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Factors Affecting Welding Seams of SS High-frequency Welded Pipes (Part One)

Posted: 02/27/2022 19:35:07 Hits: 18

Abstract: To ensure the quality of stainless steel high-frequency welded pipes and realize their quality control, the main factors affecting the process quality of stainless steel high-frequency welding were analyzed, including the physical and chemical properties of stainless steel, welding and forming processes. It is pointed out that non oxidation of the welding area is the key to achieving high-frequency welding of stainless steel pipes. On this basis, optimize the inclusion level of raw materials; optimizing welding and forming processes, and adopting advanced forming processes and welding power sources can effectively improve the weld quality of stainless steel high-frequency welded pipes.

Introduction
The development of the oil and gas industry has put forward higher and higher requirements for the corrosion resistance of oil pipes, and stainless steel with excellent corrosion resistance has gradually become a research hot topic in the oil and gas industry. With the advancement of manufacturing technology of oil and gas pipes, whether it is stainless steel seamless pipe or welded pipe, its production technology has made great progress, and the output, quality and variety have continued to increase and improve.

The welding methods of stainless steel welded pipes with straight seams can be divided into two categories. One is adopting fusion welding in gas shielded or vacuum environment and the other is high-frequency induction welding. Fusion welding includes plasma arc welding, TIG welding and laser welding. This type of stainless steel welded pipe has good welding seam and can be used in various high-end fields; the disadvantage is that the welding speed is too slow. High-frequency induction welding has the biggest advantage of fast production speeds, high efficiency and the temperature can be controlled by computer. It has the advantages of having many welding seam defects, unstable quality and not being able to meet high quality requirements in petrochemical, nuclear power and other fields with harsh operating environments.

1. Factors affecting the quality of stainless steel high-frequency welded pipes
High-frequency induction welding has been used for manufacturing carbon steel pipes for more than 50 years, but it is still a developing technology for welding stainless steel pipes. Most foreign stainless steel and high-nickel alloy pipes are seamless steel pipes, especially high-end products such as aviation pipes, nuclear drive structure pipes, and high-purity stainless steel pipes, which are mostly produced by seamless hot rolling. In recent years, the recognition of high-frequency stainless steel pipes has been continuously improved in the market, but there are still many problems that need to be solved when stainless steel high-frequency welded pipes are used in the oil and gas chemical industry.

1.1 The influence of physical and chemical properties of stainless steel on welding
1.1.1 The influence of physical and chemical properties on manufacturing processes
The physical and chemical properties of stainless steel are very different from those of carbon steel. In actual production, the manufacturing process and equipment of carbon steel welded pipes cannot be directly applied to the production of stainless steel welded pipes. The coefficient of linear expansion of stainless steel is greater than that of carbon steel. For example, the coefficient of linear expansion of austenitic stainless steel is 40% higher than that of carbon steel; the high-temperature strength of stainless steel is about 2 to 3 times greater than that of carbon steel. The elastic recovery rate of stainless steel welded pipe is also much better than that of carbon steel pipe. The design of the forming roll must be carried out according to the actual elastic recovery rate of the stainless steel. In addition, stainless steel is non-magnetic and has great resistance. In the welding process, the welding seam generates a lot of heat. The thermal conductivity of stainless steel is poor, and heat is not easily diffused in the welding process. The comprehensive result of these conditions causes high-frequency induced current to penetrate more; the high-frequency skin effect decreases, and the welding speed is greatly reduced.

The welding process parameters of stainless steel are also very different from those of carbon steel during high-frequency welding due to the difference in physical and chemical properties. Under the same power, the high-frequency welding speed of austenitic stainless steel welded pipes can reach 1.3 times that of carbon steel pipes; in other words, when the welding speed is the same, the power consumption of austenitic stainless steel is about 0.77 times more than that of low carbon steel. Under the same power, the high-frequency welding speed of ferritic stainless steel pipes is 0.8 times faster than that of carbon steel pipes. The power consumed by high-frequency welding of ferritic stainless steel pipes is 1.25 times more than that of carbon steel pipes to obtain the same welding speed.

1.1.2 Inclusions in welding seams
The biggest problem in high-frequency induction welding of stainless steel pipes is the influence of oxides on quality. If the welded area is exposed to air, a lot of oxides will be produced due to much alloy in stainless steel in the high frequency welding process. Although most of the oxides can be extruded by extrusion, a small number of oxides remain in the welding seam to form welding seam inclusions. Since the melting point of iron oxides (FeO:1420℃ and Fe2O3:1565℃) is lower than or close to that of iron (1537°C), the oxides will reach the melting point before the base metal or at the same time when welding carbon steel pipes. It’s easier to remove the oxide. The situation is different when stainless steel pipes are welded. Whether it is chromium-based or nickel-based stainless steel, there is a problem that the melting point of the oxide is higher than that of the base metal. When w(Cr) is greater than 12% in stainless steel, chromium is preferentially combined with oxygen than iron, and a dense oxide film Cr203 is formed on the surface of the base metal; the melting point of which is 2265°C, while the melting point of chromium is 1857°C. The melting point (2090°C) of nickel oxide (NiO) is also much higher than that of nickel (1446°C). Therefore, the oxides in the welding seam and their removal are the key factors affecting the welding of stainless steel pipes.

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