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, more varieties and specifications, less equipment investment, but its general strength is lower than seamless steel pipe. Since the 1930s, with the rapid development of high-quality strip continuous rolling production and the progress of welding and inspection technology, the quality of weld seam has been continuously improved, and the variety and specification of welded steel pipe has been increasing day by day. It has replaced seamless steel pipe in more and more fields, especially in heat exchanger, spare pipe, decorative pipe, medium and low pressure fluid pipe, etc. One of which is continuous on-line production of small-caliber stainless steel welded pipe. The thicker the production and wall thickness, the greater the investment of the unit and the dissolving equipment, the less economical and practical it will be. The thinner the wall thickness, the lower the input-output ratio; secondly, the process of the product determines its advantages and disadvantages. Generally, the welded steel pipe has high accuracy, uniform wall thickness, high brightness of the inner and outer surface of the pipe (the brightness of the steel pipe determined by the surface grade of the steel plate), and can be arbitrarily ruled. Therefore, it embodies its economy and beauty in the application of high precision, medium and low pressure fluids.