Welding method for lower half of low-pressure rear cylinder of steam turbine

Abstract

The application belongs to the technical field of welding, and specifically relates to a welding method for a lower half of a low-pressure rear cylinder of a steam turbine, which comprises the following steps: S1, assembling a flange frame and a bearing seat, welding a first tension bar between the flange frames on both sides of the bearing seat, the first tension bar being parallel to the axial direction of the bearing seat, welding a second tension bar between the outer wall of the bearing seat and the first tension bar, and stress detection elements being arranged on the first tension bar and the second tension bar; S2, welding the flange frame; S3, welding the bearing seat and the flange frame in a segmented back-welding mode, changing the welding sequence when the stress detected by the stress detection elements reaches a set stress threshold value, and simultaneously performing stress improvement treatment on the welded welds; S4, welding an outer cylinder body; S5, welding internal rib plates and accessories inside the outer cylinder body; and S6, welding external rib plates and accessories outside the outer cylinder body. The application can reduce the deformation amount of the bearing seat and improve the welding quality.

Description

Technical Field

[0001] This invention belongs to the field of welding technology, and in particular to a welding method for the lower half of the low-pressure rear cylinder of a steam turbine. Background Technology

[0002] The low-pressure exhaust cylinder of a steam turbine is the exhaust section of the low-pressure cylinder and is one of the largest and most complex components in the turbine. Its main task is to effectively guide the completed work steam (exhaust steam) into the condenser and create a highly efficient vacuum environment. During production, the upper and lower halves of the low-pressure exhaust cylinder are machined separately and then joined together by a flange on the split face.

[0003] The low-pressure after-cylinder of a steam turbine is mainly formed by welding. The most difficult part to weld is the lower half of the low-pressure after-cylinder. The structure of the lower half of the low-pressure after-cylinder of a steam turbine is as follows: Figure 1 As shown, it mainly includes a flange frame 100, a bearing housing 200, and an outer cylinder body 300 located on the split surface. The outer cylinder body 300 is composed of four side plates and a bottom plate. The outer wall of the bearing housing 200 and the inner wall of the outer cylinder body 300 are provided with a large number of internal stiffeners 400. The outer wall of the outer cylinder body 300 is provided with a large number of external stiffeners 500. Multiple accessories are provided both inside and outside the outer cylinder body 300.

[0004] Due to the large size of the rear cylinder, components such as the outer cylinder body 300, flange frame 100, and stiffening plate are thin-walled parts, resulting in large welding deformation and affecting product quality. In particular, when welding the flange frame 100 and the bearing housing 200, only one end of the bearing housing 200 is welded to the flange frame 100, while the other end is suspended. The bearing housing is prone to shrinkage and deformation, and the suspended end is also prone to vertical and horizontal displacement, causing the bearing housing axis to deviate from the axis of the low-pressure rear cylinder. As a result, the dimensional and positional accuracy after welding is poor.

[0005] The invention patent with application number CN201910208460.6 discloses a method for eliminating the deformation of the middle section of the outer ring of the low-pressure hollow partition of a steam turbine. By setting up tie rods, the deformation of the middle section during welding is limited. However, this technology cannot monitor the amount of deformation in real time and adjust the welding process in a timely manner.

[0006] An invention application with the application number CN202411184399.3 discloses a thin-walled weak rigid structure ring weld deformation control and correction method. The method performs three-dimensional scanning on the workpiece to be welded after positioning welding, obtains the shape and size state of the workpiece to be welded, can accurately identify the shape and size precision of the workpiece to be welded, and performs real-time monitoring on the profile degree in the process of sectional welding. The method corrects the position of the welding deformation through an effective high-energy beam heat source reverse deformation mode. After the sectional welding is completed, the method obtains the real deformation condition through stabilization treatment and performs the final shape and size correction. Although the technology can monitor the deformation position and deformation amount in real time and adjust the welding process according to the monitoring result, the technology can only adjust after the part has been deformed, and the adjustment is not timely enough. SUMMARY

[0007] The technical problem to be solved by the present application is to provide a steam turbine low-pressure rear cylinder lower half welding method, which reduces the deformation amount and improves the welding quality.

[0008] To solve the above problems, the technical scheme adopted by the present application is as follows: a steam turbine low-pressure rear cylinder lower half welding method, comprising the following steps:

[0009] S1, draw a sample line on a rigid platform, then assemble a flange frame and a bearing seat according to the sample line, fix the flange frame to the rigid platform, weld a first tension bar between the flange frames on both sides of the bearing seat, the first tension bar is parallel to the axial direction of the bearing seat, and a second tension bar is welded between the outer wall of the bearing seat and the first tension bar, and stress detection elements are arranged on the first tension bar and the second tension bar;

[0010] S2, weld the flange frame;

[0011] S3, weld the bearing seat and the flange frame in a sectional retreat welding manner, detect the stress received by the first tension bar and the second tension bar by using the stress detection elements at the same time of welding, change the welding sequence when the stress reaches a set stress threshold value, and perform stress improvement treatment on the welded welds;

[0012] S4, weld the outer cylinder body;

[0013] S5, weld the internal rib plate and the accessories inside the outer cylinder body;

[0014] S6, weld the external rib plate and the accessories outside the outer cylinder body.

[0015] Further, before step S1, input a welding model into a finite element analysis software, the welding model includes the rigid platform, the flange frame, the bearing seat, the first tension bar and the second tension bar, simulate the welding process by using the finite element analysis software, predict the change of the welding stress and the deformation of the bearing seat in the welding process of the bearing seat, determine the stress value received by the first tension bar and the second tension bar when the bearing seat is deformed and deviated, and take the stress value as the stress threshold value.

[0016] Further, in step S4, the outer cylinder body is assembled first, and the third reinforcing rib is welded on the inner wall of the outer cylinder body.

[0017] Further, in step S1, the fourth reinforcing rib is welded on the steam inlet of the flange frame.

[0018] Further, a sliding groove is arranged on the rigid platform outside the flange frame, a sliding block is arranged in the sliding groove, the sliding block is connected with a driving mechanism, a vertical hydraulic cylinder is arranged on the sliding block, a mounting block is arranged on the top of the hydraulic cylinder, a horizontal and detachable supporting shaft is arranged on the side wall of the mounting block, and a pressure roller is arranged on the supporting shaft in a rotating manner.

[0019] In step S1, the hydraulic cylinder drives the mounting block, the supporting shaft and the pressure roller to move downward, so that the pressure roller contacts the upper surface of the flange frame, and the hydraulic cylinder provides an initial pulling force to press the flange frame tightly on the rigid platform.

[0020] In steps S2 to S5, the driving mechanism drives the sliding block to move, and the pressure roller rolls on the upper surface of the flange frame.

[0021] Further, a vertical guide column is arranged on the sliding block, the guide column penetrates through the mounting block and is in sliding cooperation with the mounting block, a spring is arranged on the guide column, the upper end of the spring is fixedly connected with the mounting block, and the lower end of the spring is connected with the sliding block through a pressure sensor.

[0022] In step S1, the initial elastic force of the spring detected by the pressure sensor is recorded.

[0023] In steps S2 to S5, during the rolling of the pressure roller, the pulling force provided by the hydraulic cylinder remains the initial pulling force, the real-time elastic force of the spring is detected by the pressure sensor, when the real-time elastic force decreases to an elastic force threshold value, it is determined that the pressure roller moves to the buckling deformation position of the flange frame, at this time, the pulling force output by the hydraulic cylinder is increased, the pressure of the pressure roller on the buckling deformation position is increased, and the buckling deformation position is corrected until the real-time elastic force is greater than the elastic force threshold value; the above process is repeated.

[0024] Further, the stress detection element is a fiber Bragg grating sensor, mounting grooves are arranged on the side walls of the first reinforcing rib and the second reinforcing rib, and the fiber Bragg grating sensor is pasted in the mounting grooves through high-temperature ceramic glue.

[0025] After step S6, the lower half of the low-pressure rear cylinder of the steam turbine is subjected to heat treatment, and in the process of heat treatment, the fiber Bragg grating sensor is used to detect the stress change of the first reinforcing rib and the second reinforcing rib in real time to determine the heat treatment effect.

[0026] After heat treatment, the first reinforcing rib and the second reinforcing rib are cut off.

[0027] Further, the stress improvement treatment process for the welded weld seam comprises:

[0028] stress relieving treatment is performed on the weld seam;

[0029] After the stress relieving treatment, the stress distribution of the weld seam is detected to find the low stress weld seam area with stress lower than the set standard;

[0030] A magnetostrictive material coating is sprayed on the base material on both sides of the low stress weld seam area;

[0031] A gradient magnetic field is applied to the magnetostrictive material coating, the magnetostrictive material coating is elongated, a tensile stress is applied to the base material, and the stress of the low stress weld seam area is increased.

[0032] Further, the area with an average stress lower than 20% of the yield strength of the base material is regarded as the low stress weld seam area; the magnetostrictive material coating is a Terfenol-D alloy with a thickness of 150 μm, the strength of the gradient magnetic field is 100 kA / m, the magnetic field gradient is 15 kA / m / mm, and the gradient magnetic field application time is 0.1-2 s.

[0033] Further, a high-frequency induction heating coil is used to heat the weld seam to realize stress relieving.

[0034] The beneficial effects of the present application are as follows: welding deformation is caused by welding stress, in the present application, the stress borne by the first and second tension bars during welding is directly monitored by the stress detection element to determine the welding stress, when the stress reaches the set stress threshold, the welding sequence is immediately adjusted, and the stress improvement treatment is performed on the welded weld seam to realize the treatment before the bearing seat deformation or position deviation. The prior art monitors the deformation and then adjusts the process, compared with the prior art, the present application adjusts more timely, can more effectively reduce the bearing seat deformation or deviation, and improves the welding quality. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 is a lower half structure schematic diagram of a low-pressure rear cylinder of a steam turbine;

[0036] Figure 2 is a flowchart of the welding method of the present application;

[0037] Figure 3 is a top view schematic diagram after assembly in step S1;

[0038] Figure 4 is a sectional view schematic diagram of the first tension bar;

[0039] Figure 5 is Figure 3 is a sectional view schematic diagram of A-A in

[0040] Figure 6is a schematic view after welding the outer cylinder body;

[0041] Figure 7 is a schematic view after welding the inner rib plate and the inner accessory (remove the side plate on one side of the outer cylinder body);

[0042] Figure 8 is a schematic view after welding the outer rib plate and the outer accessory.

[0043] Fig. 1 is a first tension bar; 11 is a stress detection element; 2 is a second tension bar; 3 is a fourth tension bar; 100 is a flange frame; 101 is an inlet; 200 is a bearing seat; 300 is an outer cylinder body; 400 is an inner rib plate; 500 is an outer rib plate; 1000 is a rigid platform; 1001 is a sliding groove; 1002 is a sliding block; 1003 is a hydraulic cylinder; 1004 is a supporting shaft; 1005 is a pressure roller; 1006 is a mounting block; 1007 is a driving mechanism; 1008 is a guide column; 1009 is a spring; 10010 is a pressure sensor. DETAILED DESCRIPTION

[0044] The application will be further described below in conjunction with the drawings and examples.

[0045] The turbine low-pressure rear cylinder lower half welding method of the application, as shown in Figure 2 , comprises the following steps:

[0046] S1, draw a sample line on the rigid platform 1000, then assemble the flange frame 100 and the bearing seat 200 according to the sample line, and fix the flange frame 100 to the rigid platform 1000, weld the first tension bar 1 between the flange frames 100 on both sides of the bearing seat 200, the first tension bar 1 is parallel to the axial direction of the bearing seat 200, and the second tension bar 2 is welded between the outer wall of the bearing seat 200 and the first tension bar 1, and the stress detection element 11 is arranged on the first tension bar 1 and the second tension bar 2.

[0047] The rigid platform 1000 can be made of a metal plate with large thickness and weight, which is fixed to the ground after leveling to ensure that its upper surface is in a horizontal state. The flange frame 100 is rectangular and is spliced by multiple plates, and the flange frame 100 is horizontally placed on the rigid platform 1000, and its lower surface serves as the center surface of the upper and lower halves of the low-pressure rear cylinder. The length direction of the first tension bar 1 is consistent with the width direction of the flange frame 100, and the second tension bar 2 is inclinedly arranged, one end of which is welded with the first tension bar 1 and the other end of which is welded with the outer wall of the bearing seat 200, as shown in Figure 3 .

[0048] The first and second reinforcing ribs 1 and 2 can enhance the rigidity of the bearing seat 200 and the flange frame 100, and can reduce the welding deformation of the bearing seat 200 and the flange frame 100 to a certain extent. When the bearing seat 200 and the flange frame 100 are deformed, stress is applied to the first and second reinforcing ribs 1 and 2. By arranging the stress detection elements 11 on the first and second reinforcing ribs 1 and 2, the stress borne by the first and second reinforcing ribs 1 and 2 can be detected in real time during welding, and the detected stress size can reflect the welding stress size of the bearing seat 200 and the flange frame 100, so that timely response can be made.

[0049] The low-pressure rear cylinder of the steam turbine has an inlet port 101 and an exhaust port. The bearing seat 200 is connected to the flange frame 100 at the exhaust port. The flange frame 100 at the inlet port 101 has an inlet gap. In order to improve the overall rigidity of the flange frame 100, a fourth reinforcing rib 3 is welded across the inlet port 101 at the inlet port 101 of the flange frame 100. The fourth reinforcing rib 3 can have the same structure as the first reinforcing rib 1, and the stress detection elements 11 are also arranged.

[0050] S2, welding the flange frame 100. The plate materials constituting the flange frame 100 are sequentially welded to obtain a complete flange frame 100 in the form of a rectangular frame.

[0051] S3, welding the bearing seat 200 and the flange frame 100 by using the segmented back welding method. The stress detection elements 11 are used to detect the stress borne by the first and second reinforcing ribs 1 and 2 during welding. When the stress reaches a set stress threshold, the welding sequence is changed, and the welded welds are treated to improve the stress.

[0052] Segmented back welding means that the weld is divided into multiple segments and welded in segments. When the stress of the first and second reinforcing ribs 1 and 2 reaches a set stress threshold, it indicates that the welding stress is large, which may cause the bearing seat 200 or the flange frame 100 to deform. At this time, the welding sequence is changed in time, i.e. assuming that the detection value of the stress detection element 11 reaches the stress threshold when welding the A segment weld, the welding of the A segment weld is stopped, and the B segment weld which is the farthest from the A segment weld is welded.

[0053] Since the welding deformation is caused by welding stress, the present application directly detects the welding stress and can process before the bearing seat deforms or deviates. In the prior art, process adjustment is made after deformation is monitored. Compared with the prior art, the present application adjusts more timely, can more effectively reduce the deformation or deviation of the bearing seat, and improves the welding quality.

[0054] The stress threshold is determined according to the stress when the bearing seat 200 or the flange frame 100 is deformed. In order to accurately obtain the stress threshold, the application inputs the welding model (i.e. the lower half model of the low-pressure rear cylinder of the steam turbine) into a finite element analysis software, such as Abaqus software, SYSWELD software, etc. before welding. The welding model includes a rigid platform 1000, the flange frame 100, the bearing seat 200, the first tension bar 1 and the second tension bar 2. The finite element analysis software is used to simulate the welding process, predict the change of the welding stress of the bearing seat 200 during the welding process and the deformation of the bearing seat 200, determine the stress value borne by the first tension bar 1 and the second tension bar 2 when the bearing seat 200 is deformed and offset, and take the stress value as the stress threshold.

[0055] The existing finite element analysis software can accurately predict the stress field, strain field, temperature field and deformation condition inside the component during the welding process and after the welding. The stress value borne by the first tension bar 1 and the second tension bar 2 when the bearing seat 200 or the flange frame 100 is deformed can be determined, so as to obtain the stress threshold. The stress threshold satisfies that when the stress borne by the first tension bar 1 and the second tension bar 2 is less than the stress threshold, the bearing seat 200 and the flange frame 100 will not be obviously deformed.

[0056] S4, welding the outer cylinder body 300. Specifically, the outer cylinder body 300 is assembled first. The outer cylinder body 300 is a large-area plate. In order to prevent the outer cylinder body 300 from being deformed, the third tension bar is welded on the inner wall of the outer cylinder body 300 before welding. One end of the third tension bar can be located on the outer wall of the bearing seat 200. The structure of the third tension bar can be the same as that of the first tension bar 1. After the outer cylinder body 300 is welded, as shown in Figure 6 .

[0057] S5, welding the internal rib plate 400 and the accessories inside the outer cylinder body 300. After welding, as shown in Figure 7 .

[0058] S6, welding the external rib plate 500 and the accessories outside the outer cylinder body 300. After welding, as shown in Figure 8 .

[0059] The lower half of the low-pressure rear cylinder of the steam turbine is connected with the upper half through the flange frame 100. Therefore, the deformation of the flange frame 100 needs to be strictly controlled. The flange frame 100 is prone to warping deformation during welding, which leads to poor flatness of the center split surface. In order to reduce the warping deformation of the flange frame 100, the flange frame 100 is fixed to the rigid platform 1000. Specifically, the flange frame 100 can be fixed to the rigid platform 1000 by spot welding. However, it is difficult to separate the flange frame 100 from the rigid platform 1000 after welding.

[0060] As a preferred embodiment, as shown in Figure 1 and Figure 5As shown, the rigid platform 1000 outside the flange frame 100 is provided with a sliding groove 1001, which can be a dovetail groove, the length direction of the sliding groove 1001 is consistent with the length direction of the plate material constituting the flange frame 100, a sliding block 1002 is arranged in the sliding groove 1001, and the sliding block 1002 can slide in the sliding groove 1001. The sliding block 1002 is connected with a driving mechanism 1007, and the driving mechanism 1007 is used to drive the sliding block 1002 to slide, and common mechanisms such as hydraulic mechanisms and motor-driven screw mechanisms can be used. A vertical hydraulic cylinder 1003 is arranged on the sliding block 1002, the hydraulic cylinder 1003 is used to provide a pressing force, a mounting block 1006 is arranged at the top of the hydraulic cylinder 1003, a horizontal and detachable supporting shaft 1004 is arranged on the side wall of the mounting block 1006, the supporting shaft 1004 can be connected with the mounting block 1006 through screws, and a pressing wheel 1005 is arranged on the supporting shaft 1004 in a rotating manner.

[0061] In step S1, when the flange frame 100 and the bearing seat 200 are assembled, the supporting shaft 1004 is detached from the mounting block 1006. After the flange frame 100 and the bearing seat 200 are assembled, the supporting shaft 1004 is mounted to the mounting block 1006, it is ensured that the mounting block 1006 is located above the flange frame 100, and then the mounting block 1006, the supporting shaft 1004 and the pressing wheel 1005 are driven by the hydraulic cylinder 1003 to move downward, so that the pressing wheel 1005 contacts the upper surface of the flange frame 100, and the hydraulic cylinder 1003 provides an initial pulling force, the pulling force is transmitted to the pressing wheel 1005 through the supporting shaft 1004, and the pressing wheel 1005 can press and fix the flange frame 100 to the rigid platform 1000. The pressing wheel 1005 contacts part of the flange frame 100, and does not cover the welding seam between the outer cylinder body 300 and the flange frame 100, and does not affect the subsequent assembly and welding of the outer cylinder body 300.

[0062] In steps S2 to S5, when welding, the driving mechanism 1007 drives the sliding block 1002 to move, and the sliding block 1002 drives the supporting shaft 1004 and the pressing wheel 1005 to move synchronously when moving, so that the pressing wheel 1005 rolls on the upper surface of the flange frame 100. By continuously rolling back and forth on the upper surface of the flange frame 100, the flange frame 100 can be leveled, which is beneficial to prevent the flange frame 100 from deforming.

[0063] In order to timely find the warping deformation of the flange frame 100 and straighten it, a vertical guide column 1008 is arranged on the sliding block 1002, the guide column 1008 penetrates through the mounting block 1006 and is in sliding fit with the mounting block 1006, a spring 1009 is arranged on the guide column 1008, the upper end of the spring 1009 is fixedly connected with the mounting block 1006, and the lower end is connected with the sliding block 1002 through a pressure sensor 10010.

[0064] The spring 1009 is always in a compressed state and has a certain elastic force. The pressure sensor 10010 can detect the elastic force of the spring 1009 and calculate the length of the spring 1009 according to the detected elastic force, and then calculate the height of the mounting block 1006.

[0065] In step S1, the initial elastic force of the spring 1009 detected by the pressure sensor 10010 is recorded.

[0066] In steps S2 to S5, during the rolling of the compression wheel 1005, the pulling force provided by the hydraulic cylinder 1003 remains the initial pulling force, and the pressure sensor 10010 detects the real-time elastic force of the spring 1009. When the real-time elastic force decreases to the elastic force threshold value, it is determined that the compression wheel 1005 moves to the buckling deformation position of the flange frame 100, at this time, the pulling force output by the hydraulic cylinder 1003 is increased, the pressure of the compression wheel 1005 on the buckling deformation position is increased, and the buckling deformation position is corrected until the real-time elastic force is greater than the elastic force threshold value; repeat the above process.

[0067] When a part of the flange frame 100 buckles and deforms, after the compression wheel 1005 rolls to the buckling position, the height will increase, driving the entire mounting block 1006 to move upward, causing the spring 1009 to elongate, the elastic force to decrease, and the real-time elastic force value detected by the pressure sensor 10010 to decrease. According to the amount of decrease of the elastic force value, the elongation of the spring 1009 can be calculated, and thus the deformation amount of the buckling deformation position can be calculated. When the buckling deformation amount exceeds the set value, the pulling force output by the hydraulic cylinder 1003 is increased, so that the pressure of the compression wheel 1005 on the flange frame 100 is increased, and the buckling deformation position is straightened, reducing the buckling deformation of the flange frame 100.

[0068] The stress detection element 11 can be various commonly used elements such as a tension sensor, but in the present application, in order to ensure the anti-deformation effect of each tension bar, the tension bar is cut off after heat treatment, and the heat treatment temperature is relatively high, usually higher than 500°C. Therefore, the stress detection element 11 of the present application is a fiber Bragg grating sensor, as shown in the figure, the first tension bar 1 and the second tension bar 2 are provided with installation grooves in the side walls, and the fiber Bragg grating sensor is pasted in the installation grooves by high-temperature ceramic glue. Figure 4 The fiber Bragg grating sensor is extremely sensitive to strain and temperature, and its material is quartz glass, which has excellent high-temperature resistance and can work normally during heat treatment, monitoring the dynamic process of stress relaxation and providing direct data for optimizing the heat treatment process.

[0069] After step S6, the lower half of the low-pressure cylinder of the steam turbine is heat treated, and in the process of heat treatment, the stress change of the first tension bar 1 and the second tension bar 2 is detected in real time by the fiber Bragg grating sensor to judge the heat treatment effect. After heat treatment, the first tension bar 1 and the second tension bar 2 are cut off.

[0070] In the present application, stress improvement treatment is performed on the welded weld to reduce deformation. The conventional treatment method is to eliminate welding stress, for example, using heating, ultrasonic treatment and other methods to eliminate stress. However, the effect of stress elimination treatment is limited. Although the overall stress level is reduced, the stress distribution is uneven, and the stress difference will also cause deformation. In addition, after subsequent machining to remove part of the material, the internal uneven stress that has not been completely eliminated will be redistributed, causing new deformation. In order to further reduce the adverse effects of welding stress, the present application improves the uniformity of stress distribution while reducing welding stress. The specific process includes:

[0071] The stress relief treatment of the weld can be performed using existing technology, for example, using a high-frequency induction heating coil to heat the weld to achieve stress relief.

[0072] After stress relief treatment, the stress distribution of the weld is detected to find the low stress weld area with stress lower than the set standard. Specifically, an X-ray stress analyzer can be used to detect the stress distribution around the weld, and the area with an average stress lower than 20% of the yield strength of the base material is considered as the low stress weld area. When the average stress is lower than 20% of the yield strength of the base material, it is far from the yield state, thus having sufficient safety margin to withstand stress increase and not causing new plastic deformation or failure risk.

[0073] A magnetostrictive material coating is sprayed on the base material on both sides of the low stress weld area. A gradient magnetic field is applied to the magnetostrictive material coating, the magnetostrictive material coating is elongated, a tensile stress is applied to the base material, and the stress of the low stress weld area is increased. The magnetostrictive material coating is a Terfenol-D alloy with a thickness of 150 μm, the strength of the gradient magnetic field is 100 kA / m, the magnetic field gradient is 15 kA / m / mm, and the gradient magnetic field application time is 0.1-2 s. Using this method, a stress of not more than 40 MPa can be introduced.

[0074] The present application increases the stress of the low stress weld area by introducing new stress in the low stress weld area, and improves the stress uniformity of the entire weld. The root cause of welding deformation is uneven shrinkage. The weld area shrinks strongly during the cooling process, but is constrained by the surrounding cold base material, thereby generating an unbalanced tensile stress. By performing stress uniformity treatment, the present application eliminates the large tensile stress gradient, makes the stress state of each part consistent, greatly reduces or eliminates the unbalanced internal moment that causes distortion, and the base material naturally restores to its original, stable geometric shape, thereby reducing the existing deformation and significantly improving the dimensional stability. In addition, after the welding stress distribution is more uniform, after machining to remove part of the material, the stress of the remaining material is also more easily balanced, preventing unpredictable warping, distortion and other deformations during machining.

[0075] The present application only needs to perform stress uniformity treatment on important welds, such as the welds of the flange frame 100 and the bearing seat 200, the welds between the plates of the flange frame 100, the welds between the flange frame 100 and the outer cylinder body 300, and the like.

[0076] The above description is merely the preferred embodiments of the present application, and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, and the like made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A method of welding the lower half of a low pressure rear cylinder of a steam turbine, characterized in that, The method comprises the following steps: S1, laying out a sample line on a rigid platform (1000), then assembling a flange frame (100) and a bearing seat (200) according to the sample line, fixing the flange frame (100) to the rigid platform (1000), welding a first tension bar (1) between the flange frames (100) on both sides of the bearing seat (200), the first tension bar (1) being parallel to the axial direction of the bearing seat (200), and welding a second tension bar (2) between the outer wall of the bearing seat (200) and the first tension bar (1), the first tension bar (1) and the second tension bar (2) each being provided with a stress detection element (11); S2, welding the flange frame (100); S3, welding the bearing seat (200) and the flange frame (100) by using a segmented back-welding method, detecting the stress received by the first tension bar (1) and the second tension bar (2) by using the stress detection element (11) while welding, changing the welding sequence when the stress reaches a set stress threshold, and performing stress improvement treatment on the welded welds; The stress improvement treatment process on the welded welds comprises: Performing stress relief treatment on the welds; After the stress relief treatment, detecting the stress distribution of the welds, finding out low-stress weld areas with stress lower than the set standard, and taking the areas with average stress lower than 20% of the yield strength of the base material as the low-stress weld areas; Spraying a magnetostrictive material coating on the base material on both sides of the low-stress weld areas; the magnetostrictive material coating is a Terfenol-D alloy with a thickness of 150 μm; Applying a gradient magnetic field to the magnetostrictive material coating, elongating the magnetostrictive material coating, the strength of the gradient magnetic field being 100 kA / m, the magnetic field gradient being 15 kA / m / mm, and the gradient magnetic field application time being 0.1-2 s; applying a tensile stress to the base material to increase the stress of the low-stress weld areas; S4, welding an outer cylinder body (300); S5, welding an internal rib plate (400) and accessories inside the outer cylinder body (300); S6, welding an external rib plate (500) and accessories outside the outer cylinder body (300).

2. The method of claim 1, wherein the method further comprises: Before step S1, inputting a welding model into a finite element analysis software, the welding model comprising the rigid platform (1000), the flange frame (100), the bearing seat (200), the first tension bar (1) and the second tension bar (2), simulating the welding process by using the finite element analysis software, predicting the change of the welding stress during the welding process of the bearing seat (200) and the deformation of the bearing seat (200), determining the stress value received by the first tension bar (1) and the second tension bar (2) when the bearing seat (200) deforms and deviates, and taking the stress value as the stress threshold.

3. The method of claim 1, wherein the method further comprises: In step S4, assembling the outer cylinder body (300) and welding a third tension bar on the inner wall of the outer cylinder body (300).

4. The method of claim 1, wherein the method further comprises: In step S1, welding a fourth tension bar (3) across the steam inlet (101) at the steam inlet (101) of the flange frame (100).

5. The method of claim 1, wherein the method further comprises: The rigid platform (1000) outside the flange frame (100) is provided with a sliding groove (1001), the sliding groove (1001) is provided with a sliding block (1002) inside, and the sliding block (1002) is connected with a driving mechanism (1007); the sliding block (1002) is provided with a vertical hydraulic cylinder (1003), the top of the hydraulic cylinder (1003) is provided with a mounting block (1006), the side wall of the mounting block (1006) is provided with a horizontal and detachable supporting shaft (1004), and the supporting shaft (1004) is provided with a pressure roller (1005) in a rotating fit mode; In step S1, the hydraulic cylinder (1003) drives the mounting block (1006), the supporting shaft (1004) and the pressure roller (1005) to move downwards, so that the pressure roller (1005) contacts the upper surface of the flange frame (100), and the hydraulic cylinder (1003) provides an initial tension, and the pressure roller (1005) tightly fixes the flange frame (100) on the rigid platform (1000); In steps S2 to S5, the driving mechanism (1007) drives the sliding block (1002) to move, and the pressure roller (1005) rolls on the upper surface of the flange frame (100).

6. The method of claim 5, wherein the method further comprises: The sliding block (1002) is provided with a vertical guide column (1008), the guide column (1008) penetrates through the mounting block (1006) and is in a sliding fit mode with the mounting block (1006), and the guide column (1008) is provided with a spring (1009) in a sleeving mode, the upper end of the spring (1009) is fixedly connected with the mounting block (1006), and the lower end is connected with the sliding block (1002) through a pressure sensor (10010); In step S1, the initial elastic force of the spring (1009) detected by the pressure sensor (10010) is recorded; In steps S2 to S5, during the rolling process of the pressure roller (1005), the tension provided by the hydraulic cylinder (1003) remains the initial tension, the real-time elastic force of the spring (1009) is detected by the pressure sensor (10010), when the real-time elastic force decreases to an elastic force threshold value, it is determined that the pressure roller (1005) moves to the buckling deformation position of the flange frame (100), at this time, the tension output by the hydraulic cylinder (1003) is increased, the pressure of the pressure roller (1005) on the buckling deformation position is increased, the buckling deformation position is corrected until the real-time elastic force is greater than the elastic force threshold value; the above process is repeated.

7. The method of claim 1, wherein the method further comprises: The stress detection element (11) is a fiber Bragg grating sensor, the first tension bar (1) and the second tension bar (2) are provided with mounting grooves in the side walls, and the fiber Bragg grating sensor is pasted in the mounting grooves through high-temperature ceramic glue; After step S6, the lower half of the low-pressure cylinder of the steam turbine is subjected to heat treatment, and in the process of heat treatment, the fiber Bragg grating sensor is used to detect the stress change of the first tension bar (1) and the second tension bar (2) in real time, so as to judge the heat treatment effect; After heat treatment, the first tension bar (1) and the second tension bar (2) are cut off.

8. The method of claim 1, wherein: The welding seam is heated by using a high-frequency induction heating coil to realize stress relief.

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