Longitudinally welded steel pipes are a type of steel pipe with welds parallel to the longitudinal direction of the pipe. They are generally classified into metric electric welded steel pipes, electric welded thin-walled pipes, transformer cooling oil pipes, etc. The production process of longitudinally welded steel pipes is simple, with high production efficiency, low cost, and rapid development. Although spiral welded pipes typically have higher strength than longitudinally welded ones, larger-diameter welded pipes can be produced from narrower billets, and pipes of different diameters can be manufactured using the same billet width. However, compared to longitudinal pipes of the same length, the weld length increases by 30% and 100%, respectively, resulting in a slower production speed.
Welding Frequency
High-frequency (HF) current affects the uniformity of current distribution within the steel plate. When selecting the HF welding frequency, both the heat penetration depth and the proximity effect should be considered. Generally, increasing the current frequency appropriately can conserve electrical energy, improve weld quality, and reduce the size of the heat-affected zone (HAZ). In terms of welding efficiency, higher frequencies are preferred. For instance, an HF current of 100kHz can penetrate 0.1mm into ferritic steel, while 400kHz can only penetrate 0.04mm, meaning the current density distribution on the steel plate surface is nearly 2.5 times higher for the latter.
In production practice, a frequency range of 350450kHz is typically selected for welding plain carbon steel materials. For welding alloy steel materials with thicknesses exceeding 10mm, a frequency of 50150kHz may be adopted due to the different skin effects caused by elements such as chromium, zinc, copper, and aluminum present in alloy steel.
Welding Power
Insufficient heating of the pipe billet groove due to low power can result in welding defects such as incomplete fusion, detachment, and inclusion. On the other hand, excessive power affects welding stability, causing the heating temperature of the pipe billet groove to exceed the required welding temperature, leading to severe spattering, pinholes, slag inclusion, and other defects known as overburning defects. The input power during HF welding should be adjusted based on the pipe wall thickness and forming speed. Different forming methods, equipment setups, and steel grades require optimization through practical experimentation.
In addition to the above factors, welding speed, welding method, welding extrusion force, and the use of impedance devices are also crucial elements in controlling the quality of HF welded pipes. Mastering these quality control factors is essential for producing superior products.
