Since the 1930s, with the development of continuous rolling production of fine strip steel and the progress of welding and inspection technology, the quality of welding seam has been continuously improved, the varieties and specifications of welded steel pipes have been increasing, and more and more seamless steel pipes have been replaced. The straight seam steel pipe is a steel pipe whose weld is longitudinally parallel to the steel pipe. It is generally divided into metric electric welding steel pipe, electric welding thin-wall pipe, transformer cooling oil pipe, etc. The steel pipe shall be subject to water pressure, bending, flattening and other tests. There are corresponding requirements for surface quality. Generally, the delivery length is 4-10m, and the delivery is usually required to be fixed length (or multiple length). The strength of straight seam steel pipe is generally higher than that of straight seam welded pipe. It can be used to produce welded pipe with larger pipe diameter with narrower blank, and can also be used to produce welded pipe with different pipe diameter with the same width blank. But compared with the straight seam pipe with the same length, the weld length increases by 30~1, and the production speed is low. Therefore, the smaller diameter welded pipes are mostly straight seam welded, while the larger diameter welded pipes are mostly straight seam steel pipes.
The specification of welded pipe is expressed by the nominal diameter (mm or inch). The nominal diameter is different from the actual one. According to the specified wall thickness, welded pipe can be divided into ordinary steel pipe and thickened steel pipe. Steel pipe can be divided into threaded and unthreaded steel pipes according to the pipe end form. The pipe whose weld seam is spirally distributed relative to the pipe axis. It is mainly used for conveying pipes, pipe piles and some structural pipes. Product specification: outer diameter 159~3620mm, wall thickness 3.2~25.4mm It refers to rolling low carbon structural steel or low alloy structural steel strip into tube blank according to the angle of corresponding helix (called forming angle), and then welding the pipe seam. It can produce large-diameter steel pipe with narrow strip steel.
Analysis of terms in straight seam steel pipe:
1. Actual size: refers to the actual size in the production process, which is usually larger or smaller than the nominal size. This phenomenon of larger or smaller than the nominal size is called deviation.
2. Nominal size: it is the nominal size specified in the standard, the ideal size expected by users and manufacturers, and the ordering size specified in the contract.
3. Deviation: In the production process, because the actual size is difficult to meet the requirements of the nominal size, that is, it is often greater than or less than the nominal size, the standard stipulates that a difference is allowed between the actual size and the nominal size. A positive difference is called a positive deviation, and a negative difference is called a negative deviation.
4. Tolerance: The sum of the corresponding values of positive and negative deviations specified in the standard is called tolerance, also called "tolerance zone"
The high-frequency welding of steel pipes is precisely to use the skin effect and proximity effect of alternating current. After the steel (strip steel) is rolled and formed, it forms a circular billet with disconnected cross section. One or a group of impedors (magnetic rods) are rotated near the center of the induction coil in the billet. An electromagnetic induction loop is formed between the impedor and the opening of the billet. Under the skin effect and proximity effect, the edge of the opening of the billet produces a concentrated thermal effect, After the weld edge is heated to the temperature required for welding and extruded by the pressure roller, the molten metal can realize intergranular bonding and form a solid butt weld after cooling.
Straight seam steel pipe is mainly used for oil and natural gas transmission pipeline, and its specification is expressed by outer diameter and inner wall thickness. The straight seam steel pipe has single-sided welding and double-sided welding. The welded pipe shall be subject to hydrostatic test, and the tensile strength and cold bending performance of the weld shall meet the requirements. The straight seam welded steel pipe has the characteristics of relatively simple process and continuous production, and is widely used in civil construction, petrochemical, light industry and other departments. It is mainly used for conveying low pressure fluid or making various engineering components and light industrial products. The high-frequency welding of straight seam steel pipe is based on the electromagnetic induction principle and the skin effect, proximity effect and eddy current heat effect of AC charge in the conductor, so that the steel at the edge of the weld can be locally heated to the molten state, and the butt weld can realize the intergranular joint through the extrusion of the roller, so as to achieve the purpose of weld welding. High frequency welding is a kind of induction welding (or pressure contact welding). It does not require weld filler, does not have welding spatter, has narrow welding heat affected zone, has beautiful welding formation, and has good welding mechanical properties. Therefore, it is widely used in the production of steel pipes.
