A water-cooled converter support ring process hole inserting tube structure and its application

Abstract

The present invention discloses a water-cooled converter support ring process hole inserting pipe structure and its application, and relates to the technical field of manufacturing and maintenance of large converter support rings. The present invention comprises two layers of converter support ring webs with annular cross-sections and a plurality of support ring process holes evenly distributed after penetrating the two layers of converter support ring webs. After the support ring cover plates are fixed to the upper and lower end surfaces of the two layers of converter support ring webs, a hollow support ring box is formed. In the support ring box, both sides of each support ring process hole are fixed with support ring internal ribs; it also comprises two symmetrical segmented inserting pipes inserted from the inside of the support ring box to the inner hole and outer hole of each support ring process hole respectively; an annular hyperboloid transition ring with an arc cross-section is sleeved on the segmented inserting pipe, and a smooth transition form with a certain arc is formed between the annular hyperboloid transition ring and the converter support ring web by a weld, which can effectively reduce the stress concentration at the connection between the segmented inserting pipe and the converter support ring web, improve the structural strength of this part, and thus improve the service life of the converter support ring web.

Description

Technical Field

[0001] The invention relates to the technical field of manufacturing and repairing a large converter support ring, and more specifically to a water-cooled converter support ring process hole inserting tube structure and application thereof. Background Art

[0002] Current large box-type water-cooled converter support ring process hole structure (attached Figure 2 , 3 ) are all formed by opening a support ring process hole 2 on the converter support ring web 1, and then inserting a pipe of corresponding size and welding it. The pipe used is a straight section circular pipe, and the connection structure is the support ring web opening and the straight section circular pipe welding.

[0003] Depend on Figure 1 It can be seen that there are connecting devices (horizontal suspension mechanism 8 and vertical suspension mechanism 7) between the converter and the supporting ring on the supporting ring bottom plate and the supporting ring web 1 on both sides of the converter supporting ring process hole 2. During the steelmaking operation, the insert pipe of the converter supporting ring process hole 2 not only has to withstand the high temperature generated by the converter steelmaking, but also under the action of the drive shaft 9, due to the repeated turning motion of charging and tapping of the converter steelmaking, the insert pipe of the process hole near the connecting device between the converter and the supporting ring will produce fatigue cracks due to the long-term alternating load.

[0004] After searching, it is found that there are relevant patents that solve the above-mentioned deficiencies. For example, the Chinese patent announcement number is: CN204342824U, and the authorization announcement date is: May 20, 2015. It discloses a converter support ring that can repair the leakage of the air vent online. It is mainly composed of a support ring body, air vents on the support ring body, a water tank and an ear shaft. The leak repairer is connected to the air vent where the leak occurs. The leak repairer is a tube type or a rod type. The characteristics of this utility model are: it can take emergency measures online to remedy the problem of water leakage in the air vent, ensure safe production, and improve the unit operation rate. However, since it is only a temporary effect of repairing the leak, it cannot solve the water leakage problem from the root. Summary of the invention

[0005] 1. Technical problem to be solved by the invention

[0006] The present application aims to provide a water-cooled converter support ring process hole inserting tube structure and its application, which is advantageous over the prior art described in the background art in at least one aspect.

[0007] 2. Technical solution

[0008] In order to achieve the above object, the technical solution provided by the present invention is:

[0009] A water-cooled converter support ring process hole inserting pipe structure includes two layers of converter support ring webs with circular cross-sections and multiple support ring process holes evenly distributed after penetrating the two layers of converter support ring webs. After the support ring cover plates are fixed to the upper and lower end surfaces of the two layers of converter support ring webs, a hollow support ring box is formed. In the support ring box, both sides of each support ring process hole are fixed with support ring internal ribs; it also includes two symmetrical segmented inserting pipes inserted from the inside of the support ring box to the inner hole and outer hole of each support ring process hole respectively; the segmented inserting pipe is sleeved with an annular hyperbolic transition ring with an arc cross-section, and the hyperbolic transition ring is tightly fixed to the inner wall of the converter support ring web. A smooth transition form with a certain arc is formed between the annular hyperbolic transition ring and the converter support ring web by a weld. Through the arc transition, the stress concentration at the connection between the segmented inserting pipe and the converter support ring web can be effectively reduced, the structural strength of this part can be improved, and the service life of the converter support ring web can be improved.

[0010] According to a further technical solution, the inner end surface of the segmented insert is a weld slope surface, and after the weld slope surfaces of the two segmented inserts are welded, a double-tube weld is formed, which is convenient for welding.

[0011] A further technical solution is that the groove between the segmented insert and the process hole of the support ring is fixed by insert weld layer welding. The insert weld is generally produced by grinding and cutting before welding, which serves as a welding preparation to improve the fixing effect; the contact surface with the hyperbolic transition ring is also arc-shaped to facilitate fitting with the hyperbolic transition ring.

[0012] A further technical solution is that the hyperbolic transition ring includes an inner arc surface and an outer arc surface. The connections between the inner arc surface and the segmented insert and the converter support ring web are fixed by welding through the transition ring fixing weld layer, and triple fixation is used to enhance the stability of the fixation effect; the curvature of the outer arc surface is adapted to the converter support ring web to improve the tight fit effect; an outer chamfer of the transition ring is provided at the center of the ring to facilitate welding.

[0013] A further technical solution is that the transition ring fixing weld layer is also in an arc shape with the arc mouth pointing outward, the arc is R, and the triple arc transition further reduces stress concentration.

[0014] A further technical solution is that the hyperbolic transition ring is formed by welding two pieces that are horizontally cut from the perspective of the main view, and the end face of half of the hyperbolic transition ring is set to be sloped. When the two half hyperbolic transition rings are put together, a transition ring connecting weld is formed, which can effectively improve the tightness of the sleeve and the fit with the belly plate of the converter support ring.

[0015] A further technical solution is that the curvature R of the transition ring fixed weld layer is not less than the curvature of the inner arc surface, so as to achieve the best effect of reducing stress concentration; the internal rib plate of the support ring is perforated with transverse waist-shaped weight-reducing holes, which not only reduce weight, but more importantly, also play a role in buffering stress concentration.

[0016] An application of a water-cooled converter support ring process hole inserting structure includes the following steps:

[0017] Step 1: Production of segmented intubation: Use a combination of finished steel pipes and steel rings or a combination of steel plate rolled welded pipes and steel rings, all of which are welded and then machined into shape; forgings or steel castings can also be used for overall processing and forming.

[0018] Step 2: Through hole: Open a support ring process hole on the belly plate of the converter support ring, and process the groove required for welding on the support ring process hole;

[0019] Step 3: plugging: two symmetrical segmented plugs are inserted from the inner hole and outer hole of the support ring process hole respectively, and a double-tube weld is formed at the connection of the two segmented plugs;

[0020] Step 4: Pipe welding: fill the groove at the connection between the segmented pipe and the support ring process hole by welding to form a pipe weld layer (4); then fill the weld between the double pipes by welding.

[0021] Further applications include:

[0022] Step 5: Stress relief: Immediately after the weld is completed, the weld should be covered with insulation cotton, and a heating plate should be laid inside the inner wall of the straight section of the insert to perform stress relief annealing.

[0023] For further application, the stress relief heat treatment curve in step 5 is that the minimum interlayer temperature is not less than 150°C during the multi-layer welding process, and the stress relief heat treatment temperature is slowly increased to 580°C at a rate of ≤50°C / hour on the basis of the interlayer temperature, kept at this temperature for 5 hours, and then slowly cooled at a rate of ≤50°C / hour.

[0024] 3. Beneficial effects

[0025] Compared with the prior art, the technical solution provided by the present invention has the following beneficial effects:

[0026] (1) In the water-cooled converter support ring process hole inserting tube structure of the present invention, the symmetrically inserted segmented inserting tubes can be prefabricated in advance to improve the fixing and installation efficiency in the support ring process hole;

[0027] (2) The water-cooled converter support ring process hole insert tube structure and its application of the present invention reduce the stress concentration phenomenon at the connection between the converter support ring process hole insert tube and the support ring web, and by increasing the structural strength and fatigue crack resistance of this part and reducing the stress concentration of this part, the structural bearing capacity and fatigue crack resistance of the converter support ring are enhanced, thereby extending the service life of the converter support ring. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1It is a schematic diagram of the structure of a large box-type water-cooled converter in the prior art;

[0029] Figure 2 It is a cross-sectional view of a large box-type water-cooled converter support ring of the prior art;

[0030] Figure 3 It is a schematic diagram of the structure of a large box-type water-cooled converter support ring in the prior art;

[0031] Figure 4 It is a schematic diagram of the structure of the process hole inserting pipe of the water-cooled converter support ring of the present invention;

[0032] Figure 5 It is a stereoscopic diagram of the segmented cannula in the present invention;

[0033] Figure 6 This is a stereoscopic diagram of two segmented cannulas in the present invention after docking;

[0034] Figure 7 It is a side view of the segmented cannula in the present invention;

[0035] Figure 8 The figure is a schematic diagram of the segmented cannula to be processed;

[0036] Fig. 9 A cross-sectional view of a segmented cannula to be processed;

[0037] Fig.10 It is a schematic diagram of the structure of the hyperbolic transition ring in the present invention;

[0038] Fig.11 for Fig.10 Schematic diagram after enlarging point A;

[0039] Fig.12 for Fig.10 A top view of

[0040] Fig.13 for Fig.10 Side view of

[0041] Fig.14 for Fig.10 Axial cross-sectional view of

[0042] Fig.15 This is a magnified schematic diagram of the weld after the segmented pipe is plugged in;

[0043] Fig.16 This is the stress relief heat treatment curve.

[0044] In the figure: 1-converter support ring web; 2-support ring process hole; 3-segmented intubation; 4-intubation weld layer; 5-hyperbolic transition ring; 6-transition ring fixed weld layer; 7-vertical suspension mechanism; 8-horizontal suspension mechanism; 9-driving shaft; 11-support ring housing; 12-support ring internal rib plate; 13-support ring cover plate; 21-outer hole; 22-inner hole; 30-steel plate rolled welded pipe; 31-weld between double pipes; 33-weld slope; 50-steel ring; 51-inner arc surface; 52-outer arc surface; 53-transition ring connecting weld; 55-transition ring outer chamfer; 56-steel ring groove; 121-weight reduction hole. DETAILED DESCRIPTION

[0045] In order to further understand the content of the present invention, the invention is described in detail with reference to the accompanying drawings.

[0046] Example 1

[0047] A water-cooled converter support ring process hole inserting structure of this embodiment, such as Figures 1 to 3 As shown, it includes two layers of converter ring webs 1 arranged in a circular cross section and a plurality of ring process holes 2 evenly distributed after penetrating the two layers of converter ring webs 1. After the ring cover plates 13 are fixed to the upper and lower end surfaces of the two layers of converter ring webs 1, a hollow ring box 11 is formed, as shown in FIG. Figure 4 As shown, in the support ring box 11, both sides of each support ring process hole 2 are fixed with support ring internal ribs 12; and also include a support ring box 11 inserted into the inner hole 22 and the outer hole 21 of each support ring process hole 2, such as Figure 6 The two symmetrical segmented cannulas 3 shown; Figure 5 , 14 As shown, the segmented insert tube 3 is sleeved with an annular hyperbolic transition ring 5 with an arc-shaped cross section, and the hyperbolic transition ring 5 is tightly fixed to the inner wall of the converter support ring web 1. In a specific application, after the hyperbolic transition ring 5 is sleeved on the segmented insert tube 3, it is tightly fixed to the inner wall of the support ring box 11 of the converter support ring web 1. Through the arc transition of the hyperbolic transition ring 5, the stress concentration at the connection between the segmented insert tube 3 and the converter support ring web 1 in the support ring process hole 2 can be effectively reduced, thereby improving the service life of the converter support ring web 1.

[0048] Example 2

[0049] The present embodiment is a water-cooled converter support ring process hole inserting tube structure, the basic structure is the same as that of embodiment 1, the difference and improvement are: Figure 5 , 6As shown, the inner end face of the segmented cannula 3 is a weld slope 33. After the weld slopes 33 of the two segmented cannula 3 are welded, a double-tube weld 31 is formed, which is convenient for welding. The groove between the segmented cannula 3 and the support ring process hole 2 is fixed by welding through the cannula weld layer 4. The cannula weld is generally produced by grinding and cutting before welding, which plays a role in welding preparation to improve the fixing effect. The contact surface between the cannula weld layer 4 and the hyperbolic transition ring 5 is also arc-shaped to facilitate fitting with the hyperbolic transition ring 5. The hyperbolic transition ring 5 includes an inner arc surface 51 and an outer arc surface 52. The connection between the inner arc surface 51 and the segmented cannula 3 and the converter support ring web 1 is fixed by welding through the transition ring fixing weld layer 6, which is triple fixed to enhance the stability of the fixation effect; the curvature of the outer arc surface 52 is adapted to the converter support ring web 1 to improve the tight fitting effect, such as Figure 6 , 7 As shown, the transition ring outer chamfer 55 is arranged at the center of the hyperbolic transition ring 5 to facilitate welding. The transition ring fixed weld layer 6 is also in an arc shape with an arc mouth pointing outward, with an arc of R, a triple arc transition, and a wavy shape, which can further reduce stress concentration.

[0050] The application of the water-cooled converter support ring process hole inserting structure of this embodiment includes the following steps:

[0051] Step 1: Production of segmented cannula: Use finished steel pipe and steel ring 50 combination or as Figure 8 , 9 The steel plate rolled welded pipe 30 and the steel ring 50 combination shown are both welded and then machined; welding refers to welding the steel ring groove 56 reserved at the connection between the steel ring 50 and the rolled welded pipe 30 to form a transition ring fixed weld layer 6 with an arc of R; so as to increase the structural strength and fatigue crack resistance of this part and reduce the stress concentration of this part; mechanical processing refers to cutting and grinding the steel ring 50 into a hyperbolic transition ring 5; the segmented insert 3 can also be integrally processed and formed by forgings or steel castings;

[0052] Step 3: plugging: two symmetrical segmented inserts 3 are inserted from the inner hole 22 and the outer hole 21 of the support ring process hole 2 respectively, and a double-tube weld 31 is formed at the connection of the two segmented inserts 3;

[0053] Step 4: Pipe welding: Fig.15 As shown, the groove at the connection between the segmented insert tube 3 and the support ring process hole 2 is filled by welding to form an insert tube weld layer 4, forming an outer chamfer 55 of the transition ring; then the weld 31 between the double tubes is filled by welding; the connection between the hyperbolic transition ring 5 and the converter support ring web 1 is also welded to form a transition ring fixed weld layer 6 with an arc of R.

[0054] Step 5, stress relief: Immediately after the weld is completed, the weld should be covered with insulation cotton, and a heating plate should be laid in the segmented insert 3 for stress relief annealing. The heat treatment curve for stress relief is to use a multi-layer welding process, slowly increase the temperature to 580℃ at a rate of ≤50℃ / hour based on the interlayer temperature, keep it warm for 5 hours, and then slowly cool it down at a rate of ≤50℃ / hour.

[0055] Example 3

[0056] The water-cooled converter support ring process hole inserting structure of this embodiment has the same basic structure as that of embodiment 2, except that: Fig.10 , 13 As shown, the hyperbolic transition ring 5 is formed by welding two parts cut horizontally from the perspective of the main view, and the end surface of the half hyperbolic transition ring 5 is set with a slope. The two half hyperbolic transition rings 5 ​​are put together to form a transition ring connecting weld 53, which is convenient for welding and can effectively improve the tightness of the sleeve connection and the fit with the converter support ring web 1. Fig.10 As shown, the outer arc surface 52 is convex, as shown in FIG. Fig.12 As shown, the outer arc surface 52 is concave outward, mainly to adapt to the arc-shaped converter support ring web 1. The curvature R of the transition ring fixed weld layer 6 is not less than the curvature of the inner arc surface 51, so as to achieve the best effect of reducing stress concentration. The inner rib plate 12 of the support ring is pierced with a transverse waist-shaped weight-reducing hole 121. While reducing weight, more importantly, the transverse waist-shaped weight-reducing hole 121 has a certain elasticity and also plays a role in buffering stress concentration. Moreover, it also plays the role of a waterway and a maintenance manhole.

[0057] Example 4

[0058] The water-cooled converter support ring process hole inserting pipe structure and its application in this embodiment, the basic structure of the water-cooled converter support ring process hole inserting pipe structure is the same as that in Example 3, and the specific application is:

[0059] The process hole 2 of the support ring of a converter support ring charging side box of a steel plant was cracked and leaked during use. In order to completely solve the leakage of the process hole of the support ring box, the following treatment plan was formulated in combination with the on-site construction conditions:

[0060] 1. Main measures:

[0061] (1) Clean the cracks on the segmented insert 3 of the support ring process hole 2 on the inner side of the support ring box, and after the cracks are cleaned, repair the grooves cleared by the cracks by welding.

[0062] (2) Install the process hole insert pipe hyperboloid transition ring 5. The size of the hyperboloid transition ring is shown in Fig.11As shown, the hyperbolic transition ring 5 is installed on the upper side of the joint between the segmented insert 3 and the converter support ring web 1. After installation, the hyperbolic transition ring 5 is welded firmly all around. The weld of the transition ring fixed weld layer 6 is an arc-shaped fillet weld with a certain arc radius. The weld arc radius is determined according to the thickness of the hyperbolic transition ring 5. The weld size of this scheme is R40~100, which is not less than the curvature of the inner arc surface 51.

[0063] 2. Welding

[0064] (1) Preheating before welding

[0065] The preheating temperature is controlled at 100-120°C. The heating method adopts flame heating or crawler electric heating according to the conditions on site.

[0066] (2) Welding material: Select J507RH, specification

[0067] (3) Welding equipment: ZX7-400 welding machine is used.

[0068] (4) Welding process parameters are shown in Table 1

[0069] Table 1

[0070]

[0071] (5) Welding sequence

[0072] After installation, the process hole inserts are welded synchronously on the inside and outside, and the two process hole inserts are welded synchronously.

[0073] (6) Technical requirements and operating instructions

[0074] 1) Welding shall comply with the provisions of JB / T5000.3-2007 "General Technical Conditions for Welding", the fillet weld shall be Bk grade, and the weld joint defect shall be Level II.

[0075] 2) Welders must be certified and strictly follow the welding process.

[0076] 3) During operation, the welds have smooth transitions on both sides, the outer welds are welded flush, the inner welds are welded into a certain arc, and the support ring box adopts single-sided welding and double-sided forming technology. There shall be no defects such as cracks, slag inclusions, pores, etc. on the weld surface.

[0077] 4) Use multi-layer and multi-pass welding technology, and after each layer is welded, slag should be removed and hammering should be performed to relieve stress.

[0078] 5) The interlayer temperature is controlled between 150 and 240°C.

[0079] 3. Post-weld stress relief annealing

[0080] Immediately after the weld is completed, the weld should be covered with insulation cotton, and a heating plate should be laid inside the intubation for stress relief annealing. The specific process of stress relief heat treatment is to slowly heat up to 580℃ at a rate of ≤50℃ / hour based on the interlayer temperature, keep warm for 5 hours, and then slowly cool down at a rate of ≤50℃ / hour. The stress relief heat treatment curve is as follows Fig.16 As shown in the figure, the stress-relieving heat treatment curve is to use a multi-layer welding process, slowly raise the temperature to 580℃ at a rate of ≤50℃ / hour based on the interlayer temperature, keep it warm for 5 hours, and then slowly cool it down at a rate of ≤50℃ / hour. The heat treatment device uses a far-infrared electric heater, and the heating plate adopts a crawler type, indirect heating without direct contact, which can reduce stress while avoiding additional stress caused by direct contact.

[0081] 4. Post-welding cleaning

[0082] After welding, use an angle grinder to clean the weld surface and surrounding areas of weld scars, welding spatter, etc.

[0083] 5. Post-weld inspection

[0084] (1) Appearance inspection

[0085] No welding defects such as cracks, weld bumps, pores, slag inclusions, etc. are allowed on the weld surface.

[0086] (2) Internal testing

[0087] The welds are subjected to 100% ultrasonic testing, with inspection level B and defect assessment level not lower than level II.

[0088] 6. After the NDT is passed, remove the heating device and the anti-deformation rigid fixing device and hand over the project.

[0089] A water-cooled converter support ring process hole insert pipe structure and its application in this embodiment adopts the technical means of adding a hyperbolic transition ring 5 in the measure of reducing stress concentration at the support ring process hole 2. The advantage of this technical means is that it can enhance the structural strength of this part on the one hand, and can greatly reduce the existence of stress concentration on the other hand. At the same time, the welds connecting the two ends of the hyperbolic transition ring 5 with the support ring web 1 and the segmented insert pipe 3 are optimized and designed as arc welds of the transition ring fixed weld layer 6, which effectively avoids the stress concentration caused by a certain angle in this part. The stress situation at the position of the converter support ring process hole 2 is greatly improved, and the structural strength of this part is also greatly improved. It has been implemented on the 300T converter support ring of a steel plant. After 1 year of use verification, there is no fatigue crack in this part of the converter support ring, which effectively ensures the normal operation of the converter steelmaking operation.

[0090] The present invention and its embodiments are described schematically above, and the description is not restrictive. The drawings show only one embodiment of the present invention, and the actual structure is not limited thereto. Therefore, if a person skilled in the art is inspired by it and designs a structural method and an embodiment similar to the technical solution without creativity without departing from the purpose of the invention, they shall all fall within the protection scope of the present invention.

Claims

1. A water-cooled converter support ring process hole inserting structure, comprising two layers of converter support ring webs (1) with a circular cross-section and a plurality of support ring process holes (2) evenly distributed after penetrating the two layers of converter support ring webs (1), wherein a support ring cover plate (13) is fixed to the upper and lower end surfaces of the two layers of converter support ring webs (1) to form a hollow support ring box (11), characterized in that: In the support ring box (11), both sides of each support ring process hole (2) are fixed with support ring internal ribs (12); it also includes two symmetrical segmented inserts (3) inserted from the inside of the support ring box (11) into the inner hole (22) and the outer hole (21) of each support ring process hole (2); the segmented inserts (3) are sleeved with an annular hyperbolic transition ring (5) with an arc cross section, and the hyperbolic transition ring (5) is tightly fixed to the inner wall of the converter support ring web (1); the support ring internal ribs (12) are pierced with transverse waist-shaped weight-reducing holes (121); The inner end surface of the segmented insert pipe (3) is a weld slope surface (33), and the weld slope surfaces (33) of the two segmented insert pipes (3) are welded to form a double-tube weld (31).

2. The water-cooled converter support ring process hole inserting structure according to claim 1 is characterized in that: The groove between the segmented insert pipe (3) and the support ring process hole (2) is fixed by welding through the insert pipe weld layer (4); the contact surface between the insert pipe weld layer (4) and the hyperbolic transition ring (5) is also arc-shaped.

3. The water-cooled converter support ring process hole inserting structure according to claim 1 is characterized in that: The hyperbolic transition ring (5) comprises an inner arc surface (51) and an outer arc surface (52); the inner arc surface (51) is fixed to the segmented insert (3) and the converter support ring web (1) by welding via a transition ring fixing weld layer (6); the curvature of the outer arc surface (52) is adapted to the converter support ring web (1), and an outer chamfer (55) of the transition ring is provided at the center of the ring.

4. The water-cooled converter support ring process hole inserting structure according to claim 3 is characterized in that: The transition ring fixed weld layer (6) is also in the shape of an arc with the arc mouth facing outwards, and the arc is R.

5. The water-cooled converter support ring process hole inserting structure according to claim 3 is characterized in that: The hyperbolic transition ring (5) is formed by welding two half hyperbolic transition rings that are horizontally cut from the perspective of the main view, the end faces of the half hyperbolic transition rings are arranged in a slope, and the two half hyperbolic transition rings are brought together to form a transition ring connecting weld (53).

6. The water-cooled converter support ring process hole inserting structure according to any one of claims 3 to 5, characterized in that: The curvature R of the transition ring fixed weld layer (6) is not less than the curvature of the inner curved surface (51).

7. An application of the water-cooled converter support ring process hole inserting structure according to any one of claims 1 to 6, characterized in that: The steps include: Step 1: manufacturing the segmented intubation pipe: using a finished steel pipe and a steel ring (50) or a steel plate rolled welded pipe (30) and a steel ring (50), both of which are welded and then machined or integrally machined using forgings or steel castings; Step 2, through hole: opening a support ring process hole (2) on the converter support ring web (1), and processing the support ring process hole (2) with a groove required for welding; Step 3: plugging: two symmetrical segmented plugs (3) are inserted from the inner hole (22) and the outer hole (21) of the support ring process hole (2) respectively, and a double-tube weld (31) is formed at the connection of the two segmented plugs (3); Step 4: Pipe welding: The groove at the connection between the segmented pipe (3) and the support ring process hole (2) is filled by welding to form a pipe weld layer (4); and then the weld (31) between the double pipes is filled by welding.

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