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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 Two)

Posted: 03/08/2022 09:09:21 Hits: 30

At present, a lot of research is carried out on the high-frequency welding of stainless steel. Yudong Dong and others studied and analyzed the changes in the microstructure and properties of the welding seam by high-frequency welding of 12%Cr ferritic stainless steel. Zongyue Bi and others studied the impact of high-frequency welding process parameters on the microstructure and properties of 16Cr austenitic stainless steel welding seams. The study showed that there were oxides of Cr and Fe in the welding seams of 16Cr stainless steel. When high-frequency welding was performed, gas shielding measures were required to prevent the formation of oxides in the welding area. Naoshi Ayukawa and others have newly developed a thin-walled HF-ERW martensitic stainless steel line pipe with high dimensional accuracy by using an improved gas shielding technology in the HF-ERW welding process, which has good CO2 corrosion resistance. In the manufacturing process of this steel pipe, a new method of shielding gas has been developed to prevent the formation of Cr oxides in the high-frequency welding process, thereby ensuring high-quality welding seams. In order to prevent the occurrence of oxides, Nippon Steel Kimitsu Plant used high-purity N and high-frequency welding to weld SUS304 austenitic stainless steel in a non-oxidizing atmosphere. Scanning electron microscope inspection was carried out on the welding seam's fracture; no oxide inclusions were found, and the welding seam's quality was effectively guaranteed.

Relevant personnel have studied the welding characteristics of stainless steel under a shielding gas atmosphere. The researcher believed that the use of a protective atmosphere has significantly expanded the welding process range of stainless steel high-frequency welding, and the role of shielding gas was to reduce the oxygen content in the welding area to less than 5%. Some studies also believed that only when u(Cr) in the steel is less than 1.0%, can it be possible to carry out defect-free welding of the steel in the air. Therefore, in order to ensure the quality of the welding seam of the high-frequency stainless steel welded pipe, inert gas, H, N, and mixed gas are used abroad to protect the welding area to achieve high-speed welding without oxidation. When the steel grades are different, the protective gas types are also different. The key is to control the oxygen concentration below 0.1%.

1.2 Special requirements for the welding process and equipment
High-frequency induction welding requires a large power supply, and the average output power of the welding machine used is 150 to 450kW. According to the different materials and specifications of the welded pipe, high-frequency welding can achieve a very high welding speed, even more than 100m/min, which is more than 10 times the highest welding speed that can be achieved by TIG welding.

From the analysis of the welding mechanism, the high-frequency welded pipe was best welded in the plastic state. When the welding temperature was too high, the results of the flaring test of the post-weld specimen were very unsatisfactory. However, the welding temperature range required by the plastic state was very narrow and difficult to control, especially when the voltage fluctuated, the welding process was difficult to carry out stably. In order to remove high melting point oxides such as chromium oxide formed at the welding port at high temperatures, and to expand the applicable process range of welding, it is generally required that the edge of the steel pipe should be in a molten state in the welding process. However, we need to study and solve the problem of the stability of this process, so that the welding seam could be good.

When carbon steel pipes are welded, the welding temperature tolerance is 150°C. When stainless steel is welded, the welding temperature tolerance is much smaller depending on its chemical composition, so that any negligence in the welding process is not allowed. According to foreign information, when the stainless steel or high-alloy steel pipe is welded at high frequency, the control accuracy range of allowable input heat in industrial production is about 5%. In view of this, on the one hand, it is required that the active parts of the pipe-making unit must have strict tolerance requirements to prevent vibration, ensuring the stability of the unit, welding points and V-shaped corners, and avoiding cold welding and overheating. On the other hand, the performance of high-frequency induction welding machines must also be stable, because fluctuations in the output power will also cause similar problems in the welding spot. Foreign high-frequency welding units are generally equipped with an automatic heat input control system and a fault monitoring system, which controls the input heat of the welding process within the range of process requirements and ensures the stability of welding quality.

Japan has applied a newly developed high-frequency resistance welding automatic power input control system, which can not only control the input heat according to the welding speed corresponding to the strip and plate, but also monitor the welding phenomenon with high-precision electronic instruments to ensure the welding process, ensuring that the welding process is always in the best condition.

In order to be able to weld stainless steel tubes of different diameters, wall thicknesses and materials, such as AISI 304, 316 austenitic stainless steel and AISI 409 ferritic stainless steel, an extremely adaptable welder is required. This problem can be solved by using a welding machine with variable output frequency, such as output frequency from 100 to 200 kHz, 150 to 300kHz or 250 to 500kHz, which can output different frequencies for welding machines. Another way is to design and install several switches of different gears on the console, and the operator can select the output frequency through the switch.

1.3 Differences in the molding process
1.3.1 Influence of material properties on molding methods
The coefficient of linear expansion of stainless steel is greater than that of carbon steel, and its tensile strength, yield strength and elongation are also very different from those of carbon steel. The forming process of stainless steel welded pipes must have its own unique design characteristics due to the differences in the strength and resilience force of stainless steel and carbon steel. The practice has proved that austenitic stainless steel has good adaptability to the double-radius forming method, and the comprehensive bending deformation should be adopted for austenitic stainless steel.

Another technical problem to be solved in the forming of austenitic stainless steel welded pipes is to prevent the problem of the forming roller from sticking to the forming material. When the steel pipe is being formed, the peripheral speed of the bottom of the forming wheel hole and the two wings are different, and the peripheral speed of the two wings is faster than the moving speed of the steel pipe in the forming process. The difference between the peripheral speed of the two wings of the forming hole and the pipe-making speed will cause friction between the steel pipe and the roller, resulting in the roller sticking to the forming rod, which will leave rolling marks on the surface of the austenitic stainless steel pipe. The most effective measure to prevent rolling marks is to reduce the difference between the peripheral speed of the two wings of the hole and the moving speed of the formed welded pipe. To solve this problem, the researchers invented the split roller to replace the traditional roller in the past.

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