Longitudinal welded pipe can be roughly divided into two types: the first is double-sided submerged arc welding, and the second is high frequency welding. But the diameter of steel pipe can be greatly different between them. If the wall thickness of high frequency welding is not more than 12 mm, this situation is caused by the performance of high frequency welding, too thick welding machine can not melt steel plate can be welded. Unlike double-sided submerged arc welding, the size, diameter and thickness of the steel pipe to be processed are much larger than those of high frequency welding. Therefore, double-sided submerged arc welded pipe (dsaw pipe) can be divided into butt welded pipe (also known as annular pipe) and straight welded pipe, and invisible welded pipe.
The so-called butt welded pipe is made up of pipe winding machine (due to the impact of machine tools, the general length is 1.5 m-2.5 m) between shorter pipes, and then welded together in the period, so it can reach the required length, called butt welded pipe. Seam welded pipe is made of large sheet metal by large hydraulic forming (its length can be based on the length of steel plate, up to 14 meters), and then welded, in the school circle. Invisible welded pipes are usually made by high frequency welding with wall thickness less than 12 mm and diameter less than 402, and then expanded to the required size through the expansion of the aperture. Large diameter straight seam submerged arc welded pipes are formed by U-shaped hydraulic pressure, double-sided submerged arc welding, with a maximum diameter of 1420 mm and a maximum thickness of 80 mm.