Guide for Welding Construction of Copper Nickel Alloy Small Diameter Pipe

This paper studies the welding defect that is easy to produce pores in the welding process of copper nickel alloy small diameter pipe and analyzes them item by item from the aspects of automation equipment, welding wire diameter, welding shielding gas, welding environment, welding process parameters, welder’s operating skills, etc., judges the main reasons affecting their qualification rate, and formulates targeted countermeasures to improve the qualification rate of thin-walled copper nickel pipelines.

0. Introduction

In the natural gas compressor skid of the offshore production platform, the seawater cooling system inside the skid uses a large number of 19.05mm, 12.7mm, and 2.5mm thick small diameter thin-walled copper nickel pipelines. Copper nickel alloy has unique physical properties, and there are technical difficulties in welding, such as difficulty in fusion, large welding stress deformation, and easy occurrence of defects such as hot cracks and pores. In the actual production project, through the investigation and data statistics of the first pass rate of welding and RT sheets, it is found that pores are the main welding defect of thin wall pipelines with small diameters. According to the standard, it must be repaired if the pore diameter exceeds 1/3 of the base material thickness. Therefore, for pipelines with a wall thickness of 2.5mm, if the pore diameter reaches 0.75mm, it will exceed the standard. The welding of this small diameter pipe has a large change in the inclination angle of the welding gun, making it difficult to control the operation, which can easily lead to poor gas protection of the molten pool and the generation of pores. To solve the technical problem of small diameter thin-walled copper nickel alloy welding, the author has continuously innovated and accumulated experience in practical construction from welding environment, pre-welding assembly, welding process, deformation control, etc., and summarized a complete set of guarantee measures. Technological innovation has been carried out, especially in gas protection and pre-welding preheating, effectively ensuring the first pass rate of welding.

1. Storage of copper nickel alloy materials (pipe fittings, pipelines) and welding materials

Metal materials must be placed in designated areas, away from the polishing area to prevent contamination, and stored on rubber or skids, as shown in Figure 1. Welders should use welding materials correctly according to the requirements of the welding process regulations, and welding materials must be obtained from the warehouse. It is strictly prohibited to store welding materials without authorization.

Figure.1 Isolation and placement of copper nickel alloy materials

2. Selection of welding methods and processes

According to the welding characteristics of copper nickel alloy materials, the welding method should be argon arc welding with concentrated arc heat and high welding quality. Table 1 shows the recommended welding process parameters (2.0mm LNTCuNi30 welding wire is used for each layer of welding material, and the power polarity is DCSP).

Table.1 Welding Process Parameters

Welding method	Current/A	Arc Voltage/V	Gas flow/(L.min-1)	Welding speed/(mm.min-1)	Heat input/(kJ.min-1)
GTAW	70–90	11–13	12–20	20–35	2.3
GTAW	80–110	12–14	12–20	30–50	2.6
GTAW	80–110	12–14	12–20	30–50	2.6

3. Requirements for welding environmental conditions

• (1) Control the wind speed below 2m/s and take wind prevention measures.
• (2) Control the relative humidity of the air below 85%.
• (3) The welding area is dust-free and clean, and welding personnel are equipped with shoe covers.
• (4) The construction area should be well ventilated due to the release of argon gas during welding.

4. Preparation before welding

4.1 Groove and assembly

• (1) The groove form should comply with the requirements of the design drawing or regulations, and the groove processing should be done using mechanical processing or stainless steel grinding machine.
• (2) Before welding, manual or mechanical methods should be used to clean the inner and outer surfaces, and slag, oxide skin, oil stains, or other impurities should be carefully removed on both sides of the groove. Use stainless steel grinding wheel to polish the groove, and use stainless steel mushroom head to polish the inner side. Before welding, use a stainless steel bowl brush to clean the groove of the spot welding seam, and then use acetone to clean it.
• (3) The assembly gap should be controlled above 3mm, and the misalignment should be below 1mm. When the local gap of the joint is too large, efforts should be made to trim it to the specified size, and adding fillers within the gap is strictly prohibited.
• (4) The group correspondence is carried out on a dedicated mold protected with rubber or similar materials.
• (5) The misalignment of the inner wall of the pipeline should be ≤ 0.5mm and the misalignment of the outer wall should be ≤ 1mm.

4.2 Preheating

• (1) Electric heating must be used, as shown in Figure 2. Flame heating is not allowed to avoid contamination of the weld seam caused by flame heating, and the preheating temperature should be above 20 ℃.

Figure.2 Preheating with Electric Heating Band

• (2) The preheating range is at least 75mm around the groove. After preheating, use a thermometer to measure the temperature, and welding can only be carried out after reaching the required preheating temperature or above.
• (3) When repairing, the preheating temperature should be 50 ℃ higher than the preheating temperature when the original weld is welded.

5. Welding

5.1 Tack welding

• (1) Qualified welders must weld the weld seam of positioning welding.
• (2) The welding materials and process parameters used for positioning welding are the same as the formal welding requirements.
• (3) 20mm for positioning welding × 10mm copper nickel alloy plate; when performing spot welding and removing spot welding welds, it is necessary not to damage the base material.
• (4) To shorten the storage time after assembly, welding should be carried out within 4 hours to avoid oxidation pollution on the surface of the base material.

5.2 Back argon filling

• (1) Use blocked water-soluble paper or sponge at each end near the weld seam, with a minimum distance of 150mm from the edge of the weld seam.
• (2) Sufficient gas pressure should be maintained inside the pipe to allow oxygen to be discharged from the outside, and the pressure inside the pipe should be maintained above the external atmospheric pressure.
• (3) The transmission of protective gas can only be stopped after welding the second pass, and the thickness of the weld pass is 1/4 of the pipe wall thickness.
• (4) During welding operations, an oxygen tester should be used to detect the oxygen content in the pipe, which should be below 70 × 10-6.
• (5) The flow rate of the protective gas should gradually increase from 35% of the chamber volume to 70% within 5 minutes to avoid the formation of an air chamber.
• (6) Test the purity of argon to ensure a purity of 99.99%.

5.3 Welding process control

• (1) During the welding process, the end of the welding wire should not leave the argon protection zone. The angle between the welding wire and the weld surface should be around 15° when it is fed in, and the angle between the welding gun and the weld surface should be maintained between 80°-90°.
• (2) Arc striking and extinguishing must be carried out within the groove. During arc extinguishing, argon gas should be supplied for an additional 10 seconds at the extinguishing point, and the tungsten electrode extension should be controlled below 5mm to avoid a poor gas protection effect.
• (3) After sealing the root weld bead, check if the root weld bead is oxidized. If it is bright brown, it is qualified; If the surface of the root weld bead is oxidized, it is considered unqualified. If it is blue gray, and should be reassembled for welding.
• (4) To ensure good weld penetration or fusion, low current, short arc welding, rapid welding, and multi-layer multi-pass welding should be used, and the interpass temperature should be kept as low as possible, below 100 ℃. The oxide film, high solder joint, and other welding defect between layers shall be thoroughly cleaned by mechanical method or stainless steel wire brush.
• (5) When using copper nickel alloy welding wire for welding, the welding wire is easily oxidized after being heated. Before re-welding, the oxidized part of the welding wire should be cut off before continuing welding.
• (6) When tungsten arc welding encounters tungsten contact, welding should be stopped, and the tungsten electrode, welding wire, and arc crater should be cleaned before continuing welding. When extinguishing the arc, the welding gun is gradually lifted from the workpiece and continues to feed wire to fill the arc pit. Then, the molten pool is made into a tail shape, and the welding gun operation switch is turned off. However, the welding gun cannot be immediately evacuated, and it must wait for the arc extinguishing area of the weld to cool before evacuation so that this area continues to be protected by argon gas.
• (7) After welding, carefully clean the weld surface using a stainless steel wire brush. Suppose there are defects such as lack of fusion or porosity, re-weld.
• (8) The surface of the weld shall not be lower than the surface of the pipeline.

5.4 Deformation control

• (1) Ensure that the gap, blunt edge, and groove angle of the entire weld groove are almost the same, to a certain extent, ensuring that different positions have the same amount of metal filling, reducing deformation caused by different shrinkage amounts.
• (2) During welding, pipelines or pipe fittings should use structural frameworks such as wooden blocks to ensure good support.
• (3) For pipelines with high mass and long length, to avoid bending deformation caused by the gravity of the pipeline itself, at least 3 layers or 6mm of metal filling should be completed before moving.
• (4) When welding the root weld bead, the weld bead is divided into four parts, starting from 6-9 o’clock, 3-0 o’clock, 6-3 o’clock, and 9-12 o’clock. The author welds in sequence to control the welding deformation caused by the shrinkage stress of the root weld bead.
• (5) When the roundness deviation of deformed pipelines or pipe fittings exceeds 8% of the outer diameter, mechanical methods must be used for adjustment. The weld seam must be removed and reassembled for welding if it cannot be restored. Before cutting off the welding seam, the owner’s permission should be obtained, and the welding seam repair procedure should be followed.

5.5 Weld repair

• (1) excessive defects inside the welded joint can be repaired by patching, but the number of patching at the same position should not exceed 2.
• (2) Certified and qualified welders should carry out weld repair for the corresponding qualified projects.
• (3) Before repair, the defects’ nature, length, and width should be analyzed, and the defect location should be confirmed.
• (4) When repairing, defects should be thoroughly removed, and the groove shape should be as regular as possible to avoid sharp corners, sudden changes in contour, and burrs at the bottom of the groove.
• (5) The welding process used for repair should be the same as the formal welding process. The preheating temperature during repair should be set to the upper limit of the preheating temperature, and the heating range should also be appropriately expanded.
• (6) The surface of the repair welding area must be repaired and polished to ensure its appearance is consistent with the original weld seam.
• (7) After the repair is completed, the repair welding area shall be inspected according to a quality standard not lower than the original weld seam. If any unacceptable defects are found, they shall be repaired again.
• (8) For defects that cannot be repaired by local repair welding, the weld joint can be cut off, and the groove can be reworked before welding.

6. Conclusion

In the domestic petroleum and petrochemical industry, copper-nickel alloy pipelines are widely used, and due to the different weldability and service environment of copper-nickel alloys, different manufacturers have also encountered similar problems. The author summarizes the welding characteristics and welding process methods of small diameter thin-walled copper nickel alloy, which can effectively reduce the generation of welding pores and improve the first pass rate of welding. After further research and improvement, it can be applied to practical production.
Author: Liu Jiyao