Why does the welded pipe have wall thickness eccentricity?

Posted by chase Liu on March 24th, 2023

The most common defect of welded steel tube is the uneven wall thickness of the pipe. The relationship between the minimum deviation of the average wall thickness and the pipe-making process is as follows: the pipe-making equipment consists of a piercing machine, a pipe rolling machine, and a tension reducing machine. Take stainless steel tube billet as an example: the wall thickness deviation of the capillary tube from the piercing machine is at least 7.5%, which is reduced to 4.5% after stretching and rolling, increased to 7% after diameter expansion, and reduced to 4.5% after tension reduction.

The eccentricity of the wall thickness of the welded pipe will bring a series of adverse effects on the strength of the steel pipe and further cold working, thus affecting the economic benefits. When rolling large-diameter thick-walled welded steel pipes, the local reduction increases, resulting in cracks and mechanical failure of the rolling mill.

Causes and Influencing Factors of Wall Thickness Eccentricity of Welded Pipe

1.cross rolling piercing

During the cross-rolling piercing process, the tube blank advances while rotating. Spiral wall thickness eccentricity occurs in the perforated capillary, and the wall thickness distribution of the perforated capillary has multiple fluctuations with different amplitudes and different periods, showing a complex wall thickness deviation curve. The wall thickness distribution is analyzed by Fourier as follows, and the factors that cause the wall thickness eccentricity of the welded pipe are deduced.

According to the respective cycle analysis, the wall thickness eccentricity of the welded pipe is mainly caused by the following factors:

1) The temperature of the billet section

2) Vibration of the plug (the center of the plug is inconsistent with the center of the ejector rod)

2.Wall thickness eccentricity of welded pipe caused by temperature difference of billet section

When there is uneven temperature in the blank section, the deformation resistance will be poor in the circumferential direction, and the pressure on the left and right sides will lose balance. According to this calculation method, the wall thickness eccentricity rate caused by the temperature difference of the billet section is ±1.7%, which is equivalent to the situation where there is a temperature difference of 20 °C.

3.The accuracy of the plug and the eccentricity caused by the vibration of the plug

It is shown by the tube blank piercing capillary test of small batch welded pipes. If the center of the plug is misaligned or out of round, when threading a capillary with a size of 136 x 8mm, a plug with a maximum eccentricity of 1.5mm is compared with a non-eccentric plug. It can be concluded that the wall thickness eccentricity caused by the accuracy of the plug will reach 0.3-0.8mm, indicating that the accuracy of the plug has a great influence on the eccentricity of the wall thickness of the stainless steel welded pipe. The vibration of the plug leads to uneven stress in the cross-rolling area during the cross-rolling piercing process, resulting in poor deformation resistance, resulting in uneven thickness of the surrounding walls.