Spherical tank welding stress reduction process coupling preheating and interlayer temperature control

Abstract

The invention relates to the technical field of pressure vessel manufacturing, and discloses a spherical tank welding stress reduction process with coupling preheating and interlayer temperature control, which comprises the following steps: preheating a to-be-welded area of a spherical shell plate before welding; multi-layer and multi-pass welding is conducted through a small heat input welding method, in the welding process, self-adaptive heat input control is executed based on geometrical characteristics of a standard linear area, a T-shaped intersection area and the like of a weld joint path, and meanwhile the interlayer temperature between all welding passes is strictly controlled; a symmetric welding method and a segmented back welding method are adopted to balance deformation; and finally, a tempering welding bead with low linear energy is welded on the cover surface layer. According to the method, through in-situ, zoning and multi-stage coordinated regulation and control of welding heat input, formation of hard and brittle structures is effectively restrained, the performance of a heat affected zone is optimized, the residual stress peak value of a welding joint, especially a geometric discontinuous region, is remarkably reduced, and the process does not need overall heat treatment after welding, so that the production cost is reduced. The manufacturing period is shortened; and the production cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of pressure vessel manufacturing technology, specifically to a stress reduction process for spherical tank welding that couples preheating and interlayer temperature control. Background Technology

[0002] Spherical tanks, as large storage and transportation equipment widely used in petroleum, chemical and other fields, are typically assembled from multiple high-strength thick steel plates through welding. The quality of the welded joints directly determines the overall safety and service life of the spherical tank. During the thick plate welding process, uneven local heating and subsequent cooling inevitably introduce residual stress into the structure, which may lead to microstructural degradation.

[0003] To control welding stress and deformation, existing technologies typically employ heat input management methods such as preheating and controlling interpass temperature. However, when welding spherical tanks with complex curved surfaces and geometric discontinuities, traditional welding processes often use a constant welding line energy. This single process parameter setting fails to consider the differences in heat dissipation conditions and stress concentration in different areas along the weld path, leading to excessive heat accumulation in specific regions. This results in a significant increase in residual stress peaks, increasing the risk of welding cracks.

[0004] Furthermore, the industry generally relies on post-weld heat treatment as the primary technical means to eliminate residual stress. While this method can reduce stress to some extent, for large spherical tanks, it not only consumes a huge amount of energy and has a long cycle, significantly increasing manufacturing costs, but also, if temperature control is not properly implemented, it may adversely affect the overall mechanical properties of the base material and the weld.

[0005] Therefore, existing technologies lack a process method that can perform real-time, zoned control based on weld geometry during the welding process and systematically integrate multiple stress reduction measures to achieve full-process, in-situ control of welding residual stress. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a stress reduction process for spherical tank welding that couples preheating and interlayer temperature control. This solves the problem that existing welding processes for thick-plate pressure vessels such as spherical tanks are difficult to precisely and regionally control when facing complex geometric structures. This leads to excessive concentration of residual welding stress in specific areas, increasing the risk of welding cracks and deformation, and affecting the safety and reliability of the final structure.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a stress reduction process for spherical tank welding that couples preheating and interlayer temperature control, comprising the following steps: Step S1, Pre-welding preparation: Separate and install the columns and shell plates of the spherical tank to reduce the overall structural rigidity during the welding process.

[0008] Step S2, Preheating before welding: The area of ​​the spherical shell plate to be welded is heated. In a specific scheme, the preheating temperature is 100-150℃.

[0009] Step S3, Multi-layer Multi-pass Welding: A low heat input welding method is used, and welding is performed according to a preset welding sequence. In a specific embodiment, the low heat input welding method is shielded metal arc welding (SMAW), and its process parameters are: welding current 170-210A, arc voltage 22-28V, and welding speed 10-15cm / min.

[0010] During welding, line energy control is performed based on the geometric characteristics of the weld path. This control specifically includes dividing the weld into a standard linear zone, a high-curvature annular zone, a T-junction zone, and an opening reinforcement zone. The line energy of the standard linear zone is set as the baseline line energy, and different line energy adjustment coefficients are preset for other geometric feature zones. Specifically, the line energy adjustment coefficient for the high-curvature annular zone is 0.95-1.05; for the T-junction zone, it is 0.85-0.95; and for the opening reinforcement zone, it is 0.80-0.90.

[0011] Interpass temperature control is performed between each weld pass. In a specific scheme, the interpass temperature is controlled within the range of 100-250℃.

[0012] The welding sequence includes: a symmetrical welding method in which at least two sets of welding units are simultaneously welded at symmetrical positions on the spherical tank, and a segmented back-welding method in which welding is performed from the middle to both ends within a single weld seam.

[0013] Step S4: Forming the tempered weld bead: Apply a tempered weld bead to the final cap coat. In one specific embodiment, the heat transfer line energy of the tempered weld bead is 70%-85% of the baseline heat transfer line energy.

[0014] In one specific embodiment, the spherical shell plate is made of 16MnDR steel plate, and the welding material used is E5015-N1 welding rod.

[0015] This invention provides a stress reduction process for spherical tank welding that couples preheating and inter-pass temperature control. It has the following beneficial effects: 1. This invention performs line energy control based on the geometric characteristics of the welding path, which can apply a lower welding line energy than the reference value in predetermined high stress concentration areas such as T-junction areas and hole reinforcement areas, while maintaining the reference line energy in the standard linear area. This method enables differentiated and precise distribution of welding heat input in different sections of the weld, thereby suppressing the peak level of residual stress in key geometric parts and optimizing the stress distribution of the entire weld joint.

[0016] 2. This invention combines preheating before welding with interpass temperature control during welding, placing the thermal cycle of the entire welding process within a preset temperature range. This slows down the cooling rate of the weld and heat-affected zone, avoiding the formation of brittle and hard structures due to excessively rapid cooling. At the same time, it reduces the temperature gradient between the molten pool and the base material, thereby effectively reducing thermal stress and improving the microstructure and properties of the weld heat-affected zone.

[0017] 3. By adopting separate installation of the column and the spherical shell plate, the present invention removes the external rigid constraint on the spherical shell during the welding thermal cycle. At the same time, by using symmetrical welding and segmented back welding methods, the welding deformation is balanced within the overall structure and individual welds. This combination of measures effectively controls the macroscopic welding deformation and overall residual stress caused by structural constraints and uneven heat. Detailed Implementation

[0018] Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Example 1 This embodiment provides a stress reduction process for spherical tank welding that couples preheating and inter-pass temperature control, including: Test materials: The spherical shell plate is made of 16MnDR steel plate with a thickness of 40mm; the welding material is E5015-N1 welding rod with a diameter of 4.0mm.

[0020] The process steps are as follows: Pre-welding preparation: Separate and install the uprights and shell plates of the spherical tank, and complete the assembly.

[0021] Preheating before welding: The weld bevel and the area within 100mm on both sides are heated by thermocouples and monitored by infrared thermometers to ensure that the temperature of the area to be welded reaches 125℃ and is maintained.

[0022] Multi-layer, multi-pass welding: SMAW (Shielded Metal Arc Welding) is used for filler welding. The baseline welding process parameters for the standard linear zone are set as follows: welding current 190A, arc voltage 25V, and welding speed 12.5cm / min. During welding, line energy control is implemented: the line energy adjustment coefficient is set to 1.00 when the welding path enters the large curvature annular zone; 0.90 when entering the T-shaped intersection zone; and 0.85 when entering the opening reinforcement zone. The welding sequence employs symmetrical welding and segmented back-welding methods. Two groups of welders begin at symmetrical positions along the equator of the spherical tank, welding in opposite directions from the middle of the weld to both ends in segments. After each weld pass is completed, the slag is cleaned, and the weld temperature is monitored using an infrared thermometer to ensure that the initial welding temperature of the next weld pass does not exceed 175℃.

[0023] Forming a tempered weld bead: After all filler and capping weld beads are completed, another tempered weld bead is applied on top of the final capping weld bead, with a line energy of 75% of the baseline line energy of the standard linear zone.

[0024] Example 2 This embodiment provides a stress reduction process for spherical tank welding that couples preheating and inter-pass temperature control, including: Test materials: The spherical shell plate is made of 16MnDR steel plate with a thickness of 40mm; the welding material is E5015-N1 welding rod with a diameter of 4.0mm.

[0025] The process steps are as follows: Preparation before welding: Same as in Example 1.

[0026] Preheating before welding: The weld bevel and the area within 100mm on both sides are heated by thermocouples and monitored by infrared thermometers to ensure that the temperature of the area to be welded reaches 100℃ and is maintained.

[0027] Multi-layer, multi-pass welding: SMAW (Shielded Metal Arc Welding) is used for filler welding. The baseline welding process parameters for the standard linear zone are set as follows: welding current 170A, arc voltage 22V, welding speed 10cm / min. During welding, line energy control is implemented: when the welding path enters the large curvature annular zone, the line energy adjustment coefficient is set to 0.95; when entering the T-shaped intersection zone, the line energy adjustment coefficient is set to 0.85; when entering the opening reinforcement zone, the line energy adjustment coefficient is set to 0.80. The welding sequence is the same as in Example 1. After each weld pass is completed, the weld slag is cleaned, and the weld temperature is monitored using an infrared thermometer to ensure that the initial welding temperature of the next weld pass does not exceed 100℃.

[0028] Forming a tempered weld bead: After all filler and capping weld beads are completed, another tempered weld bead is applied on top of the final capping weld bead, with a line energy of 70% of the baseline line energy of the standard linear zone.

[0029] Example 3 This embodiment provides a stress reduction process for spherical tank welding that couples preheating and inter-pass temperature control, including: Test materials: The spherical shell plate is made of 16MnDR steel plate with a thickness of 40mm; the welding material is E5015-N1 welding rod with a diameter of 4.0mm.

[0030] The process steps are as follows: Preparation before welding: Same as in Example 1.

[0031] Preheating before welding: The weld bevel and the area within 100mm on both sides are heated by thermocouples and monitored by infrared thermometers to ensure that the temperature of the area to be welded reaches 150℃ and is maintained.

[0032] Multi-layer, multi-pass welding: SMAW (Shielded Metal Arc Welding) is used for filler welding. The baseline welding process parameters for the standard linear zone are set as follows: welding current 210A, arc voltage 28V, and welding speed 15cm / min. During welding, heat input control is implemented: the heat input adjustment coefficient is set to 1.05 when the welding path enters the large curvature annular zone; 0.95 when entering the T-shaped intersection zone; and 0.90 when entering the opening reinforcement zone. The welding sequence is the same as in Example 1. After each weld pass is completed, the slag is cleaned, and the weld temperature is monitored using an infrared thermometer to ensure that the initial welding temperature of the next weld pass does not exceed 250℃.

[0033] Forming a tempered weld bead: After all filler and capping weld beads are completed, another tempered weld bead is applied on top of the final capping weld bead, with a line energy of 85% of the baseline line energy of the standard linear zone.

[0034] Comparative Example 1: Compared with Example 1, the difference is that the conventional high heat input welding method is used, but preheating is not performed, heat input control is not performed, interpass temperature is not controlled, and tempering welds are not applied. All other aspects are the same.

[0035] Comparative Example 2: Compared with Example 1, the difference is that in multi-layer multi-pass welding, the baseline energy of the standard linear zone is always used for welding, and line energy control is not performed for geometric feature areas such as T-shaped intersections. Everything else is the same.

[0036] Comparative Example 3: Compared with Example 1, the difference is that welding was stopped immediately after the cover weld was completed, and the operation of applying a tempering weld was not performed; all other aspects were the same.

[0037] Comparative Example 4: Compared with Example 1, the difference is that the welding sequence does not use the symmetrical welding method and the segmented back welding method, but is instead continuously welded from one end of the weld to the other by a single welding unit, while the rest are the same.

[0038] Comparative Example 5: Compared with Example 1, the difference is that the preheating operation is not performed, but all other operations are the same.

[0039] Test Example 1: Residual Stress Test of Welded Joints Experimental steps Sample acquisition: Welded samples for residual stress testing were prepared according to the process methods and parameters defined in Examples 1, 2, 3 and Comparative Examples 1 to 5, respectively.

[0040] Surface treatment: Before testing, the surface of the weld cap layer in the T-junction area of ​​each specimen was electropolished. The purpose of this step is to remove the surface deformation layer that may have been generated during welding and machining to ensure that the measured stress value is the true residual stress inside the material.

[0041] Test execution: X-ray diffractometer was used to measure the stress on the surface of the treated T-shaped intersection weld. The test target was a Cr target with an α-Fe(211) crystal plane. Measurements were taken at the target measurement points along the direction parallel to and perpendicular to the weld.

[0042] Data recording: For each specimen, the maximum principal stress value at the measuring point is calculated based on the stress components in the two directions, in megapascals (MPa), and summarized in Table 1.

[0043] Experimental data Table 1. Test results of maximum principal stress on the surface of T-intersection welds under different process schemes. Experimental Results Analysis The test data in Table 1 show that the maximum principal stress values ​​on the surface of the T-junction weld of the samples prepared using the process methods of Examples 1-3 are all lower than those of the samples prepared using the process methods of Comparative Examples 1-5. This result is attributed to the systematic combination of steps such as preheating before welding, line energy control, interpass temperature control, and tempering of the weld beads included in the process of the examples, which jointly regulate the thermal cycle and uneven shrinkage during the welding process, thereby suppressing the formation of high residual tensile stress.

[0044] The test results of Example 1 differ from those of Comparative Example 2, and this difference is directly related to whether line energy control was performed in the T-junction zone. The geometry of the T-junction zone results in poorer heat dissipation conditions than the standard linear zone. During constant line energy welding, heat accumulation in this region is more significant, leading to a larger temperature gradient and shrinkage strain during cooling. The process in Example 1, by reducing the welding line energy in the T-junction zone, reduces the peak temperature and total heat input in this local area, resulting in a more uniform temperature field distribution and thus reducing uncoordinated deformation during cooling. Therefore, the maximum principal stress value measured in Example 1 is lower than that in Comparative Example 2.

[0045] The test results of Example 1 also differ from those of Comparative Examples 3 and 5, corresponding to the presence or absence of the tempering weld step and the preheating step, respectively. Compared to Comparative Example 3, the tempering weld in Example 1 applied a compressive stress field to the underlying cap weld and heat-affected zone during its own cooling and contraction. This compressive stress field partially offset the tensile stress generated by the previous weld layer. Compared to Comparative Example 5, the preheating step in Example 1 reduced the initial temperature gradient of the weldment, slowed down the overall cooling rate, and made the temperature change in the entire weld area more gradual, thereby reducing the residual stress caused by inconsistent thermal expansion and contraction.

[0046] Test Example 2: Hardness Test of Welded Joints Experimental steps Sample acquisition: Welded samples for hardness testing were prepared according to the process methods and parameters defined in Examples 1, 2, 3 and Comparative Examples 1 to 5, respectively.

[0047] Section specimen preparation: Metallographic specimens were cut perpendicular to the weld direction from each weld specimen. The specimens were then inlaid, ground, and polished. Subsequently, the polished sections were etched with a 4% (v / v) nitric acid alcohol solution to clearly reveal the microstructure boundaries of the weld, heat-affected zone (HAZ), and base metal.

[0048] Test Execution: The prepared cross-sectional specimens were tested for hardness using a Vickers hardness tester. The applied test force was 9.8 N (HV1), and the holding time was 15 seconds. The test path was a straight line parallel to the specimen surface, 2 mm below the surface, spanning the weld, heat-affected zone, and base material. Measurements were taken continuously at 0.5 mm intervals along this path.

[0049] Data recording: For each specimen, record the Vickers hardness values ​​at all measuring points within its heat-affected zone, and select the maximum value as the highest hardness of the heat-affected zone for that specimen. Summarize all data in Table 2.

[0050] 2. Experimental Data Table 2. Test results of the highest hardness of the weld heat-affected zone under different process schemes. Experimental Results Analysis The test data in Table 2 show that the highest hardness values ​​of the heat-affected zone of the samples prepared using the processes of Examples 1-3 are all lower than those of the samples prepared using the processes of Comparative Examples 1, 3, and 5. This result corresponds to the thermal cycling control measures in the process of the examples, that is, by combining preheating before welding with interpass temperature control, the cooling rate of the weld heat-affected zone is reduced, thereby inhibiting the transformation of austenite to high-hardness martensite.

[0051] The test results of Example 1 differ from those of Comparative Example 5, and this difference is directly related to whether or not a preheating step was performed. The preheating step in Example 1 reduced the initial temperature difference between the welding area and the surrounding base material, resulting in a more gradual temperature drop during post-weld cooling. This slow cooling process provides more time for the austenite to transform into the more ductile ferrite and bainite structures, thus avoiding the formation of a large amount of hard and brittle martensite. Therefore, the highest hardness value of the heat-affected zone measured in Example 1 is lower than that of Comparative Example 5, which did not undergo preheating.

[0052] The test results of Example 1 and Comparative Example 3 also differed, corresponding to the presence or absence of the tempered weld bead formation step. In Example 1, the heat generated by the final tempered weld bead applied heat-treated the cooled coarse-grained heat-affected zone below. The temperature and time of this heat treatment were sufficient to decompose or transform the hard and brittle structure already formed in the coarse-grained zone into a stable structure with lower hardness. This process is equivalent to a local tempering, therefore the highest hardness value of its heat-affected zone is lower than that of Comparative Example 3, which did not perform the tempered weld bead step.

[0053] Test Example 3: Impact Toughness Test of Welded Joints Experimental steps Sample acquisition: Welded samples for impact toughness testing were prepared according to the process methods and parameters defined in Examples 1, 2, 3 and Comparative Examples 1 to 5, respectively.

[0054] Impact test specimen preparation: In accordance with GB / T229-2020 standard, standard Charpy V-notch impact test specimens with dimensions of 10mm×10mm×55mm were prepared from each welded specimen. During preparation, the location of the heat-affected zone (HAZ) was determined by etching the specimen cross-section, and the position of the notch was precisely controlled to ensure that the V-notch was located in the coarse-grained region of the HAZ.

[0055] Test execution: A set of processed samples were placed in a cryogenic bath and cooled to -20°C using a mixture of alcohol and liquid nitrogen. Subsequently, the samples were quickly removed from the cryogenic bath, placed on the support of a pendulum impact testing machine, and the pendulum was immediately released to conduct an impact test on the samples.

[0056] Data recording: Record the impact energy absorbed by the specimen upon fracture, as displayed on the dial or sensor of the testing machine, in joules (J). The average value of the test results for three specimens under each process scheme is summarized in Table 3.

[0057] Experimental data Table 3. Results of impact energy test of the weld heat-affected zone at -20℃ under different process schemes. Experimental Results Analysis The test data in Table 3 show that the impact energy values ​​measured at -20℃ for the samples prepared using the process methods of Examples 1-3 are all higher than those for the samples prepared using the process methods of Comparative Examples 1-5. This result indicates that the welded joints prepared by the process methods of the examples have higher low-temperature toughness. This is because the process optimizes the final microstructure of the heat-affected zone and reduces the formation of brittle phases through systematic control of heat input and heat cycling.

[0058] The test results of Example 1 differ from those of Comparative Example 5, and this difference is directly related to whether or not the preheating step was performed. The preheating and interpass temperature control steps included in the process of Example 1 work together to reduce the cooling rate of the weld heat-affected zone. The slower cooling rate inhibits the transformation of austenite into high-hardness martensite and promotes the formation of tougher microstructures such as bainite. Therefore, the microstructure and toughness of the heat-affected zone of the Example 1 specimen are superior to those of the Comparative Example 5 specimen, enabling it to absorb more energy in the low-temperature impact test.

[0059] The test results of Example 1 and Comparative Example 3 also differed, corresponding to the presence or absence of the tempered weld bead formation step. The tempered weld bead formed on the cap coat in Example 1, through a thermal cycle, performed in-situ heat treatment on the cooled cap coat bead and heat-affected zone below. The peak temperature and duration of this heat treatment process were sufficient to temper any hard and brittle microstructure that might form in the underlying coarse-grained region, transforming it into a more ductile tempered microstructure. This improvement in microstructure is the direct reason why the impact energy value of the Example 1 specimen was higher than that of the Comparative Example 3 specimen, which did not undergo this step.

Claims

1. A stress reduction process for spherical tank welding that couples preheating and inter-pass temperature control, characterized in that, Includes the following steps: S1. Pre-welding preparation: Separate and install the uprights and shell plates of the spherical tank; S2. Preheating before welding: The area of ​​the spherical shell plate to be welded is heated. S3, Multi-layer multi-pass welding: Low heat input welding method is used, and welding is performed according to the preset welding sequence; During the welding process, line energy control is performed based on the welding path geometry of the weld, and interpass temperature control is performed between each weld pass; S4. Forming a tempered weld bead: Apply a tempered weld bead to the final capping layer.

2. The stress reduction process for spherical tank welding with coupled preheating and interlayer temperature control according to claim 1, characterized in that, In step S3, line energy control is performed based on the geometric characteristics of the welding path of the weld, specifically including: dividing the weld into a standard linear zone, a large curvature annular zone, a T-shaped intersection zone, and an opening reinforcement zone, and setting the line energy of the standard linear zone as the baseline line energy.

3. The stress reduction process for spherical tank welding with coupled preheating and interlayer temperature control according to claim 2, characterized in that, Line energy adjustment coefficients are preset for other geometric feature areas. The line energy adjustment coefficients are set as follows: 0.95-1.05 for the large curvature annular area; 0.85-0.95 for the T-shaped intersection area; and 0.80-0.90 for the opening reinforcement area.

4. The stress reduction process for spherical tank welding with coupled preheating and interlayer temperature control according to claim 1, characterized in that, The low heat input welding method in step S3 is shielded metal arc welding (SMAW).

5. The stress reduction process for spherical tank welding with coupled preheating and interlayer temperature control according to claim 4, characterized in that, The process parameters for the shielded metal arc welding (SMAW) are: welding current of 170-210A, arc voltage of 22-28V, and welding speed of 10-15cm / min.

6. The stress reduction process for spherical tank welding with coupled preheating and interlayer temperature control according to claim 1, characterized in that, The temperature for preheating before welding in step S2 is 100-150℃.

7. The stress reduction process for spherical tank welding with coupled preheating and interlayer temperature control according to claim 1, characterized in that, The temperature range for interlayer temperature control in step S3 is 100-250℃.

8. The stress reduction process for spherical tank welding with coupled preheating and interlayer temperature control according to claim 1, characterized in that, The welding sequence in step S3 includes: a symmetrical welding method in which at least two sets of welding units are simultaneously welded at symmetrical positions on the spherical tank, and a segmented back-welding method in which welding is performed from the middle to both ends within a single weld seam.

9. The stress reduction process for spherical tank welding with coupled preheating and interlayer temperature control according to claim 1, characterized in that, The spherical shell plate is made of 16MnDR steel plate; the welding material is E5015-N1 welding rod.

10. The stress reduction process for spherical tank welding with coupled preheating and interlayer temperature control according to claim 2, characterized in that, In step S4, the line energy of the tempering weld bead is 70%-85% of the baseline line energy.

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