Technical problems of heat treatment of welding seam of welded pipe

The welding process of high-frequency welded steel pipe (erw) is carried out under the condition of fast heating rate and high cooling rate. The rapid temperature change causes a certain welding stress, and the structure of the weld also changes. The structure in the welding center area along the weld is Low-carbon martensite and small area of free ferrite; the transition region 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 index of the weld, while the plasticity index decreases, and the process performance deteriorates. In order to change the performance of the steel pipe, heat treatment must be used to eliminate the microstructure difference between the weld and the parent metal, so that the coarse grains are refined, the structure is uniform, the stress generated during cold forming and welding is eliminated, and the quality of the weld and the steel pipe are guaranteed. Technological and mechanical properties, and adapt to the production requirements of the subsequent cold working process.

There are generally two types of heat treatment processes for precision welded pipes:

(1) Annealing: It is mainly to eliminate the welding stress state and work hardening phenomenon and improve the weld plasticity of the welded pipe. The heating temperature is below the phase transition point.
(2) Normalizing (normalizing treatment): It is mainly to improve the inhomogeneity of the mechanical properties of the welded pipe, so that the mechanical properties of the parent metal and the metal at the weld are similar, so as to improve the metal microstructure and refine the grains. The heating temperature is air-cooled at a point above the phase transition point.

According to the different use requirements of precision welded pipes, 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 seam heat treatment: After the steel pipe is welded, a set of intermediate frequency strip induction heating devices are used for heat treatment along the axial direction of the weld seam, and the diameter is directly sized after air cooling and water cooling. This method only heats the weld area, does not involve the steel tube 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 a rectangular sensor. The device is equipped with an automatic tracking device for a temperature measuring 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. The biggest disadvantage is the heating area. The temperature difference with the non-heated zone can lead to significant residual stress, and the working line is long.

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

1) On-line heat treatment:

After the steel pipe is welded, use two or more sets of intermediate frequency ring induction heating devices to heat the whole pipe, heat it to the temperature required for normalization in a short time of 900-920 °C, keep it for a certain period of time, and then air-cool it to below 400 °C. Normal cooling, so that the whole tube organization is improved.

2) Heat treatment in off-line normalizing furnace:

The overall heat treatment device for welded pipes includes chamber furnace and roller hearth furnace. Nitrogen or hydrogen-nitrogen mixed gas is used as a protective atmosphere to achieve no oxidation or bright state. Due to the low production efficiency of chamber furnaces, roller hearth type continuous heat treatment furnaces are currently used. The characteristics of the overall heat treatment are: during the treatment process, there is no temperature difference in the tube wall, no residual stress will be generated, the heating and holding time can be adjusted to adapt to more complex heat treatment specifications, and it can also be automatically controlled by a computer, but the roller bottom type. The furnace equipment is complex and the operating cost is high.