UNS S32750 Super Duplex Stainless Steel Small Pipe Diameter Welding Process and Difficulty Control

In a 1500m deep water manifold super duplex stainless steel pipeline of a certain project, the control system pipelines are all small pipe diameters less than 1 inch, which requires extremely high welding proficiency from welders. The use of TIG welding for bottoming is prone to the risk of root blockage. By using the method of track self fusion welding or self fusion welding + wire-filling welding, the root of the weld bead has excellent formation. It can obtain good backing welds in batches, achieving ideal welding results. Due to the high requirements for the ellipticity of the base material and the accuracy of the weld assembly in self-fusion welding, combined with the welding difficulties encountered during the project production process, an in-depth analysis was conducted, and effective solutions were taken. Summarize the welding defect with the most frequent occurrence of small diameter UNS S32750 pipes, and propose error correction plans and precautions to provide a good reference for subsequent project production.

0. Introduction

The main fluids inside submarine gas and oil pipelines are hydraulic control or chemical agents. These fluids have strong corrosiveness and require high corrosion resistance at the root of the weld seam, i.e., on the inner surface of the pipeline. Additionally, long-term contact with seawater containing chloride ions on the outer surface of the pipeline also requires certain requirements for the material’s pitting resistance. Super dual-phase steel combines the advantages of the dual-phase structure of austenite and ferrite with a high corrosion resistance coefficient and good mechanical properties. It is considered an important material for pipelines used in deep-sea underwater pipelines.
UNS S32750 super duplex stainless steel is mainly used in the manufacturing fields of submarine pipeline laying, ship manufacturing, and offshore gas field exploration with high strength and corrosion resistance requirements. There is much discussion on the welding process of UNS S32750, but it mainly focuses on large pipelines with a diameter of>720mm. The welding of small diameter pipelines is rarely produced in China, and there are few reference resources. The welding technology of this type of product in foreign countries is mainly mastered by several large ultra-double welding companies, such as One Subsea, Akersolu Ion, Baker Hughes, etc. The welding of ultra double small pipes can only be explored by oneself; the exploration of small pipe welding by marine engineering is committed to solving the current gap. This article analyzes the weldability of UNS S32750 material, summarizes the selection of welding methods, defects such as porosity and lack of fusion that may occur during the welding process, and corresponding measures.

1. Weldability analysis of UNS S32750 super duplex stainless steel

Based on ordinary duplex stainless steel containing Cr and Ni elements, UNS S32750 has added elements such as Mo, N, Cu, etc. Its chemical composition and mechanical properties are shown in Table 1 and Table 2, respectively. In oxidizing media, with the increase of Cr content, the corrosion resistance of steel increases because Cr can generate a stable and dense Cr₂O₃ passivation film on the surface of the steel.
Table.1 ASTM A790 UNS S32750 Chemical Composition (Mass Fraction, %)

Table.2 Mechanical Properties of ASTM A790 UNS S32750

UNS S32750 adds a significant amount of N (0.24% -0.32%), an element that forms austenite and expands the austenite zone. During welding, it should also be ensured that the weld metal contains the corresponding N content to match its metal properties with the base metal. Therefore, welding materials with the corresponding N content should be selected. There is no need to fill metal during self-fusion welding, and Sandvik25.10.4. L welding wire is used for filling during manual cover welding.
UNS S32750 has good weldability but has very high welding requirements. The mechanical properties and corrosion resistance of the joint are very sensitive to heat input. Excessive or too small heat input can affect the formation of biphasic during the welding process. Due to the embrittlement of the weld metal caused by overheating the base material and weld metal, it is necessary to control the interlayer temperature during welding to reduce the residence time in the overheating range (475 ℃). The sensitivity to cold cracks during the welding process is low, but there is a tendency to produce hot cracks.

2. Welding method and welding process

The project production includes various specifications and thicknesses of pipe fittings, such as DN20mm and DN10mm. Due to the small diameter of the pipe, it isn’t easy to use traditional TIG welding, and the welding quality cannot be guaranteed. Two welding methods were ultimately determined by analyzing the pipe diameter size and thickness and conducting continuous experiments, as shown in Table 3. According to the standard AWS D10.18, it is recommended to use the rail self fusion welding method when the thin-walled thickness of small pipes is less than 3mm, and the outer diameter of pipes is less than 25mm; When the thickness of the base material is too large to form a qualified weld shape through self fusion welding, a combination process of self fusion welding and TIG covering can be used to ensure that the appearance of the welding surface is qualified. The schematic diagram of the I-shaped groove assembly with 0 gaps is shown in Figure 1. Before assembly, the ovality and wall thickness on both sides of the groove should be carefully checked to ensure that the misalignment is less than 0.3mm and the assembly gap is 0mm. These technical parameters are by the machine usage specifications provided by the manufacturer when purchasing the welding machine. Clean the 50mm oxide skin on both sides of the weld seam, and use alcohol or acetone to wipe the outer and inner surfaces to ensure no impurities or contamination near the weld seam.

Figure.1 Schematic diagram of I-shaped groove
The self-fusion welding adopts P4 or P6 welding machines produced by POLYSOUDE. During prefabrication, the 5G position is used. As the product cannot be flipped, the on-site welding of the weld junction is mostly in the 2G and 5G positions. The welding parameters are shown in Table 3, with peak and base values accounting for 200ms and 300ms, respectively.
Table.3 Welding Process Parameters

3. Common welding defect and control measures

During the on-site welding of small-diameter pipelines, due to the complex layout of the pipeline, there have been cases of back inflation and incorrect welding, resulting in root oxidation defects. Moreover, the on-site welding position is far from the gas cylinder, and the long gas pipeline impacts the purity and airflow speed of the protective gas. According to statistics, among the welding defect of super double small tubes in recent years, the porosity accounts for 50%, the lack of fusion accounts for 25%, and the root oxidation account for 10%. Other defects, such as foreign matters in the tube, film misjudgment caused by appearance leakage, excessive height, etc., account for a small proportion and can be controlled in real-time by emphasizing the on-site production discipline. This article will not discuss them.

3.1 Porosity

Porosity is the most common defect in welding ultra double small pipes, which may occur at the root of the pipeline and on the welding surface. Gas purity, gas flow rate, groove cleanliness, and inadequate protective measures during welding can all cause porosity defects, as shown in Figure 2.

Figure.2 Pore defects
Control measures: After the material groove is processed, it must be cleaned to remove impurities such as surface oxide film and ensure that the surface within 50mm of the groove is clean and free of moisture, grease, and other impurities. Before welding, clean the surface of the weld seam with acetone or methanol, then preheat the weld seam with a hot air gun to remove surface moisture. According to the recommendation of AWS D10.18, the preheating temperature should not exceed 65 ℃. When filling protective gas before welding, use an oxygen content analyzer to determine that the oxygen content cannot exceed 500 × For cylinders standing for a long time, they should be rolled and shaken before use. Additionally, during transportation, they should be placed as flat as possible. Spot welding also requires back inflation, with an oxygen content of less than 500 × Welding can only begin after 10 to 6 days. Timely inspect the sealing of the self-fusion welding machine head to ensure that welding is carried out in a good protective atmosphere. For damaged welding machine heads, replace accessories promptly. When filling the back protective gas, avoid turbulence in shorter pipe sections and ensure a uniform and consistent gas flow rate inside. When conducting on-site welding seam inflation protection, ensure that one end of the pipeline is inflated and the other has an exhaust hole for deflation. It is prohibited to deflate at the weld seam or set an exhaust hole at the same end of the inflation.

3.2 Root incomplete fusion

The main reasons for the lack of fusion during the welding of the UNS S32750 ultra double small tube are the excessive size of the misalignment during assembly and the deviation of the tungsten electrode from the center of the weld seam during the rotation of the welding machine head at the 5G position during welding.

Figure.3 Root Incomplete Fusion
Control measures: During welding, special attention should be paid to preventing deformation caused by uneven heating of the pipeline. The welding sequence should be adjusted promptly according to the size of the deformation, and the orientation of the prefabricated pipeline should be adjusted. Before welding, the welder should self-check the root gap of the pipeline assembly to be 0mm and the misalignment size is less than 0.3mm. Pipelines with uneven ovality of pipe diameter should be rejected for assembly. For track self-fusion welding, it is necessary to ensure that the wall thickness and misalignment size of the pipeline is within the specified range. At the 5G position, during the self fusion welding process, the welder should use their hands to support the self fusion welding head, ensuring that the tungsten electrode inside the welding head is aligned with the gap between the assembly grooves. When the welding machine rotates in the 3 o’clock direction, this position is for vertical downward welding. Due to the weight of the welding machine, the downward sliding speed is easily too fast, which may lead to partial, incomplete fusion at the welding junction.

3.3 Root oxidation of weld seam

The main reason for the oxidation of the weld root is that when there is no protective gas on the back, the high-temperature deposited metal corrosion-resistant elements during welding will react with oxygen, resulting in oxidation defects at the weld root, as shown in Figure 4.

Figure.4 Root oxidation
Control measures: During the welding process, the internal protective gas of the pipe diameter should be detected at any time, and the pipeline direction of the inflation hose should be sorted out. If necessary, eye-catching markings can be made on the pipeline to ensure the consistency between the welding pipeline and the inflation pipeline, to avoid the situation of incorrect inflation due to too many welding joints on site, and to avoid personnel stepping on the inflation pipeline. All argon meters for welding equipment are dedicated meters and cannot be replaced by other meters. In the operation of self fusion welding and wire-filling welding, after the completion of self fusion welding, the pipe’s internal diameter should still be kept inflated during wire-filling welding.

3.4 Cover depression

During the self-fusion welding process, there may also be concave cover surfaces. When welding a DN20mm pipe at a 5G position, the welding parameters are appropriate, but after welding, there is a surface depression with a depth of less than 10% of the wall thickness, as shown in Figure 5. Analysis of the cause: The assembly has unqualified straight groove grinding, which leads to gaps after assembly, resulting in the base metal filling into the gaps during self fusion welding and causing post-welding depressions.

Control measures: Strictly control the accuracy of pipeline assembly, with a root gap of 0mm and a misalignment size of less than 0.3mm. Before welding, simulate the production conditions and adjust the welding parameters to the optimal state, including the nitrogen flow rate on the back. During the construction of multiple pieces of equipment on site, the voltage of the 220 V power supply welding machine may be unstable, so the method of adding a voltage regulator is adopted; For appearance dents, a combination process of self-fusion welding and manual covering can be used to repair the dents, ensuring that the weld appearance is qualified.

Figure.5 Surface Depression

4. Other precautions

• (1) Carbon steel contamination should be avoided when welding super duplex steel. The welding workshop should be separated from the carbon steel workshop, and it should be a closed and dust-free workshop. When entering the workshop, shoe covers must be worn, and the workshop should be cleaned using a vacuum cleaner. Grinding dust should be wiped dry with a mop with water after work. The tools used for production and quality inspection are special tools that should be distinguished from the inspection tools used in the carbon steel workshop. Mixing is prohibited to avoid contamination of ferrite and other C elements after welding. The paint pen used for marking should be a stainless steel special low chlorine pen to avoid contamination of halogen elements.
• (2) For super duplex steel materials, if the ferrite content is too low after welding, the weld seam is prone to cracking when heated. If the ferrite content is too high, the weld seam will lose toughness and ductility. During the welding process, it is necessary to prevent excessive welding heat input, which should not exceed the range specified in the welding process specification. At the same time, the interlayer temperature should be strictly controlled, and the lower of the double specified 100 ℃ or PQR test should prevail. This should be recorded in the welding process specification. If the temperature is too high, air cooling or other methods can be used for cooling.
• (3) When welding, pure nitrogen gas is used as the protective gas inside the pipeline, which promotes austenite transformation compared to pure argon gas and can better protect the corrosion resistance of the weld zone. Positive protective gas selection φ (Ar) 98%+ φ (N₂) 2% high-purity argon nitrogen mixture.

5. Conclusion

• (1) In the absence of reference materials for small diameter super duplex stainless steel pipes in China, combined with the 1500 meter deep water manifold super duplex stainless steel pipeline project, research and development were carried out on the small diameter welding process of UNS S32750 material. A combination of self fusion welding and self fusion 90 weldings + wire filling welding was successfully developed and applied to ongoing projects, achieving high qualification rates in welding appearance and internal inspection during project construction.
• (2) Introducing the equipment selection and welding parameters in the UNS S32750 material small tube self fusion welding and self fusion welding + wire filling welding. Combined with the analysis of welding defects and specific problems that are easy to cause in the process of project welding, the causes of defects are screened through directional tests, and effective methods are taken to solve them. Summarize the welding process and difficulty control of UNS S32750 super duplex stainless steel small pipe diameter, providing guidance and reference for future super duplex steel small pipe diameter welding.

Author: Zhang Lianshuang