Basic knowledge of steel pipes

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Steel pipes are hollow, long-shaped steel products that are widely used as pipelines for transporting fluids such as oil, natural gas, water, gas, and steam. Additionally, since they have a lower weight while maintaining the same bending and torsional strength, they are also extensively used in the manufacture of mechanical parts and engineering structures. It is also commonly used in the production of various conventional weapons, gun barrels, artillery shells, etc. 1. Classification of steel pipes: Steel pipes are divided into two main categories – seamless steel pipes and welded steel pipes (seamed pipes). Based on the cross-sectional shape, they can be further divided into round pipes and shaped pipes. Round steel pipes are widely used, but there are also shaped steel pipes such as square, rectangular, semi-circular, hexagonal, equilateral triangular, and octagonal shapes. All steel pipes subjected to fluid pressure must undergo hydraulic testing to evaluate their pressure resistance and quality; they are considered qualified if they do not leak, become wet, or expand under the specified pressure. Some steel pipes also need to go through tests such as curling tests, flaring tests, and crushing tests, in accordance with standards or the requirements of the client. 2. Seamless steel pipes: Seamless steel pipes are made by piercing steel ingots or solid tube billets to form rough tubes, which are then produced through hot rolling, cold rolling, or cold drawing. The specifications of seamless steel pipes are expressed as outer diameter * wall thickness in millimeters. Seamless steel pipes are divided into two categories: hot-rolled and cold-rolled (drawn) seamless steel pipes. Hot-rolled seamless steel pipes are classified into ordinary steel pipes, low and medium pressure boiler steel pipes, high pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, geological steel pipes, and other types of steel pipes. Cold-rolled (drawn) seamless steel tubes include, in addition to ordinary steel tubes, low and medium pressure boiler tubes, high pressure boiler tubes, alloy steel tubes, stainless steel tubes, oil cracking tubes, and other types of steel tubes, carbon thin-walled steel tubes, alloy thin-walled steel tubes, stainless thin-walled steel tubes, and shaped steel tubes as well. The outer diameter of hot-rolled seamless pipes is generally greater than 32 mm, with wall thicknesses ranging from 2.5 to 75 mm. The outer diameter of cold-rolled seamless pipes can be as low as 6 mm, and their wall thickness can reach 0.25 mm. The outer diameter of thin-walled pipes can be up to 5 mm, with wall thicknesses less than 0.25 mm. Cold-rolling yields higher dimensional precision compared to hot-rolling. Seamless steel pipes are generally manufactured by hot-rolling or cold-rolling high-quality carbon structural steels such as 10, 20, 30, 35, 45, as well as low-alloy structural steels like 16Mn and 5MnV, or alloyed structural steels such as 40Cr, 30CrMnSi, 45Mn2, and 40MnB. Seamless pipes made of low-carbon steels such as 10 and 20 are primarily used for fluid transport pipelines. Seamless tubes made of medium-carbon steels such as 45 and 40Cr are used to manufacture mechanical parts, such as the load-bearing components in automobiles and tractors. Generally, seamless steel pipes require strength and flattening tests to be ensured. Hot-rolled steel pipes are delivered in the hot-rolled state or after heat treatment ； It is delivered in a cold-rolled or heat-treated state. Seamless steel tubes for low and medium pressure boilers: Used in the manufacture of various low and medium pressure boilers, superheater tubes, feedwater tubes, water wall tubes, as well as superheater tubes, large flue tubes, small flue tubes, and arch brick tubes for locomotive boilers. Seamless pipes made of high-quality carbon structural steel, hot-rolled or cold-rolled (drawn). It is mainly manufactured from grades 10 and 20 steel; in addition to ensuring the chemical composition and mechanical properties, it is subject to hydrostatic testing, as well as tests such as curling, flaring, and crushing. Hot-rolled products are delivered in their hot-rolled state, while cold-rolled (drawn) products are delivered in their heat-treated state. High-pressure boiler steel pipes: These are high-quality carbon structural steel, alloy structural steel, and stainless heat-resistant steel seamless pipes used primarily for manufacturing steam boiler tubes operating under high pressures and above. Such boiler tubes are exposed to high temperatures and pressures, and they are also subject to oxidation and corrosion as a result of exposure to hot flue gases and water vapor; therefore, they require high endurance strength, excellent oxidation resistance, and good structural stability. The relevant steel grades include 20G, 20MnG, and 25MnG for high-quality carbon structural steel ； Alloy structural steel grades such as 15MoG, 20MoG, 12CrMoG, 15CrMoG, 12Cr2MoG, 12CrMoVG, 12Cr3MoVSiTiB, etc ； For rust-resistant heat-resistant steel, 1Cr18Ni9 and 1Cr18Ni11Nb high-pressure boiler tubes are commonly used. In addition to ensuring the chemical composition and mechanical properties, each tube must undergo a hydrostatic test, as well as flaring and flattening tests. The steel pipes are delivered in the heat-treated condition. In addition, there are also certain requirements regarding the microstructure, grain size, and decarburized layer of the finished steel pipes. Seamless steel tubes for geological drilling and oil drilling control ； Drilling rigs are used to explore the structure of underground rock layers, as well as groundwater, oil, natural gas, and mineral resources. Oil and gas extraction relies even more on drilling. Seamless steel pipes used in geological drilling and oil drilling are the main equipment for drilling, including core outer tubes, core inner tubes, casings, drill pipes, etc. Since drilling pipes have to operate at depths of several kilometers in the earth’s strata, the working conditions are extremely complex. These pipes are subjected to stresses such as tension, compression, bending, torsion, and uneven impact forces; in addition, they are exposed to wear caused by mud and rocks. Therefore, the pipes must possess sufficient strength, hardness, wear resistance, and impact toughness. The steel used for these pipes is designated using the code “DZ” (the pinyin initials for ‘geology’) followed by a number that indicates the yield strength of the steel. Common steel grades include 45MnB and 50Mn, corresponding to DZ45 ； 40Mn2, 40Mn2Si of DZ50 ； 40Mn2Mo, 40MnVB of DZ55 ； 40MnMoB for DZ60, 27MnMoVB for DZ65. All steel pipes are delivered in a heat-treated condition. Petroleum cracking tubes: seamless tubes used for furnace tubes, heat exchanger tubes, and pipelines in oil refineries. It is commonly manufactured from high-quality carbon steel (10, 20), alloy steel (12CrMo, 15CrMo), heat-resistant steel (12Cr2Mo, 15Cr5Mo), and stainless steel (1Cr18Ni9, 1Cr18Ni9Ti). In addition to proving its chemical composition and various mechanical properties, steel pipes must also pass tests such as hydrostatic pressure, flattening, and flaring tests, as well as meet requirements regarding surface quality and non-destructive testing. The steel pipes are delivered in the heat-treated condition. Stainless steel pipes: These are made from various types of stainless steel that have been hot-rolled or cold-rolled, and they are widely used in pipelines for petroleum and chemical equipment as well as in various stainless steel structural components. In addition to meeting the requirements regarding chemical composition and mechanical properties, pipes that are used to withstand fluid pressure must also pass hydraulic testing. Various specialized steel pipes must meet the specified requirements. 3. Welded steel pipes: Also known as welded tubes, they are made by bending steel plates or strips and then welding them together. Based on the weld pattern, they are divided into straight-seam welded pipes and spiral-welded pipes. Based on their purpose, they are further divided into ordinary welded pipes, galvanized welded pipes, oxygen-blown welded pipes, wire sleeves, metric welded pipes, idler roller pipes, deep well pump pipes, pipes for use in automobiles, transformer pipes, thin-walled welded pipes, shaped welded pipes, and spiral welded pipes. Ordinary welded pipes: Ordinary welded pipes are used to transport low-pressure fluids. Made of Q195A, Q215A, and Q235A steel. It can also be manufactured from other soft steels that are easy to weld. Steel pipes are subject to tests such as hydrostatic pressure testing, bending tests, and flattening tests, and certain requirements are placed on their surface quality. The typical delivery length is 4–10 meters, and it is often required that the pipes be delivered in fixed lengths (or in multiples of that length). The specifications of welded pipes are indicated by the nominal diameter (in millimeters or inches). The nominal diameter differs from the actual size. Welded pipes are available in two types according to the specified wall thickness: regular steel pipes and thick-walled steel pipes. Additionally, they can be divided into threaded and unthreaded types based on the shape of their ends. With the improvement of welded steel pipe technology and its ease of processing, welded pipes are now used to replace some seamless pipes, and welded pipes can now be produced in sizes of 219 mm (8 inches) and larger. Galvanized steel pipe: To improve the corrosion resistance of steel pipes, ordinary steel pipes (black pipes) are galvanized. Galvanized steel pipes are divided into hot-dip galvanized and electro-galvanized types; hot-dip galvanizing results in a thicker zinc coating, while electro-galvanizing is less costly. Oxygen blowing welding pipes: Used for oxygen blowing in steelmaking; they are typically small-diameter welded steel pipes, available in eight sizes ranging from 3/8 inch to 2 inches. Made from 08, 10, 15, 20, or Q195-Q235 steel strips. To prevent corrosion, some are treated with aluminum infiltration. Wire sleeves: Also made of ordinary carbon steel welded pipe, they are used in concrete and various structural power distribution projects; the common nominal diameters range from 13 to 76 mm. The wall thickness of wire insulation sleeves is relatively thin; they are usually coated or galvanized before use, and a cold bending test is required. Metric welded pipes: These are welded steel pipes specified in the form of seamless tubes, indicated by outer diameter * wall thickness in millimeters. They are manufactured by hot-rolling or cold-rolling ordinary carbon steel, high-quality carbon steel, or low-alloy steel, or by first hot-rolling them and then processing them through cold drawing. Metric welded pipes are divided into heavy-wall and thin-wall types; the heavy-wall type is commonly used as structural components such as drive shafts, or for transporting fluids, while the thin-wall type is used in the production of furniture, lighting fixtures, etc. It is necessary to ensure the strength of these pipes through bending tests. Idler tube: An electric-welded steel tube used for idlers in belt conveyors, generally manufactured from Q215, Q235A, B steels, and 20 steel, with a diameter of 63.5–219.0 mm. There are certain requirements regarding the pipe’s bendability, the perpendicularity of its end faces to the center line, and its ellipticity; generally, hydrostatic and flattening tests are conducted. Transformer tubes: Used to manufacture cooling tubes for transformers and other heat exchangers; they are made from ordinary carbon steel and require processes such as flattening, flaring, bending, and hydraulic testing. Steel pipes are delivered in fixed lengths or in multiples of such lengths, and there are certain requirements regarding their degree of bending. Special-shaped pipes: Square pipes, rectangular pipes, cap-shaped pipes, and steel pipes for glass doors and windows made by welding ordinary carbon structural steel and steel strips such as 16Mn; they are primarily used for agricultural machinery components, steel doors, and windows. Welded thin-walled tubes: mainly used to manufacture furniture, toys, lighting fixtures, etc. In recent years, thin-walled tubes made from stainless steel strips have been widely used in high-end furniture, decorations, fences, and more. Spiral welded pipe: It is made by rolling low-carbon carbon structural steel or low-alloy structural steel strips into a pipe billet at a certain spiral angle (referred to as the forming angle), and then welding the seam of the pipe together. It allows for the production of large-diameter pipes using narrower strips of steel. Spiral welded pipes are primarily used for oil and gas transmission pipelines, and their specifications are indicated by outer diameter * wall thickness. Spiral welded pipes come in single-sided and double-sided welded versions. The pipes must meet the specified requirements regarding hydraulic testing, the tensile strength of the welds, and their cold bending properties.