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　　Welded steel pipe is referred to as welded pipe, which is made by welding common steel or steel strip after being curled and formed by machine set and die. Welded steel pipe has the advantages of simple production process, high production efficiency, many varieties and specifications and less equipment, but its general strength is lower than that of seamless steel pipe. Since the 1930s, with the rapid development of high-quality strip rolling production and the progress of welding and inspection technology, the quality of weld seams has been continuously improved, and the variety and specification of welded steel tubes have been increasing. Seamless steel pipe have been replaced in more and more fields, especially in the fields of heat exchanger, spare pipe, decorative pipe, medium and low pressure fluid tubes.

　　Longitudinal welded steel pipe has the advantages of simple production process, high production efficiency, low cost and rapid development. The strength of spiral welded steel pipe is generally higher than that of straight welded pipe. It can produce ASTM A53 30 Inch Welded Steel Pipe with bigger diameter with narrower blank, and welded pipe with different diameter can also be produced with blank of the same width. However, compared with the straight seam pipe of the same length, the length of the weld increases by 30-100%, and the production speed is lower.

　　Generally speaking, the diameter of pipe can be divided into outer diameter, inner diameter and nominal diameter. The outer diameter of a pipe made of spiral steel pipe is expressed by the letter D, followed by the size and wall thickness of the outer diameter, such as seamless steel pipe with an outer diameter of 108.

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