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　　The welded steel pipe can be roughly divided into three categories according to the forming process.

　　1. ERW

　　Resistance welded steel pipe

　　Resistance welded steel pipe, English name ERW (Electric resistance welded Pipe), the type of weld is straight seam. Resistance welding is a kind of pressure welding without filler metal. There is no filler metal in the welding seam. By the skin effect and proximity effect of high frequency current, the edge of the plate is instantly heated to the welding temperature, and the weld with forged structure is formed by extrusion roller.

　　ERW pipes are mainly used to transport oil, natural gas and other vapor-liquid objects, which can meet various requirements of high and low pressure. At present, ERW pipes play an important role in the field of transportation in the world.

　　2. SSAW

　　Spiral submerged-arc welding pipe, English name SSAW (Spiral submerged-arc welding pipe), weld type is spiral seam. The method of submerged arc welding is adopted. Submerged arc welding (including submerged arc surfacing and electroslag surfacing) is an important welding method. It has the advantages of stable welding quality, high welding productivity, no arc light and little smoke and dust.

　　The spiral welded steel pipe has a large diameter and can reach more than 3000mm. It is suitable for large diameter pipeline transportation and building structure.

　　3. LSAW

　　Longitudinal Submerged Arc Welded Steel Pipe (LSAW) is a kind of longitudinal weld. The method of submerged arc welding is also adopted. The straight seam steel pipe has large wall thickness and is similar to spiral welded steel pipe.

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