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Impact of Normalizing Temperature on High-Strength Bainitic Steel Pipe

Understanding the influence of normalizing temperature on the microstructure and properties of high-strength bainitic steel pipe is crucial for optimizing its performance in various applications. In this...

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Factors Affecting Welding Seams of SS High Frequency Welded Pipes (Part Three)

Posted: 03/30/2022 14:03:01 Hits: 30

1.3.2 Influence of opening angle and extrusion force
It is very important to ensure that the edges of the stainless steel strip are flat, smooth and have as few burrs as possible for subsequent welding. Zesheng Zhang and others found that whether the burning process of the edge is stable or not during welding is related to the size of the opening angle, the position of the opening angle and the state of being in contact with the pipe edge in the test. When the opening angle is too small or the contact state is not good, a larger protrusion is formed since the molten metal moves toward the solder spot after forming. A pit that is difficult to press will be formed after the explosion. By changing the size of the opening angle, the process can be adjusted from an unstable burning process to a stable burning process, that is, a critical opening angle value can be found. When it is smaller than this value, the burning process will become unstable. In the test, the test can be carried out according to the specific product specification to find the corresponding optimal opening angle. In the study of induction welding, the relationship between the V-shaped angle and the welding quality is studied, and the measures to obtain the ideal V-shaped area are proposed. The research shows that in the production of high-frequency straight seam welded pipes, the position of the top of the V-shaped corner in the welding zone, the size of the opening angle and the parallelism of the two sides of the V-shaped mouth all affect the welding quality of the steel pipe.

The HFW welded pipe is welded; when the two edges of the tube are heated to the welding temperature, the molten metal with oxides will be squeezed out of the welding seam, and then the two edges of the tube will continue to be extruded and co-extruded, form common metal grains to penetrate and crystallize each other, and finally form a firmed welding seam. The size of the extrusion force has a direct impact on the quality of the welding seam. Through proper temperature adjustment and control, sufficient heating and proper extrusion force, high-quality forging welding effect can be achieved. The impact toughness of the welding seam and the pipe body can be obtained by heat treatment after welding.

Various factors cause various quality defects in the production of welded pipes. The characteristic morphology of the welding seam such as the waist drum shape of the welding seam and the rising angle of the streamline can directly reflect whether the selection and cooperation of the welding process parameters are reasonable, and can judge the welding quality better. Major welded pipe manufacturers at home and abroad still adjust and control the process elements such as the opening angle, welding temperature, welding speed and welding extrusion force of the welding plate edge through the observation of the weldings seam’s shape and metallographic analysis to ensure the welded pipe with the best quality.

1.4 Burrs of welding seams and online quality monitoring
Application industries of welded pipes such as chemical, food and nuclear industries do not accept high-frequency welded stainless steel pipes with internal welding burrs. When high-frequency welding is used for high-speed production, the internal burr removal of ferritic stainless steel welded pipes is relatively easy, while that of austenitic stainless steel welded pipes is very difficult. Burrs in such welded pipes generally do not affect the use of decorative, furniture, mechanical and structural pipes. However, they are not acceptable for the food and petrochemical industries.

In the production of stainless steel seamless pipes, ultrasonic flaw detection is recognized as an effective non-destructive testing method. However, for stainless steel welded pipes, especially for austenitic stainless steel, its flaw detection effect is not ideal. The main reason is that there are relatively coarse austenite columnar crystals in the welding seam metal of austenitic stainless steel, which is easy to cause the ultrasonic beam to be scattered and produce obvious forest reflection waves, resulting in the weakening of the defect signal, the decrease of the SN ratio, and the difficulty of defect identification. On-line eddy current inspection is used in the production of stainless steel welded pipes; the SN ratio increases, and the inspection effect is better.

2. Gas protection in the high-frequency welding area of stainless steel
2.1 Sealing gas protection devices in the welding area
The principle of high-frequency welding gas shielding sealing device is shown in Figure 1. Welding in a protective manner in the welding area is considered to be one of the key factors in improving the quality of high-frequency welding of stainless steel. In order to ensure the welding seam’s quality of HFW alloy steel pipes, non-oxidative high-speed welding is adopted abroad under the protection of inert gas, H2, N2, and mixed gas.

Figure 1 The schematic diagram of the principle of stainless steel high-frequency welding gas protection sealing devices

After the device seals the V-shaped corner area of high-frequency welding, the welding area is filled with the protective gas, so that the whole welding process is carried out in an oxygen-free environment, thereby improving the purity of the welding seam and ensuring high-quality welding seam.

2.2 Plasma gas jet protection devices
A plasma gun that produces a stable mixed gas stream of argon plus nitrogen plus hydrogen is developed. The technology of using low-temperature gas to protect the welding area at room temperatures is very effective for obtaining a stable welding area with low oxygen content, and preventing the generation of oxide inclusions. Compared with the low-temperature gas protection technology, the plasma has a higher temperature state (up to 6 000 K), and the decomposed gas will react with the entrained oxygen. The plasma gas flow enhances the protective effect due to less entrained gas. In addition, the oxides at the edge of the steel strip may also be melted or degraded under the action of the high-temperature plasma. The test material is high-strength pipeline steel with a yield strength of grade X65, and the effect of plasma gas jet protection in the welding area is observed. It is found that through the welding test using this new protection technology, HF-ERW welding seam has few welding defects and good low-temperature toughness. Figure 2 is a high-speed photograph of plasma jets emitted by plasma guns of different sizes.

Figure 2 The high-speed photographic picture of plasma gas flow emitted by plasma guns of different sizes

Studies have shown that high-frequency welding by using plasma gas for protection not only has a wider process window, but also decreases the defect rate of the welding seam. Also, the low-temperature impact toughness of the welding seam is also significantly improved.

3. Conclusion
High-frequency welding of stainless steel is much more difficult than that of carbon steel. Because the physical and chemical properties of stainless steel are very different from those of carbon steel, the manufacturing process of stainless steel high-frequency welded pipes is also very different from that of carbon steel welded pipes. In order to ensure the quality of stainless steel welded pipes, it is necessary to start from the composition design of raw materials, and to realize the quality control of stainless steel high-frequency welded pipes by means of raw material design, molding process innovation and welding process control, and to produce stainless steel high-frequency welded pipes that meet the needs of the petrochemical industry.

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