welded steel pipe
Structural steel pipes are divided into two categories: hot-rolled seamless steel pipes and welded steel pipes. Welded steel pipes are made of steel strips and are divided into straight seam welding and spiral welding according to the pipe diameter.
Seamless steel pipes for structures can be divided into hot-rolled and cold-drawn. Cold-drawn pipes are limited to small diameters. The outer diameter of hot-rolled seamless steel pipes ranges from 32 to 630 mm, and the wall thickness ranges from 2.5 to 75 mm. The steel grades used are mainly high-quality carbon structural steel grades of 10, 20, 35, 45 and low-alloy high-strength structural steel Q355. Seamless steel pipes used in building steel structures are mainly 20 steel (equivalent to Q235), with a diameter of more than 180 mm and a length of 3 to 12 m.
The specifications of straight seam electric welded steel pipes range from 32mm to 152mm in outer diameter, and the wall thickness ranges from 2.0 to 5.5mm. The pipe diameter (nominal diameter) ranges from 6mm to 150mm, and the steel grades used include Q195, Q215, Q235 and Q355.
Straight seam steel pipe, spiral steel pipe and seamless steel pipe are common pipes in engineering. They are used in industrial and civil construction and municipal engineering construction. So what is the difference between straight seam steel pipe, spiral steel pipe and seamless steel pipe? How should we choose in engineering applications?
The spiral steel pipe is made of steel coils, extruded at room temperature, and welded by automatic double-wire double-sided buried welding. The spiral steel pipe feeds the steel strip into the welding pipe unit, and after multiple roller rolling, the steel strip gradually rolls up to form a round pipe with an open gap. The pressing amount of the extrusion roller is adjusted to control the weld gap to 1~3mm, and the two ends of the weld are flat.
Commonly used in pipelines for transporting fluids such as rainwater, sewage, and petroleum, it is usually called a fluid pipe.
A seamless steel pipe is a long steel strip with a hollow cross-section and no seams around it. It is made by perforating a steel ingot or solid tube into a rough tube, and then hot-rolling, cold-rolling or cold-drawing. It is widely used as a pipeline for conveying fluids, especially pressure pipelines and structural load-bearing rods in construction projects, and is suitable for various engineering conditions.
Thermal expansion pipe is a type of large diameter thick wall seamless steel pipe. The rolled thick wall seamless steel pipe is heated and then extruded and expanded. During the expansion process, the wall thickness increases unevenly due to uneven temperature and different expansion and stretching speeds. Generally, the wall thickness of seamless steel pipes of 4~20mm is allowed to have a deviation of 15%~12.5%, while the thermal expansion pipe of 20mm thickness is allowed to have a deviation of 18%, and the 30mm thickness is allowed to have a deviation of 25%. If thermal expansion is encountered, the design must leave room for the increase in deviation.
Straight seam steel pipe is an iron plate produced through processes such as bending, closing and welding, and has a longitudinal weld.
Structural steel pipes should be the same in theory, and the difference is not very big. Straight seam welded pipes are not as regular as seamless pipes. The centroid of welded pipes may not be in the center, so they should be used as pressure components. The probability of defects in welded pipe welds is relatively high. Seamless pipes cannot be replaced in important parts. The wall thickness of seamless pipes cannot be made very thin due to the limitations of processing technology (the average wall thickness of seamless pipes with the same diameter is thicker than that of welded pipes). In many cases, the material utilization efficiency of seamless pipes is not as good as that of welded pipes, especially large diameter pipes. The biggest difference between seamless pipes and welded pipes is that they are used for pressure gas or liquid transmission.
At present, there are many fluid pipes, generally No. 20 steel, which is a high-quality steel pipe, mainly subjected to lateral force and good anti-seepage.
However, after the spiral steel pipe is pulled, the force direction is inconsistent with the rolling direction. More importantly, the spiral seam is connected with the hollow ball and the cone head and the main and secondary pipes of the intersecting nodes, or the connection with the steel beam when used as a steel column is cross-welded. This part is often a weak link for brittle fracture and fatigue damage. Cross welding should be avoided, and the weld is not beautiful.
During the manufacturing process, fluid pipes are bent and twisted, and the free edge is forced to deform due to excessive deformation, as well as residual stress caused by internal and external spiral welding. It has been measured that the residual stress near the weld reaches the yield strength, so fluid pipes are not suitable for use in structural components of construction projects.
Some engineering structure designs require the use of seamless steel pipes, which is a great waste, because straight seam steel pipes have fully met the structural requirements and should be used as much as possible.
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