Why are there pores in the weld of welded steel pipe?

Pores in welded seam of welded steel tube not only affect the tightness of pipe weld and cause pipe leakage, but also become the induction point of corrosion, which seriously reduces the strength and toughness of weld.

The factors that cause pores in the weld are: moisture in the flux, dirt, scale and iron filings, welding composition and coverage thickness, surface quality of the steel plate, steel plate edge treatment, welding process and steel pipe forming process, etc.

Measures to prevent pores in welded steel pipes:

1) Flux composition: When the welded steel pipe contains an appropriate amount of CaF2 and SiO2, it will react and absorb a large amount of H2 to generate HF with high stability and insoluble in liquid metal, thereby preventing the formation of hydrogen pores.

2) The accumulation thickness of flux is generally 25-45mm. If the flux particle size is large and the density is small, the accumulation thickness shall be the maximum value, otherwise the minimum value shall be taken; for high current and low welding speed, the accumulation thickness shall be the maximum value, otherwise the minimum value shall be taken. In addition, in summer or when the air humidity is high, the recovered flux should be dried before use.

3) Steel plate surface treatment: In order to prevent sundries such as oxide scales falling off from decoiling and leveling from entering the forming process, a plate surface cleaning device should be installed.

4) Steel plate edge treatment: rust and burr removal devices should be installed on the steel plate edge to reduce the possibility of air holes. The position of the cleaning device is preferably installed behind the edge milling machine and the disc cutter. The structure of the device is two active steel wire wheels with adjustable gaps on one side, which press the edge of the plate up and down.

5) Weld profile: The forming coefficient of the weld is too small, the shape of the weld is narrow and deep, the gas and inclusions are not easy to float out, and it is easy to form pores and slag inclusions. Generally, the weld shape coefficient is controlled at 1.3-1.5, the maximum value is taken for thick-walled welded pipes, and the minimum value is taken for thin-walled welded pipes.

6) Reduce the secondary magnetic field: In order to reduce the influence of magnetic deflection, the connection position of the welding cable on the workpiece should be as close as possible away from the welding terminal, so as to avoid the secondary magnetic field generated by some welding cables on the workpiece.

7) In terms of technology: the welding speed should be appropriately reduced or the current should be increased to delay the crystallization speed of the weld pool metal so that the gas can escape. At the same time, if the delivery position of the strip steel is unstable, it should be adjusted in time to avoid frequent fine-tuning of the front shaft or rear shaft to maintain the shape, which will cause difficulty in gas escape.

Welded steel pipe refers to a steel pipe made of steel plate or strip steel after being crimped and welded, generally with a length of 6 meters. Its production process is simple, its production efficiency is high, its cost is low, and its development is rapid, but its strength is generally lower than that of seamless pipes.

Article source: https://article-realm.com/article/Finance/37767-Why-are-there-pores-in-the-weld-of-welded-steel-pipe.html