatively large, and the operation is prone to confusion;

[0009] II. The heat exchange tubes are densely distributed in a cluster, and it is difficult to measure the thermal elongation amount of the heat exchange tubes in the middle;

[0010] III. The temperature of the heat exchange tubes after being heated and elongated is relatively high, especially the temperature in the middle is more concentrated, which has a greater impact on the measurement accuracy of the electronic measurement components, and the high temperature affects the service life of the measurement components. Summary of the Invention

[0011] In view of the above-mentioned all or part of the technical problems existing in the prior art, the present invention provides a low-stress welding method for tube-tube sheet joints.

[0012] To achieve the above object, the present invention provides the following technical solutions:

[0013] Provide a low-stress welding method for tube-tube sheet joints, including the following steps:

[0014] A pre-positioning step, according to the length of the heat exchange tube, fixedly mounting the first tube sheet and the second tube sheet side by side on the tooling, one end of multiple heat exchange tubes is respectively inserted and welded and fixed in multiple tube holes of the first tube sheet, and the other end of multiple heat exchange tubes is respectively movably inserted into multiple tube holes of the second tube sheet;

[0015] Partitioning step: An identification mark is set on the outer side of the second tube sheet, and multiple weld joint areas formed by the ends of multiple heat exchange tubes and multiple tube holes are divided into multiple groups of welding areas according to a preset quantity.

[0016] Heating step: Taking each group as a unit, multiple groups of heat exchange tubes in multiple groups of welding areas are respectively heated by multiple groups of heating devices, so that the multiple heat exchange tubes thermally expand in a direction away from the first tube sheet.

[0017] Stress measurement and control step: The heating device is electrically connected to a control module and a laser flatness measuring instrument. The laser flatness measuring instrument is located beside the outer side of the second tube sheet and is used to measure the flatness of the outer side surface of the second tube sheet in real time. The control module controls the heat output of each heating device according to the measured flatness change signal, so as to control the flatness of the outer side surface of the second tube sheet to be stable within 0.01 - 0.8 mm.

[0018] Welding step: After the flatness of the outer side surface of the second tube sheet is stable, the weld joint areas of each group of welding areas are successively welded and fixed. And before welding the next group of welding areas, the above stress measurement and control step is re-implemented.

[0019] Heat removal step: After all the welding areas are completely welded, the multiple groups of heating devices stop heating the heat exchange tubes.

[0020] As a further optional solution, in the stress measurement and control step, the flatness of the outer side surface of the second tube sheet is controlled to be ≤ 0.5 mm.

[0021] As a further optional solution, the heating device is an electric heating rod inserted into multiple heat exchange tubes from the outer side of the first tube sheet. Multiple electric heating rods in the same welding area are electrically connected to the same control module to control their synchronous temperature rise and fall.

[0022] As a further optional solution, the heating device further includes a support plate. Multiple electric heating rods corresponding to the same welding area are fixedly arranged side by side on the support plate, so that multiple electric heating rods in the same welding area can be synchronously inserted into the corresponding heat exchange tubes. The support plate is fitted and positioned with the outer side surface of the first tube sheet.

[0023] Specifically, a guiding support protrusion matching the inner wall of the heat exchange tube is arranged on the outer side of the electric heating rod.

[0024] Specifically, the arrangement mode of multiple electric heating rods on the support plate is adjustable, and it is realized as follows: The support plate is provided with multiple positioning holes, the number of positioning holes is greater than the number of electric heating rods, and the electric heating rods can be selectively and detachably connected to different positioning holes; or the support plate is provided with multiple long strip holes, and the ends of multiple electric heating rods can be installed in different long strip holes in a loose and tight manner.

[0025] As a further optional solution, in the partitioning step, the identification mark is a pattern, shape or number provided on the outer side surface of the second tube sheet.

[0026] As a further optional solution, the heating device is a flexible electric blanket that wraps multiple heat exchange tubes corresponding to each welding area.

[0027] Specifically, the electric blanket includes an inner heating core and an outer heat insulation layer, and the heat insulation layer is used to block the heat transfer of the electric blankets corresponding to different welding areas.

[0028] Specifically, the electric blanket includes a blanket body for wrapping multiple heat exchange tubes and a traction part extending outside all the heat exchange tubes. Pulling the traction part with an external force can pull the electric blanket away from the heat exchange tubes.

[0029] Advantages of the present invention:

[0030] A low-stress welding method for tube-tube sheet joints of the present invention has the following advantages compared with the prior art:

[0031] First, by dividing the joint to be welded into multiple welding areas, the temperature of multiple heat exchange tubes in each welding area is controlled simultaneously. The number of circuit paths is small, not easily confused, and convenient for operation;

[0032] Second, by using a laser flatness measuring instrument arranged beside the second tube sheet to measure the flatness of the outer side of the second tube sheet to indirectly feedback the thermal elongation of the heat exchange tubes in each welding group, there is no need to install a linear displacement sensor on the side wall of the heat exchange tube as in the prior art, avoiding the problem of high installation difficulty of the elongation measuring device, and the disassembly and assembly are relatively simple;

[0033] Third, electronic components such as the laser flatness measuring instrument are separated from the heat exchange tubes, avoiding the influence of the high temperature of the heat exchange tubes on the measurement accuracy of the electronic components, extending their service life, and saving costs. Description of the drawings

[0034] Figure 1 It is a schematic diagram of the welding method in the embodiment.

[0035] Figure 2 It is a schematic diagram of the partition of multiple joints to be welded in the embodiment.

[0036] Figure 3 It is a schematic diagram of the heating device in one of the embodiments.

[0037] Figure 4 It is a schematic diagram of the heating device in another embodiment.

[0038] Reference signs:

[0039] Heat exchange tube 1, first tube sheet 2, second tube sheet 3, identifier 4;

[0040] Heating device 5;

[0041] Electric heating rod 51, support plate 52, guiding support protrusion 53;

[0042] Electric blanket 61, heating core 62, heat insulation layer 63, blanket body 64, traction part 65;

[0043] Laser flatness measuring instrument 7. Specific implementation manner

[0044] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Without conflict, the following embodiments and the features in the embodiments may be combined with each other.

[0045] The low-stress welding method of this embodiment is used for welding between the tube bundle and the tube sheet of the shell-and-tube heat exchanger as Figure 1 shown. The tube bundle includes a plurality of heat exchange tubes 1 arranged side by side. Both ends of the plurality of heat exchange tubes 1 are respectively inserted into a plurality of tube holes opened in the first tube sheet 1 and the second tube sheet 2. The right end of the heat exchange tube 1 is Figure 1 welded and fixed to the tube holes of the first tube sheet 2 on the right side in Figure 1 The welding method of this embodiment is applied to Figures 1 to 3 weld the left end of the heat exchange tube 1 in

[0046] to the tube holes of the second tube sheet 3 on the left side. As shown in

[0047] the pre-positioning step: According to the length of the heat exchange tube 1, the first tube sheet 2 and the second tube sheet 3 are fixedly arranged side by side on the tooling. The right ends of the plurality of heat exchange tubes 1 are respectively inserted and welded and fixed to a plurality of tube holes of the first tube sheet 2. The left ends of the plurality of heat exchange tubes 1 are respectively movably inserted into a plurality of tube holes of the second tube sheet 3 as the joints to be welded. The second tube sheet 2 is a flexible tube sheet, and its periphery is provided with an arc-shaped flexible part. Figure 3 The zoning step: An identifier 4 is arranged on the outer side of the second tube sheet 3. The plurality of joints to be welded formed by the ends of the plurality of heat exchange tubes 1 and the plurality of tube holes are divided into multiple welding zones according to a preset quantity. As

[0048] shown by the dotted line, nine groups are divided. Of course, in practice, it can be divided into other quantities according to process requirements. The identifier 4 is a pattern, shape or number arranged on the outer side of the second tube sheet 3. For example, each welding zone is circled, different numbers and shapes are marked for each welding zone (for example, all of one welding zone are marked with triangles, and all of another welding zone are marked with squares, and so on).

[0049] Steps for stress measurement and control: The heating device 5 is electrically connected to a control module and a laser flatness measuring instrument 7. The laser flatness measuring instrument 7 is located on the outer side of the second tube sheet 3 and is used to measure the flatness of the outer side of the second tube sheet 3 in real time. After a software engineer programs the control module, the following is achieved by constructing a computer function module: The control module controls the heat generation amount of each heating device 5 according to the measured flatness change signal to control the flatness of the outer side of the second tube sheet 3 to be stable within 0.01 - 0.8 mm. Preferably, the flatness of the outer side of the second tube sheet 3 is ≤ 0.5 mm. When the outer side of the second tube sheet 3 bulges outwards ( Figure 1 in the leftward direction is considered outwards), it indicates that the thermal elongation of the heat exchange tube 1 corresponding to this welding zone is too large, then the heat generation amount of the corresponding heating device is reduced, and vice versa, the heat generation amount is increased. The heating device 5 controls the heating temperature by controlling the heating power.

[0050] Welding steps: After the flatness of the outer side of the second tube sheet 3 is stable, the weld joints of each group of welding zones are welded and fixed in sequence. And before welding the next group of welding zones, the above stress measurement and control steps are re-implemented. In practice, it can be automatically welded by a robot or manually welded.

[0051] Heat removal steps: After all the welding zones are completely welded, multiple groups of heating devices 5 stop heating the heat exchange tubes 1, and the heating devices 5 are removed.

[0052] It should be noted that the laser flatness measuring instrument 7, also known as the laser leveling instrument, is an instrument for measuring flatness in the prior art and will not be elaborated here. However, it should be emphasized that in the prior art, either each heat exchange tube 1 is heated and welded one by one, which cannot achieve the effect of eliminating the influence of thermal stress; or all heat exchange tubes 1 are heated independently at the same time, which cannot achieve the effects of convenient installation and high detection accuracy. And in this application, the heating of the weld joints to be welded is divided into zones and the thermal elongation of the heat exchange tube 1 is indirectly fed back through the flatness of the second tube sheet 3, achieving the effects of convenient measurement and high accuracy. The two complement each other and are indispensable. If the heating device 5 is not set in zones, the temperature cannot be controlled according to the flatness. If the temperature is controlled according to the flatness without zone heating, the equipment will be more complex. Therefore, the two are inseparable as a whole. According to the overall judgment principle for evaluating creativity, in the field of tube-tube sheet joint welding technology, when the prior art does not disclose both the zonal heating of the weld joints to be welded and the indirect feedback of the thermal elongation of the heat exchange tube 1 through the flatness of the second tube sheet 3, this application has outstanding substantive features and significant progress.

[0053] In this embodiment, the heating device 5 is an electric heating rod 51 inserted into multiple heat exchange tubes 1 from the outside of the first tube sheet 2, that is Figure 1 in the direction from right to left, multiple electric heating rods 51 in the same welding zone are electrically connected to the same control module to control their synchronous temperature rise and fall.

[0054] Specifically, the heating device 5 further includes a support plate 52. A plurality of heating rods 51 corresponding to the same welding zone are fixedly arranged side by side on the support plate 52, so that the plurality of heating rods 51 in the same welding zone can be synchronously inserted into the corresponding heat exchange tubes 1, and the support plate 52 is fitted and positioned on the outer side surface of the first tube sheet 2. Specifically, a guiding and supporting protrusion 53 that matches the inner wall of the heat exchange tube 1 is arranged on the outer side of the heating rod 51, and the guiding and supporting protrusions 53 of adjacent heating rods 51 are arranged staggeredly in the circumferential direction. In practice, the guiding and supporting protrusions 53 can be arranged on only some of the heating rods 51, or can be arranged on all the heating rods 51. Of course, the plurality of heating rods 51 in all welding zones can also use a single support plate 52 at the same time, as long as they can be inserted at one time, but in this case, the heating rods 51 need to be precisely aligned with the ports of the heat exchange tubes 1.

[0055] Optionally, the heating rod 51 can be designed as a telescopic umbrella bone structure to be applicable to heat exchange tubes 1 of different lengths or to change the position of the heat exchange tubes 1.

[0056] Optionally, the heating rod 51 can be designed to be tubular, and a high-temperature resistant water-cooled jacket is arranged inside it for circulating refrigerant to quickly adjust the temperature of the heating rod 51.

[0057] In practice, a heat insulation blanket can be provided to wrap the plurality of heat exchange tubes corresponding to each group of welding zones to avoid the temperature influence between the heat exchange tubes of different welding zones and make it more convenient to control the temperature.

[0058] Specifically, the arrangement of the plurality of heating rods 51 on the support plate 52 is adjustable. According to different division methods, the installation positions of the heating rods 51 are adjusted so that the heating rods 51 are applicable to the arrangement modes of the heat exchange tubes 1 of different heat exchangers. It is realized as follows: The support plate 52 is provided with a plurality of positioning holes, the number of the positioning holes is greater than the number of the heating rods 51, and the heating rods 51 can be selectively and detachably connected to different positioning holes, such as threaded connection, snap connection, interference fit, etc. Or the support plate 52 is provided with a plurality of long holes, and the ends of the plurality of heating rods 51 can be loosely installed in different long holes, and after adjusting the heating rods 51 to a preset position along the length direction of the long holes, they can be fixed.

[0059] Another embodiment of the heating device 5 is as Figure 4 shown, which is a flexible electric heating blanket 61 that wraps the plurality of heat exchange tubes 1 corresponding to each group of welding zones. Before the pre-positioning step, when welding and fixing the plurality of heat exchange tubes 1 to the first tube sheet 2, after welding the heat exchange tubes 1 corresponding to the welding zone to the first tube sheet 2, the electric heating blanket 61 is wrapped outside the plurality of heat exchange tubes 1 corresponding to the welding zone.

[0060] Specifically, the electric blanket 61 includes an inner heating core 62 and an outer heat insulation layer 63. The heat insulation layer 63 is used to block the heat transfer of the electric blanket 61 corresponding to different welding areas, reduce the mutual interference effect of the temperatures of different welding areas, and make the temperature more controllable. The heating core 62 is in close contact with each heat exchange tube 1 corresponding to the welding area, and the heating wire of the heating core 62 is attached to the outer wall of the heat exchange tube 1 to heat it.

[0061] Specifically, the electric blanket 61 includes a blanket body 64 for wrapping a plurality of heat exchange tubes 1 and a traction part 65 extending outside all the heat exchange tubes 1. The traction part 65 does not generate heat, and it extends out of the manhole of the shell-side housing. After heat dissipation, pulling the traction part 65 externally can pull the electric blanket 61 away from the heat exchange tubes 1.

[0062] In the description of the present invention, obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. The components of the embodiments of the present invention usually described and illustrated in the drawings here can be arranged and designed in various different configurations.

[0063] Therefore, the above detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed present invention, but only represents the selected embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0064] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "in", "upper", "lower", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of this invention is usually placed during use. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation to the present invention. In addition, the terms "first", "second", etc. are only used for descriptive distinction and cannot be understood as indicating or implying relative importance.

[0065] In the description of the present invention, it should be noted that unless otherwise clearly specified and limited, the terms "set", "connected", "connected to" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection. It can be a mechanical connection or an electrical connection. It can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

Claims

1. A low-stress welding method for tube-to-tubesheet joints, characterized in that: The method includes the following steps: Pre-positioning step: According to the length of the heat exchange tubes, the first tube sheet and the second tube sheet are fixedly arranged side by side on the tooling. One end of each of the multiple heat exchange tubes is inserted and welded to the corresponding tube holes of the first tube sheet, and the other end of each of the multiple heat exchange tubes is movably inserted into the corresponding tube holes of the second tube sheet; Partitioning step: Markings are set on the outer side of the second tube sheet, and the multiple weld joints formed by the ends of the multiple heat exchange tubes and the multiple tube holes are divided into multiple groups of welding areas according to a preset quantity; Heating step: Taking each group as a unit, the multiple groups of heating devices respectively heat the heat exchange tubes in the multiple groups of welding areas to cause the multiple heat exchange tubes to thermally expand in a direction away from the first tube sheet; Stress measurement and control step: The heating device is electrically connected to a control module and a laser flatness measuring instrument. The laser flatness measuring instrument is located beside the outer side of the second tube sheet and is used to measure the flatness of the outer side surface of the second tube sheet in real time. The control module controls the calorific value of each heating device according to the measured flatness change signal to control the flatness of the outer side surface of the second tube sheet to be stable within 0.01 - 0.8 mm; Welding step: After the flatness of the outer side surface of the second tube sheet is stable, the weld joints in each group of welding areas are welded and fixed in sequence. Before welding the next group of welding areas, the above stress measurement and control step is re-implemented; Heat removal step: After all the welding areas are completely welded, the multiple groups of heating devices stop heating the heat exchange tubes.

2. The low-stress welding method for a tube-tubesheet joint according to claim 1, characterized in that: In the stress measurement and control step, the flatness of the outer side surface of the second tube sheet is controlled to be ≤ 0.5 mm.

3. A low-stress welding method for a tube-tubesheet joint according to claim 1, characterized in that: The heating device is an electric heating rod inserted into the multiple heat exchange tubes from the outer side of the first tube sheet. The multiple electric heating rods in the same welding area are electrically connected to the same control module to control their synchronous temperature rise and fall.

4. A low-stress welding method for a tube-tubesheet joint according to claim 3, characterized in that: The heating device further includes a support plate. The multiple electric heating rods corresponding to the same welding area are fixedly arranged side by side on the support plate so that the multiple electric heating rods in the same welding area can be synchronously inserted into the corresponding heat exchange tubes, and the support plate is fitted and positioned with the outer side surface of the first tube sheet.

5. A low-stress welding method for a tube-tubesheet joint according to claim 4, characterized in that: A guiding and supporting protrusion matching the inner wall of the heat exchange tube is arranged on the outer side of the electric heating rod.

6. A low-stress welding method for a tube-tubesheet joint according to claim 4, characterized in that: The arrangement mode of the multiple electric heating rods on the support plate is adjustable, which is realized as follows: The support plate is provided with a plurality of positioning holes, the number of the positioning holes is greater than the number of the electric heating rods, and the electric heating rods can be selectively and detachably connected to different positioning holes; or the support plate is provided with a plurality of long strip holes, and the ends of the multiple electric heating rods can be installed in different long strip holes in a loose and tight manner.

7. A low-stress welding method for a tube-tubesheet joint according to claim 1, characterized in that: In the partitioning step, the marking is a pattern, shape or number provided on the outer side surface of the second tube sheet.

8. A low-stress welding method for a tube-tubesheet joint according to claim 1, characterized in that: The heating device is a flexible electric heating blanket that wraps the multiple heat exchange tubes corresponding to each group of welding areas.

9. A low-stress welding method for a tube-tubesheet joint according to claim 7, characterized in that: The electric heating blanket includes an inner heating core and an outer heat insulation layer, and the heat insulation layer is used to block the heat transfer between the electric heating blankets corresponding to different welding areas.

10. A low-stress welding method for a tube-tubesheet joint according to claim 7, characterized in that: The electric heating blanket includes a blanket body for wrapping the multiple heat exchange tubes and a traction part extending outside all the heat exchange tubes. By pulling the traction part with an external force, the electric heating blanket can be pulled away from the heat exchange tubes.

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