Specification for welded pipe process design

Specification for welded pipe process design

Steel and Welding Materials

1. Before welding, the steel grade of the material to be welded must be identified in order to correctly select the corresponding welding material and determine the appropriate welding process and heat treatment process.
2. The steel products must meet the national standards (or ministerial standards, professional technical standards), and the imported steel products must meet the national standards of the country or the technical standards stipulated in the contract.
3. The quality of welding materials [welding rod, welding wire, tungsten rod, argon, oxygen, acetylene gas (calcium carbide) and flux] meets the national standard (or relevant standard).
4. Steel materials, welding rods, welding wires, etc. should have the quality certificate of the manufacturer. Where there is no quality certificate or if there is any doubt about its quality, it should be randomly checked and tested according to the batch number. It can be used only after passing the test.

5. The selection of welding rods and wires should be based on the chemical composition and mechanical properties of the base metal, the crack resistance of the welded joints, carbon diffusion, preheating before welding, heat treatment after welding and service conditions.

5.1 When the same kind of steel is welded, the selection of welding rod (welding wire) generally meets the following requirements: the properties and chemical composition of the weld metal are equivalent to those of the base metal.
5.2. The process performance is good.
5.3 When welding dissimilar steel materials, the selection of welding rod (welding wire) generally meets the following requirements:
(1) When one of the two sides is austenitic stainless steel, a stainless steel electrode (welding wire) with a higher nickel content can be selected.
(2) When the steel materials on both sides are non-austenitic stainless steel, the electrode (welding wire) whose composition is between the two or the side with low alloy content can be selected.

6. The electrodes used for argon tungsten arc welding should use cerium tungsten rods, and the purity of argon gas used should not be lower than 99.95%.
7. The purity of oxygen used in oxy-acetylene welding should be above 98.5%. The purity of acetylene gas should meet the requirements of GB6819-86 "Dissolved Acetylene". If acetylene gas is produced by a power plant, the calcium carbide should have a factory certificate, and its quality can be determined by checking the sulfur and phosphorus content in the weld metal (according to the standard of the welded metal); the acetylene gas used for welding should be filtered , Calcium carbide that has not been inspected or whose impurity content exceeds the standard shall not be used for welding of supervised parts.
8. The flux used for submerged arc automatic welding should meet the technical requirements of relevant standards.

Preparation before welding

1. The position of the welding joint should avoid the stress concentration area and be convenient for welding and heat treatment. Generally, the following requirements should be met:

1.1 The center line of the pipe welding joint on the heating surface of the boiler should be at least 70mm away from the pipe bending starting point or the steam drum, the outer wall of the header and the edge of the support and hanger, and the distance between the two butt welding joints should not be less than 150mm.
1.2 The distance between the center line of the pipe butt joint and the bending starting point of the pipe shall not be less than the outer diameter of the pipe, and shall not be less than 100mm (except welding, forging, octg casing,and casting pipe fittings), and the distance from the edge of the support hanger shall be at least 50mm. The distance shall not be less than the diameter of the pipe and shall not be less than 150mm.
1.3 Pipe joints and instrument sockets are generally not allowed to be placed in welds or heat-affected zones.
1.4 For the butt weld of the cylinder, the distance from the center line to the bending starting point of the head is not less than the wall thickness plus 15mm, and not less than 25mm and not more than 50mm.
1.5 For cylinders and heads with longitudinal welds, the distance between two adjacent longitudinal welds should be greater than 3 times the wall thickness and not less than 100mm
1.6 The pipe hole of the welded pipe should be avoided as much as possible on the weld seam, and the pipe hole weld seam should not overlap with the heat-affected zone of the adjacent weld seam. If the hole must be opened on or near the weld seam, the following conditions should be met:
(1) The welds on both sides of the pipe hole that are larger than the hole diameter and not larger than 60mm shall pass the non-destructive inspection according to the requirements of Table 8.0.3.
(2) The edge of the hole is not on the weld defect.
(3) Pipe joints need to be heat treated after welding to relieve stress.
1.7 The lap joint size of lap welds should not be less than 5 times the thickness of the parent metal, and not less than 30mm.

2. The groove form of pipes, pipelines, pressure vessels and steel structures shall be processed according to the design drawings. If there is no regulation, the type and size of the groove shall ensure welding quality, less filler metal, improve working conditions and facilitate operation , Reduce welding stress and deformation, and adapt to the requirements of flaw detection.

3. Welding blanking and groove processing shall be carried out according to the following requirements:

3.1 It is advisable to use mechanical methods for weldment blanking. For alloy steels with a large hardening tendency, pipes with a nominal diameter of <100mm and steam and water pipes with a nominal diameter of >100mm and a working pressure of >3.9Mpa, mechanical methods should be used in particular.
3.2 If the material is cut by thermal processing (such as gas cutting), a machining allowance should be left in the cut part to remove the hardened layer and overheated metal. After the alloy steel with a large tendency of hardening is blanked by thermal processing, the cut part should be annealed first before processing.
3.3 The preparation of the groove should be carried out by mechanical processing. If flame cutting is used to cut the groove, the oxides and slag splashes on the surface of the cut should be cleaned up according to the requirements of Article 5, and the unevenness should be cleaned. Repair level.

4. After blanking and groove processing, the weldment shall be inspected according to the following requirements, and the assembly can only be carried out after passing the pass.
(1) For steel with high hardenability, if flame cutting is used to cut the groove, it must pass the surface flaw detection inspection after processing.
(2) The base metal at the groove has no defects such as cracks, heavy skin, groove damage and burrs.
(3) The size of the groove processing meets the requirements of the drawing.
(4) There are no defects such as cracks and interlayers within the scope of cleaning specified in Article 5.

5. Before the weldment is assembled, the oil, paint, dirt, rust, etc. on the surface of the weld joint and the inner and outer walls of the nearby base metal should be cleaned until it emits a metallic luster. The scope of cleanup is as follows:
(1) Manual arc welding butt joint, each side is 10~15mm
(2) Submerged arc welding joint: each side is 20mm.
(3) Welding joint of fillet joint: the K value of welding angle is +10mm

6. The end face of the butt joint should be perpendicular to the center line of the pipe. Its deflection △f shall not exceed the following regulations:
(1) When φ≤60mm, △f≤0.5mm
(2) When 60<φ≤159mm, △f≤1mm
(3) When 159<φ≤219mm, △f≤1.5mm
(4) When φ＞219mm, △f≤2mm

7. When the weldment is aligned, the inner wall should generally be flush. If there is a misalignment, the misalignment value should meet the following requirements:
(1) The local stagger value of butt single-sided welding shall not exceed 10% of the wall thickness, and shall not be greater than 1mm.
(2) The local stagger value of butt double-sided welding shall not exceed 10% of the thickness of the weldment, and shall not be greater than 3mm.

8. When weldments with different thicknesses are matched, the thickness difference can be handled according to the prescribed method.
9. When the local gap of the welding joint is too large, try to trim it to the specified size, and it is strictly forbidden to add fillers in the gap.
10. Welding rods, welding wires and fluxes should be stored in a dry, well-ventilated warehouse with a temperature greater than 5°C and a relative air humidity of less than 60%. Electrodes and fluxes should be baked no more than twice according to their instructions before use. Before the wire is used, the rust and oil should be removed until the metallic luster is exposed. The welding rods for welding important parts should be placed in a special insulated box with a temperature of 100~150°C when used, and can be taken as needed. When the electrodes stored for more than one year are used for welding important parts, if there is any doubt about their quality, they should be re-appraised and used only after they meet the requirements.

11. When welding and assembling, the workpiece to be welded should be placed firmly to prevent deformation and additional stress during welding and heat treatment.
12. Except for the cold-drawn joint specified in the design, it is forbidden to use strong joints for other welding joints, and it is not allowed to use thermal expansion method to prevent additional stress.
13. The welding place should take measures to prevent wind, rain, snow and cold.
14. The welding construction process includes four important processes of counterpart assembly, welding, heat treatment and inspection. The next process can only be carried out after this process meets the requirements, otherwise the construction of the next process is prohibited.
15. Qualified welders should carry out simulation exercises adapted to the actual conditions before welding the heating surface tubes of the boiler, and formal welding can only be performed after the fracture surface inspection meets the requirements.

Welding process

1. The minimum ambient temperature allowed during soldering is as follows:
Carbon steel: -20°C.
Low alloy steel, common low alloy steel: -10℃.
Medium and high alloy steel: 0°C.

2. The recommended preheating temperature for various steel materials before welding is as follows:

2. 1 The preheating requirements are put forward according to the welding procedure qualification.
2.2 When alloy steel pipes, pipe fittings (such as elbows, tees, etc.) and large-thickness plates with wall thickness ≥ 6mm are welded at negative temperature, the preheating temperature can be increased by 20~50°C according to the specified value in Table 1.
2.3 Low alloy steel pipes with wall thickness <6mm and carbon steel pipes with wall thickness >15mm should also be properly preheated when welding at negative temperature.
2.4 When dissimilar steels are welded, the preheating temperature should be selected according to the side with poorer welding performance or higher alloy composition.
2.5 The preheating temperature stipulated by the supervisor shall prevail when the connection seat is welded with the supervisor.
2.6 When welding non-pressure-bearing parts and pressure-bearing parts, the preheating temperature should be selected according to the pressure-bearing parts.

3. The preheat width starts from the center of the opposite door and is not less than three times the thickness of the weldment on each side. The temperature rise rate during preheating of welded joints with a thickness greater than 35 mm shall comply with the provisions of Article 605.
4. During the welding process, the temperature between layers should not be lower than the lower limit of the specified preheating temperature, and not higher than 400°C.
5. When welding pressure-bearing pipes, in order to ensure the cleanliness of the inner wall of the pipe (or pipe) and the quality of the root layer of the weld, the welding method used should comply with the regulations in the table below.
6. For medium and high alloy steel (chromium content ≥ 3% or total alloy content > 5%) pipes and pipe welds, in order to prevent root layer oxidation or overburning, the inner wall should be protected by argon or mixed gas during welding.
7. It is strictly forbidden to ignite the arc, test the current or randomly weld temporary supports on the surface of the workpiece to be welded, and the surface of high-alloy steel materials must not be welded with fixtures for counterparts.
8. When the pipe is welded, there must be no hallway inside the pipe.

9. During spot welding, except that the welding material, welding process, welder and preheating temperature should be the same as the formal welding time, the following requirements should also be met:
9.1 When spot welding at the root of the counterpart, the quality of each solder joint should be checked after spot welding. If there is any defect, it should be removed immediately and spot welding should be carried out again.
9.2 If the thick-walled large-diameter pipe is spot-fixed by the method of filling, when removing the temporary spot solid, the base metal should not be damaged, and the residual welding scars should be cleaned and polished.

10. After the inspection of the root layer welds made by tungsten-argon arc welding, the welding of the sub-layer welds should be carried out in time to prevent cracks. When multi-layer and multi-pass welds are welded, they should be inspected layer by layer. After passing the self-inspection, the second layer can be welded until it is completed.

11. The welding of thick-walled and large-diameter pipes should adopt multi-layer multi-pass welding. When the wall thickness is greater than 35mm, it should also meet the following requirements:
11.1 The thickness of the welding layer for argon arc welding is not less than 3mm.
11.2 The thickness of a single layer of other weld bead shall not be greater than the diameter of the electrode used plus 2mm.
11.3 The swing width of a single weld bead shall not be greater than 5 times the diameter of the electrode used.

12. In order to reduce welding deformation and joint defects, two-person symmetrical welding should be adopted for the butt joints of pipes with a diameter greater than 194mm and densely arranged boiler pipes (pipe spacing ≤ 30mm). ]
13. Welding of steel structures shall be carried out in accordance with the provisions of the work instructions.
14. During welding, special attention should be paid to the quality of joints and arc extinguishing. The molten pool should be filled up when arc extinguishing, and the joints of multi-layer multi-pass welding should be staggered.
15. The welding process shall be completed continuously, except that welding is required in batches in the process and inspection. If it is forced to be interrupted, measures to prevent cracks (such as post-cooling, slow cooling, heat preservation, etc.) should be taken. When re-welding, check carefully and confirm that there are no cracks before continuing to weld according to the process requirements.

16. The butt joints of pipes or vessels with a nominal diameter greater than or equal to 1000mm shall be welded on both sides, and root cleaning measures shall be taken to ensure the quality of back sealing welding. After root cleaning, the oxides should be removed according to the requirements of Article 405.
17. For the concealed welds that need to be inspected, other processes can only be carried out after passing the inspection.
18. After welding, the weld should be cleaned, and after passing the self-inspection, the welder's code stamp or permanent mark should be stamped near the weld.
19. For steels that are prone to delayed cracks, heat treatment should be carried out immediately after welding, otherwise post-heat treatment should be carried out, and the temperature should be 300~350℃, and the constant temperature time should not be less than two hours. The heating range of post heat treatment is the same as that of heat treatment.

20. When the welded joint has defects exceeding the standard, it can be repaired by digging and repairing. However, the number of digging and mending at the same position shall generally not exceed three times, and the number of times for medium and high alloy steel shall not exceed two times, and the following regulations shall be complied with:
20.1 Thoroughly remove defects.
20.2 When repairing welding, specific repairing welding measures should be formulated and carried out according to the process requirements.
20.3 For welded joints that require heat treatment, heat treatment should be repeated after repair.

21. The loading tool used to install the pipe cold-drawn joint cannot be unloaded until the entire joint welding and heat treatment are completed.

22. Heating corrections shall not be performed on welded joints.