Detailed Information of Straight Seam Steel Pipes

Straight seam steel pipe refers to the steel pipe whose weld seam is parallel to the length direction of steel pipes. It is usually divided into metric welded steel pipe, thin-walled electric welding pipe, transformer cooling oil pipe, etc. The production process of longitudinal welded pipe is simple, with high production efficiency, low cost and rapid development. The strength of spiral welded pipe is usually higher than that of longitudinal welded pipe. The narrow blank can be used to produce welded pipes with larger diameter, while the billet with the same width can also be used to produce welded pipes with different diameter. However, compared with the straight seam pipes with the same length, the welding length increases by 30-100%, and the production speed is reduced.

Brief introduction
Straight seam steel pipes are used for conveying low pressure steel pipes. Water pressure and bending tests are required for steel pipes, which have certain requirements for surface quality. Generally, the delivery length is 4-10m, which usually requires fixed length (or double length) delivery. The specification of welded pipe is expressed in nominal diameter (mm or inch). The nominal diameter is different from the actual diameter. According to the specified wall thickness, welded pipe is divided into ordinary steel pipe and thickened steel pipe. According to the different processes of submerged arc welding and submerged arc welding, the pipe can be divided into straight seam welding and submerged arc welding.

High frequency welding
Longitudinal welded steel pipe is a kind of steel pipe formed by rolling long strip steel strip of certain specification into circular tube and longitudinal welding by high frequency welding unit. The shape of steel pipe can be round, square or special-shaped, which depends on the sizing rolling after welding. The main materials of welded steel pipe are: low carbon steel and low alloy steel with σ s ≤300N/mm2 and σ s ≤500N/mm2 or other steels. The production process of high frequency welding of straight seam steel pipes is as follows:

High frequency welding is based on the principle of electromagnetic induction and the skin effect, proximity effect and eddy current heating effect of AC charge in the conductor. Therefore, the steel at the edge of the weld is locally heated to molten state, and the butt weld is extruded by rollers together so as to achieve the purpose of seam welding. High frequency welding is a kind of induction welding (or pressure contact welding), which does not need weld filler, no welding spatter, narrow welding heat affected zone, beautiful welding shape and good welding mechanical properties. Therefore, it has many advantages in the production of steel pipes and has a wide range of applications.

The high frequency welding of steel tube makes use of the skin effect and proximity effect of alternating current to roll the steel (strip) into a round tube blank with cross section, which is rotated in the tube blank near the center of induction coil. And it can work with a set of resistors (magnetic rods), which form an electromagnetic induction loop with the opening of the tube blank. Under the action of skin effect and proximity effect, a strong and concentrated heat effect is produced at the edge of tube blank opening. After the edge of the weld is heated to the required temperature and extruded by the pressure roller, the molten metal realizes intergranular bonding and forms a firm butt weld after cooling.