China used pipe welding machines often consider many key technologies when producing precision welded pipes, such as ① quality requirements for raw materials. ② welded pipe forming process. ③ formation and removal of weld burrs. ④ weld heat treatment. ⑤ non-destructive testing. ⑥ correct Equipment requirements for welded pipe unit. For this reason, we introduce the  weld heat treatment to be considered in the production of precision welded pipes.

Precision welded pipes are more accurate in geometric dimensions than general welded pipes, and the welds and inner and outer surfaces are of good quality, and the wall thickness is uniform. It can be used in equipment components directly or with a small amount of processing. It is widely used in chemical, textile machinery, aviation, instrumentation, automotive and other sectors.

The welding process of high-frequency welded steel pipes in Chinese used pipe welding machines is carried out under the conditions of high heating speed and high cooling speed. The sharp temperature changes cause certain welding stress, and the structure of the weld seam also changes. Welding along the weld seam The structure in the central area is low-carbon martensite and a small area of free ferrite; the transition area is composed of ferrite and granular pearlite; and the parent structure is ferrite and pearlite. Therefore, the performance of the steel pipe is due to the difference between the metallographic microstructure of the weld and the parent body, which leads to an increase in the strength of the weld, while a decrease in the plasticity index, and the process performance deteriorates. In order to change the performance of the steel pipe, it is necessary to use heat treatment to eliminate the microstructure difference between the weld and the parent metal, refine the coarse grains, make the structure uniform, remove the stress generated during cold forming and welding, and ensure the quality of the weld and the quality of the steel pipe. Process and mechanical properties, and adapt it to the production requirements of the subsequent cold processing process.

There are generally two types of heat treatment processes for precision welded pipes of Chinese used pipe welding machines:

(1) Annealing: mainly to eliminate welding stress and work hardening, and improve the weld ductility of welded pipes. The heating temperature is below the phase transition point.

(2) Normalizing (normalizing treatment): It is mainly to improve the unevenness of the mechanical properties of the welded pipe, make the mechanical properties of the parent metal and the metal at the weld close, and improve the metal microstructure and refine the grain. The heating temperature is air-cooled at a certain point above the phase transition point.

According to the different use requirements of the precision welded pipe produced by the Chinese used pipe welding machines, it can be divided into weld heat treatment and overall heat treatment.

(1)Weld heat treatment: It can be divided into online heat treatment and offline heat treatment.

Weld heat treatment: After the steel pipe is welded, a set of intermediate frequency strip-shaped induction heating devices are used for heat treatment along the axial direction of the weld, and the diameter is directly sizing after air cooling and water cooling. This method only heats the weld area, does not involve the steel pipe matrix, and aims to improve the weld structure and eliminate welding stress, without the need to fix the heating furnace. The welding seam is heated under the rectangular sensor. The device is equipped with a temperature measuring device automatic tracking device. When the welding seam is deflected, it can automatically center and perform temperature compensation. It can also use the welding waste heat to save energy. Its biggest disadvantage is the heating zone. The temperature difference between and the non-heated zone will cause significant residual stress, and the operating line is longer.

(2)Overall heat treatment: it can be divided into online heat treatment and offline heat treatment

On-line heat treatment: After the steel pipe is welded, use two or more intermediate frequency ring-shaped induction heating devices to heat the whole pipe, heating to the normalized temperature of 900~920℃ in a short time, holding for a certain period of time, and air cooling to 400℃Normally cool down after the temperature is below, so that the whole tube organization is improved.

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