Welded steel pipes refer to steel pipes with seams on the surface that are welded after bending and deforming steel strips or steel plates into round or square shapes. The billet used for welded steel pipe is steel plate or strip steel. According to different welding methods, it can be divided into arc welded pipe, high frequency or low-frequency resistance welded pipe, gas welded pipe, furnace welded pipe, Bondi pipe, etc. 

Electric welded steel pipe: used in oil drilling and machinery manufacturing.

Furnace welded pipe: can be used as a water gas pipe, etc., large diameter longitudinally welded pipe is used for high-pressure oil and gas transportation, etc.; a spiral welded pipe is used for oil and gas transportation, pipe piles, bridge piers, etc. According to the weld shape, it can be divided into straight seam welded pipe and spiral welded pipe.
Longitudinal welded pipe: simple production process, high production efficiency, low cost, and rapid development.
Spiral welded pipe: The strength is generally higher than that of straight seam welded pipes. A narrower blank can be used to produce a larger diameter welded pipe, and the same width of the blank can be used to produce a different diameter welded pipe. However, compared with the straight seam pipe of the same length, the weld length is increased by 30-100%, and the production speed is lower. Therefore, most of the smaller diameter welded pipes adopt straight seam welding, and the large diameter welded pipes mostly adopt spiral welding.
Spiral seam welded steel pipes are divided into automatic submerged arc welded steel pipes and high frequency welded steel pipes.
a. Spiral longitudinally welded pipe seam automatic submerged arc welded steel pipes are divided into two types: Class A pipe and Class B pipe according to the pressure of the conveying medium. Class A pipes are generally welded with ordinary carbon steel Q235, Q235F, and ordinary low-alloy structural steel 16Mn. Class B pipes are welded with steels such as Q235, Q235F, Q195, etc., and are used as low-pressure fluid conveying pipes.
b. Spiral seam high-frequency welded steel pipes. Spiral seam high-frequency welded steel pipes have no unified product standards. They are generally made of ordinary carbon steel Q235, Q235F, and other steels.
According to the purpose, it is divided into general welded pipe, galvanized welded pipe, electric welded special-shaped pipe and spiral welded pipe, etc.
General welded pipe: General welded pipe is used to transport low-pressure fluid. Made of Q195A, Q215A, Q235A steel. It can also be made of other mild steel that is easy to weld. Steel pipes need to be subjected to water pressure, bending, flattening and other experiments, and there are certain requirements for surface quality. Usually, the delivery length is 4-10m, and fixed-length (or double-length) delivery is often required. The specifications of welded pipes are expressed by nominal diameter (mm or inches). The nominal diameter is different from the actual. Welded pipes are divided into two types: ordinary steel pipe and thickened steel pipe according to the specified wall thickness. The steel pipe is divided into two types with thread and without thread according to the pipe end form.
Galvanized steel pipe: To improve the corrosion resistance of the steel pipe, the general steel pipe (black pipe) is galvanized. Galvanized steel pipes are divided into two types: hot-dip galvanizing and electric steel zinc. The hot-dip galvanizing layer is thick and the cost of electro-galvanizing is low.
Metric welded pipe: The specification is in the form of seamless pipe, and the welded steel pipe expressed by outer diameter * wall thickness in millimeters, using ordinary carbon steel, high-quality carbon steel or general energy low-alloy steel for hot and cold welding, or after hot welding Then made by cold dialing method. Metric welded pipes are divided into ordinary energy and thin-walled and are commonly used as structural parts, such as drive shafts or conveying fluids. Thin-walled pipes are used to produce furniture, lamps, etc., and the strength and bending test of the steel pipe must be guaranteed.
Special-shaped pipes: square pipes, rectangular pipes, hat-shaped pipes, hollow rubber steel doors and windows welded by ordinary carbon structural steel and 16Mn steel strips, mainly used for agricultural machinery parts, steel windows and doors, etc.
Welded thin-wall pipe: mainly used to make furniture, toys, lamps, etc. At present, thin-walled pipes made of stainless steel belts are widely used, such as high-end furniture, decoration, and fences.
Spiral welded pipe: the low carbon structural steel or low alloy structural steel strip is rolled into a tube blank at a certain helix angle (called forming angle), and then the tube seam is welded together. It can be made with a narrower Strip steel that produces large-diameter steel pipes. Spiral welded pipes are mainly used for oil and natural gas pipelines, and their specifications are expressed by outer diameter * wall thickness. Spiral welded pipes can be welded on one side and both sides. Welded pipes should be guaranteed to meet the requirements for the hydraulic test, tensile strength, and cold bending performance of the weld.
According to the shape of the end, it is divided into round welded pipe and special-shaped (square, flat, etc.) welded pipe