Tube-tube plate low-stress welding expansion forming method for large tube type heat exchanger

Abstract

The invention belongs to the technical field of heat exchanger manufacturing, and relates to a low-stress welding expansion forming method for a tube-tube plate of a large tubular heat exchanger. The method comprises the steps of preparation before welding, weld reinforcement size design, connection of a pipe and a pipe plate, welding quality evaluation, adhesion and expansion load determination and implementation and the like. The preparation before welding relates to a groove form, surface pretreatment, preheating and the like; the influence of the welding seam size on residual stress and mechanical strength is disclosed, and a surplus height design method based on residual stress control and meeting the strength requirement is provided so as to reduce stress concentration; carrying out full penetration strength welding on the tube and the tube plate by adopting automatic argon arc welding; the welding quality is ensured through nondestructive testing; hydraulic expansion connection is adopted in the adhesion expansion process, the influence of expansion connection loads on residual stress and sealing performance is revealed, an optimal expansion connection load design method is provided, and meanwhile the pipe expansion rate is evaluated; through the welding expansion method, the risk of stress corrosion cracking is reduced, the service life of equipment is prolonged, and the reliability of the large tubular heat exchanger is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of heat exchanger manufacturing, and specifically discloses a low-stress welding and expansion forming method for tubes and tube sheets of a large shell-and-tube heat exchanger. Background Art

[0002] In industrial applications, large shell-and-tube heat exchangers are widely used due to their excellent corrosion resistance and heat conduction efficiency. Typical ones include equipment such as ethylene oxide reactors, EO / EG recycle gas coolers, C2 reactors, floating head heat exchangers, and packed column heat exchangers. As heat exchangers develop towards larger sizes, the tube sheet diameter reaches 7 - 9 meters. After surfacing, tens of thousands of heat exchange tubes need to be welded. The operation difficulty of connecting the tubes and the tube sheet is large, and the quality of the welded joints and expansion joints has an extremely important impact on the safe and stable operation of the reactor.

[0003] Strength welding + caulking is one of the common connection methods for tubes and tube sheets of large shell-and-tube heat exchangers. The following difficulties are faced during the connection process: the welding grooves are small and dense, it is not easy to penetrate the root of the weld, and the tube wall is easy to be welded through; the caulking accuracy requirements are high. Too large or too small will affect the sealing effect and mechanical strength, and it is easy to cause relatively large residual stresses, which puts forward higher technical requirements and stricter quality control standards for the manufacture of large shell-and-tube heat exchangers. At present, there is no clear standard for determining the weld size, which will affect the welding residual stress and deformation. However, the existing methods do not consider this problem. The welding residual stress and deformation will seriously affect the service performance of the equipment and reduce the service life, and are prone to induce stress corrosion cracking phenomena. In addition, the magnitude of the caulking load during the caulking process after welding the tubes and the tube sheet is also crucial. The traditional process only focuses on the caulking rate. In fact, due to the plastic deformation generated during the caulking process, the residual stress can be released, but there is no method to determine the appropriate caulking load. If the caulking load is too small, the stress elimination effect is poor, and gaps are likely to appear, leading to failure problems such as crevice corrosion; if the caulking load is too large, plastic deformation is likely to occur, and the component is prone to work hardening effect, the brittleness sensitivity increases, and it is easy to crack. Summary of the Invention

[0004] In order to solve the above problems, the present invention discloses a low-stress welding and expansion forming method for tubes and tube sheets of a large shell-and-tube heat exchanger. This method aims to regulate the residual stress after welding, optimizes the welding and expansion process of the tubes and the tube sheet, reduces the residual stress and deformation at the root of the weld, improves the welding and caulking quality, and reduces the risk of failure problems such as stress corrosion.

[0005] The present invention includes the following technical solutions:

[0006] A low-stress welding and expansion forming method for tubes and tube sheets of a large shell-and-tube heat exchanger, comprising the following steps:

[0007] a) Preparation before welding: The groove form of the tube-sheet adopts a semi-U groove and the tube extension structure. The surface of the tube-sheet groove is inspected by penetration and cleaned. The outer wall of the tube and the tube-sheet holes are ground and polished, rust is removed, and oil stains are wiped off with acetone. The area to be welded and its surrounding area are preheated, and the preheating temperature is 50 - 100 °C;

[0008] b) Design of the weld reinforcement size: Establish a finite element calculation model and method for the welding residual stress of large tube-sheets and tube bundles, and reveal the plastic deformation and residual stress distribution under different weld reinforcement conditions. Based on the fact that when the plastic deformation is the largest, the release effect of the residual stress is the best, so select the weld reinforcement size in the case where the plastic deformation is the largest and the residual stress is the smallest, and meet the weld size requirements of the standard and the requirement of the connection pull-off force, so as to establish a method for designing the weld reinforcement size;

[0009] c) Connection between the tube and the tube-sheet: Two-layer welding is carried out using an automatic argon arc welding machine. The welded joint is a full penetration strength weld. During the welding process, the preheating temperature, interpass temperature, and interlayer temperature are strictly controlled. The intermediate inspection process is fully carried out in accordance with relevant technical documents, regulations, and standards. Slow cooling is carried out after welding.

[0010] d) Welding quality assessment: 100% radiographic testing and 100% penetrant testing are carried out according to the welding procedure qualification standard for pressure-bearing equipment to confirm that there are no internal defects;

[0011] e) Expanding and adhering: Adopt the hydraulic expanding process method, and establish a design method for the expanding and adhering load based on the control of martensite phase and residual stress. On the one hand, the expanding rate after expanding and adhering should meet the quality requirements. On the other hand, use the plastic deformation generated during the expanding and adhering process to release a part of the residual stress, and avoid excessive expanding load causing shape and phase changes to generate martensite and prevent the occurrence of work hardening effect.

[0012] Furthermore, in the above-mentioned low-stress welding and expanding forming method for the tubes and tube-sheets of a large shell-and-tube heat exchanger, in step b), the weld size standard for the strength weld of the heat exchanger tube-sheet is specified in GB / T 151 - 2014, and the fillet weld height meets the requirement of the connection pull-off force for the connection between the tube and the tube-sheet in Article 7.4.7 of this standard, that is where [q] is the allowable pull-off force, MPa; are the allowable stresses of the tube material and the tube-sheet material at the design temperature, MPa, respectively, and it should be ensured that the fillet weld height ≥ the tube diameter.

[0013] Furthermore, in the above-mentioned low-stress welding and expanding forming method for the tubes and tube-sheets of a large shell-and-tube heat exchanger, in step e), the method for establishing the design method for the expanding and adhering load based on the control of martensite phase and residual stress includes the following steps:

[0014] 1) Determine the critical maximum value of the available expansion load: If the heat exchange tube material contains an austenite phase, establish a finite element calculation model and method for surface expansion, reveal the martensite formation law during the deformation and phase transformation process under the expansion load, and use the critical expansion load value when no deformation and phase transformation occur exactly, that is, the content of newly formed martensite is 0, as the maximum value of the expansion load range selected in the subsequent simulation process;

[0015] 2) Use the finite element software ABAQUS to simulate the plastic deformation and residual stress distribution at the root of the weld under different expansion loads, select the expansion load in the case of the maximum plastic deformation and the minimum residual stress for expansion, and calculate the expansion rate after expansion. The calculation formula for the expansion rate is:

[0016]

[0017] Among them, η is the expansion rate, d2 is the inner diameter of the tube after expansion, in mm, d1 is the inner diameter of the tube before expansion, in mm, a is the clearance between the tube and the tube plate hole before expansion, in mm, and δ is the wall thickness of the tube before expansion, in mm.

[0018] Furthermore, for the above-mentioned method for low-stress welding and expansion forming of tubes and tube sheets of a large shell-and-tube heat exchanger, for tubes made of carbon steel or stainless steel, the surface expansion rate value η = 1.5 - 3%, calculate the expansion rate after expansion, and if it is within the range, it meets the requirements.

[0019] The present invention also discloses the application of the above-mentioned welding and expansion forming method in the manufacture of large shell-and-tube heat exchangers.

[0020] The present invention also discloses a large shell-and-tube heat exchanger manufactured by the welding and expansion forming method described in any one of the above.

[0021] Compared with the prior art, the present invention has the following outstanding beneficial effects:

[0022] The present invention discloses a method for low-stress welding and expansion forming of tubes and tube sheets of a large shell-and-tube heat exchanger. In the dimension design of the welded joint, select an appropriate weld reinforcement height to reduce the concentration of welding residual stress, and regulate the residual stress on the basis of meeting the weld size standard and strength requirements to improve the welding quality. During the surface expansion process, select an expansion load that can release the post-weld residual stress as much as possible. Regulate the residual stress on the basis of meeting the expansion process evaluation to improve the expansion quality.

[0023] Specifically, the beneficial effects are as follows:

[0024] (1) Improve the tube expansion process to ensure machining accuracy. On the one hand, it can avoid too small expansion load and reduce the probability of failure problems such as stress corrosion cracking caused by the gap between the tube and the tube sheet. On the other hand, it can avoid too large expansion load, reduce the occurrence of deformation and phase transformation, thereby reducing the martensite content and avoiding the occurrence of work hardening effect.

[0025] (2) Reasonably utilize the plastic deformation generated by the expansion load to release the residual stress at the root of the tube-to-tube sheet weld, reduce the risk of stress corrosion cracking, and thus improve the service life of the equipment.

[0026] (3) Reasonably design the size of the weld reinforcement to ensure that the expansion load generates sufficient plastic deformation to release the residual stress, and at the same time ensure the strength and load-bearing capacity of the welded joint, thereby improving the reliability of the equipment and ensuring safety. Description of the Drawings

[0027] Figure 1 Schematic diagram of the tube and the tube sheet;

[0028] Figure 2 Contour map of the plastic strain distribution at the root of the welded joint after welding and tube expansion;

[0029] Figure 3 Contour map of the residual stress distribution of the welded joint after welding and tube expansion;

[0030] Figure 4 Circumferential residual stress curve of the center line of the welded joint after welding and tube expansion. Detailed Implementation Modes

[0031] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0032] Example 1

[0033] This example discloses a low-stress welding and expansion forming method for the tube-tube sheet of a large shell-and-tube heat exchanger. The material of the tube sheet is SA508-3, the surfacing layer material is 309L + 308L, the tube material is S31803, the welding material between the tube and the tube sheet is 309Mo, the tube diameter is 44.45 mm, the gap width between the tube and the tube sheet is 0.25 mm, the surfacing layer thickness is 5 mm, the tube expansion length is 214 mm, and the part 2-3 mm away from the tube end is not expanded. The tube-to-tube sheet welding process and the tube expansion process specifically include the following steps:

[0034] 1. Preparation before welding

[0035] (1) The groove form of the tube and tube sheet is a semi-U groove and the tube extension structure.

[0036] (2) Conduct penetrant inspection on the surface of the tube sheet groove and clean it. Use a semi-automatic polishing disc to polish the outer wall of the tube and the tube sheet holes. It is necessary to thoroughly remove rust and wipe off oil stains with acetone to prevent contamination and ensure a high level of cleanliness of the tube and tube sheet before welding, preventing the generation of pores and cracks during the welding process.

[0037] (3) Preheat the area to be welded and its surrounding area, with the preheating temperature being 50 - 100 °C.

[0038] 2. Design of the weld reinforcement size

[0039] (1) Weld size and connection pull-off force assessment: According to GB / T 151 - 2014, a fillet weld size of 4 mm meets the requirements of the pull-off force of the tube and tube sheet connection in Article 7.4.7 of this standard and is greater than the tube wall thickness of 3 mm.

[0040] (2) Simulation: Use the finite element software ABAQUS to select a value every 0.1 mm between 1 - 2 mm as the weld reinforcement size for simulation. Control the weld reinforcement size as a variable, with other parameters such as the tube and tube sheet sizes and welding process remaining unchanged. Simulate the tube and tube sheet welding process in Example 1, and select the weld reinforcement in the case with the largest plastic deformation and the smallest residual stress as the final size.

[0041] In summary, when the weld reinforcement is 1.5 mm, the final plastic deformation value obtained is the largest and meets the standard requirements. Therefore, the weld reinforcement size is selected as 1.5 mm.

[0042] 3. Connection between the tube and the tube sheet

[0043] The welding of the tube and tube sheet joint is carried out in two layers using an automatic argon arc welding machine, with one pass of autogenous welding and one pass of filler wire addition. The welded joint is a full penetration strength weld.

[0044] (1) For the first layer, autogenous welding without filler wire is carried out. The welding parameters are:

[0045] No welding material, the current polarity is DCEN, the welding current is 95 - 100 A, the welding voltage is 15 V, the welding speed is 3 cm / min, the shielding gas is 99.999% Ar, the gas flow rate is 12 - 15 L / min, and the interlayer temperature is 35 °C;

[0046] (2) For the second layer, welding with filler wire is carried out. The welding material is S31803, and the welding parameters are:

[0047] The current polarity is DCEN, the welding current is 100 - 105 A, the welding voltage is 16 - 17 V, the welding speed is 5 cm / min, the shielding gas is 99.999% Ar, the gas flow rate is 18 L / min, and the interpass temperature is 120 °C.

[0048] 4. Welding Quality Assessment

[0049] Perform 100% RT and 100% PT non-destructive testing in accordance with NB / T 47014-2011 "Welding Procedure Qualification for Pressure Equipment" to confirm no internal defects.

[0050] 5. Expanding and Swaging

[0051] (1) Adopt the hydraulic expanding and swaging process method

[0052] (2) Determination of the expanding and swaging load

[0053] First, use the finite element software ABAQUS to change the expanding and swaging load magnitude to simulate multiple groups of expanding and swaging processes, and analyze the change in martensite content to obtain the maximum expanding and swaging load when the content of newly formed martensite is 0, which is 500 MPa.

[0054] Use the finite element software ABAQUS to select a value every 50 MPa between 200 - 500 MPa as the expanding and swaging load for simulation. Ensure that the tube-to-tube sheet dimensions, gaps, and the welding process before expanding and swaging remain unchanged, and control the expanding and swaging load as a variable to obtain the final plastic deformation and residual stress results. Through comparison, select the expanding and swaging load magnitude when the plastic deformation is the largest and the residual stress is the smallest. Therefore, the expanding and swaging load is determined to be 400 MPa.

[0055] (3) Assessment of the expanding ratio

[0056] The inner diameters of the tube before and after expanding and swaging are 44.45 mm and 44.81 mm respectively. The gap between the tube and the tube sheet hole before expanding and swaging is 0.25 mm, and the wall thickness of the tube before expanding and swaging is 3 mm. It can be calculated that the expanding ratio is 1.83%, which meets the requirements.

[0057] Test Example

[0058] Conduct finite element simulation on Example 1 to analyze the plastic strain and residual stress distribution after welding and after expanding and swaging of the welded joint. Figure 2 It can be seen that the peak value of the plastic strain after welding of the welded joint in the example is approximately 0.58, and the peak value of the plastic strain after expanding and swaging is approximately 0.85, both of which are located at the weld root. The process of increasing plastic strain will be accompanied by the release of a part of the residual stress after welding, achieving the effect of regulating the residual stress.

[0059] From Figure 3It can be seen that the peak values of the residual stresses at the weld root after welding in the embodiment (before expansion joint): the radial stress is 399.1 MPa, the axial stress is 256.5 MPa, and the circumferential stress is 359.1 MPa. The peak values of the residual stresses at the weld root after expansion joint: the radial stress is -329.6 MPa, the axial stress is -217.7 MPa, and the circumferential stress is -348.1 MPa. The magnitudes of the peak values of the radial, axial, and circumferential stresses all decrease, and the stress state changes from tensile stress to compressive stress, which can offset part of the tensile stress caused by the external load during the service state, thereby extending the service life of the equipment. Conduct a specific analysis of the circumferential residual stress among them. From Figure 4 It can be seen the changing trend of the circumferential residual stress at the center line of the welded joint after welding and after expansion joint in the embodiment. After welding, the circumferential residual stress of the welded joint changes gently, and it is overall tensile stress and the magnitude is concentrated around 290 MPa; after expansion joint, the state of the circumferential residual stress has changed significantly. At the weld root, it changes from tensile stress to compressive stress, and gradually increases along the path to a smaller tensile stress, and the overall magnitude of the stress decreases.

[0060] From the above embodiments and test examples, it can be seen that the method of the present invention can well reduce the residual stress at the weld root, ensure the bearing capacity of the welded joint, improve the strength of the welded joint, optimize the expansion joint process, ensure the processing accuracy, thereby reducing the occurrence of failure problems such as stress corrosion cracking, reducing the occurrence of deformation and phase transformation, thereby reducing the martensite content, avoiding the occurrence of work hardening effect, extending the service life, and improving the reliability of the manufacture and operation of the heat exchanger.

[0061] The above are only several limited preferred embodiments of the present invention, and the description thereof is relatively specific and detailed, but it should not be construed as a limitation to the scope of the patent of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention.

Claims

1. A low-stress welding and expanding forming method for tubes and tube sheets of a large shell-and-tube heat exchanger, characterized in that, It includes the following steps: a) Preparation before welding: The groove form of the tube and tube sheet adopts a semi-U groove and the tube extension structure. The surface of the tube sheet groove is inspected by penetration and cleaned. The outer wall of the tube and the tube sheet holes are ground and polished, rust is removed, and oil stains are wiped off with acetone. The area to be welded and its surrounding area are preheated, and the preheating temperature is 50 - 100 °C; b) Design of the weld reinforcement size: Establish a finite element calculation model and method for the welding residual stress of the large tube sheet - tube bundle, reveal the plastic deformation and residual stress distribution under different weld reinforcement conditions, select the weld reinforcement size in the case with the largest plastic deformation and the smallest residual stress, and meet the weld size requirements of the standard and the requirement of the connection pull-off force, so as to establish a method for designing the weld reinforcement size; c) Connection of the tube and the tube sheet: Two-layer welding is carried out using an automatic argon arc welding machine. The welded joint is a full penetration strength weld, and slow cooling is carried out after welding; d) Welding quality assessment: 100% radiographic testing and 100% penetrant testing are carried out according to the welding procedure qualification standard for pressure-bearing equipment to confirm no internal defects; e) Expanding and fitting: The hydraulic expanding process method is adopted, and a method for designing the expanding and fitting load based on the control of martensite phase and residual stress is established.

2. A low-stress welding and expanding forming method for tubes and tube sheets of a large shell-and-tube heat exchanger according to claim 1, characterized in that In step b), the weld size standard for the strength weld of the heat exchanger tube and tube sheet is specified in GB / T 151-2014. The fillet weld height meets the requirement of the pull-off force for the connection between the tube and the tube sheet in Article 7.4.7 of this standard, that is where [q] is the allowable pull-off force, MPa; are the allowable stresses of the tube material and the tube sheet material at the design temperature, MPa, respectively, and it should be ensured that the fillet weld height ≥ the tube diameter.

3. A method for low-stress welding and expanding forming of tubes and tube sheets of a large shell-and-tube heat exchanger according to claim 1, characterized in that In step e), the method for designing the expanding and fitting load based on the control of martensite phase and residual stress includes the following steps: 1) Determine the critical maximum value of the available expanding load: If the heat exchange tube material contains austenite phase, establish a finite element calculation model and method for expanding and fitting, reveal the martensite formation law during the deformation and phase transformation process under the expanding load, and take the critical expanding load value when no deformation and phase transformation occur exactly, that is, the new martensite content is 0, as the maximum value of the expanding load range selected in the subsequent simulation process; 2) Use the finite element software ABAQUS to simulate the plastic deformation and residual stress distribution at the weld root under different expanding loads, select the expanding load in the case with the largest plastic deformation and the smallest residual stress for expanding, and calculate the expanding rate after expanding. The calculation formula for the expanding rate is: where η is the expanding rate, d2 is the inner diameter of the tube after expansion, in mm, d1 is the inner diameter of the tube before expansion, in mm, a is the clearance between the tube and the tube sheet hole before expansion, in mm, and δ is the wall thickness of the tube before expansion, in mm.

4. A low-stress welding and expanding forming method for tubes and tube sheets of a large shell-and-tube heat exchanger according to claim 3, characterized in that, For tubes made of carbon steel or stainless steel, the expanding rate value η of expanding and fitting is 1.5 - 3%, and the expanding rate is calculated after expanding. If it is within the range, it meets the requirements.

5. A method for low-stress welding and expanding forming of tubes and tube sheets of a large shell-and-tube heat exchanger according to claim 1, characterized in that, The welding procedure qualification standard for pressure-bearing equipment in step d) is NB / T47014 - 2011 "Welding Procedure Qualification for Pressure-bearing Equipment".

6. Application of the welding and expanding forming method according to any one of claims 1 - 5 in the manufacture of large shell-and-tube heat exchangers.

7. A large shell-and-tube heat exchanger, characterized in that, Manufactured by the welding and expanding forming method according to any one of claims 1 - 5.

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