Steel pipe is a kind of hollow long steel, which is widely used as a pipeline for conveying fluids, such as oil, natural gas, water, gas, steam, etc. in addition, when the bending and torsional strength are the same, the weight is relatively light, so it is also widely used to manufacture mechanical parts and engineering structures, and also commonly used to produce various conventional weapons, gun barrels, shells, etc.

Classification of steel pipes: steel pipes are divided into seamless steel pipes and welded steel pipes (seamed pipes). According to the section shape, it can be divided into circular pipe and special-shaped pipe. Circular steel pipe is widely used, but there are also some special-shaped steel pipes such as square, rectangular, semicircular, hexagonal, equilateral triangle, octagon, etc.

Hydraulic test shall be carried out for the steel pipes bearing fluid pressure to test their pressure resistance and quality. It is qualified if there is no leakage, wetting or expansion under the specified pressure. Some steel pipes shall be subject to crimping test, flaring test, flattening test, etc. according to the standards or the requirements of the demander.

Seamless steel pipe: seamless steel pipe is made of steel ingot or solid tube blank through perforation, and then hot rolled, cold rolled or cold drawn. The specification of seamless steel pipe is expressed in mm of outer diameter * wall thickness. Seamless steel tubes are divided into hot rolled and cold rolled (rolled) seamless steel tubes.

Hot rolled seamless steel pipes are divided into general 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 steel pipes.

Cold rolled (drawn) seamless steel pipes include carbon thin-walled steel pipes, alloy thin-walled steel pipes, stainless steel pipes, stainless steel thin-walled steel pipes and special-shaped steel pipes in addition to general steel pipes, low and medium pressure boiler steel pipes, high-pressure boiler steel pipes, alloy steel pipes, petroleum cracking steel pipes and other steel pipes.

The outer diameter of hot-rolled seamless pipe is generally greater than 32mm, and the wall thickness is 2.5-75mm; The outer diameter of cold-rolled seamless steel pipe can reach 6mm, the wall thickness can reach 0.25mm, and the outer diameter of thin-walled pipe can reach 5mm, and the wall thickness is less than 0.25mm. Cold rolling has higher dimensional accuracy than hot rolling.

Generally, seamless steel pipes are made of (10, 20, 35, 45) and other excellent carbon bonded steels, 16Mn, 5mnv and other low alloy structural steels, or (40Cr, 30CrMnSi, 45Mn2, 40MnB) and other bonded steels by hot rolling or cold rolling. (10, 20) and other low carbon steel seamless pipes are mainly used for fluid transmission pipelines. (45, 40Cr) and other seamless pipes made of medium carbon steel are used to manufacture mechanical parts, such as stressed parts of automobiles and tractors.

Generally, the strength and flattening test of seamless steel pipe shall be ensured. Hot rolled steel pipes shall be delivered in hot rolling state or heat treatment state; Cold rolling is delivered in heat treated condition.

Seamless steel tubes for low and medium pressure boilers: used for manufacturing various low and medium pressure boilers, superheated steam tubes, boiling water tubes, water wall tubes and superheated steam tubes, large smoke tubes, small smoke tubes and arch brick tubes for locomotive boilers. Hot rolled or cold rolled (rolled) seamless steel tubes of high-quality carbon structural steel for low and medium pressure boilers are mainly made of (10, 20) steel. In addition to ensuring chemical composition and mechanical properties, hydrostatic test, crimping, flaring, flattening and other tests shall be conducted. Hot rolling shall be delivered in hot rolling state, and cold rolling (dialing) shall be delivered in heat treatment state.

High pressure boiler steel pipe: it is mainly used to manufacture excellent carbon structural steel, alloy structural steel and stainless heat-resistant steel seamless steel pipes for steam boiler pipes with high pressure and above. These boiler pipes often work under high temperature and high pressure. The pipes will also be oxidized and corroded under the action of high temperature flue gas and steam. Therefore, the steel pipes are required to have high endurance strength, high oxidation resistance and good organizational stability, The adopted steel grades are: excellent carbon structural steel grades are (20g, 20mng, 25mng); Alloy structural steel grades (15mog, 20mog, 12crmog, 15CrMoG, 12CR2MOG, 12crmovg, 12Cr3MoVSiTiB), etc; Rusty heat-resistant steels are commonly used (1Cr18Ni9, 1cr18ni11nb). In addition to ensuring the chemical composition and mechanical properties, the high-pressure boiler tubes shall be subject to hydrostatic test, flaring and flattening test one by one. Steel pipes shall be delivered in heat treated condition. In addition, there are certain requirements for the microstructure, grain size and decarburization layer of the finished steel pipe.

Seamless steel pipes for geological drilling and oil drilling: in order to explore the underground rock structure, groundwater, oil, natural gas and mineral resources, drilling rigs are used to drill wells. Oil and natural gas exploitation is inseparable from drilling. Seamless steel pipes for oil drilling are the main equipment for geological drilling, mainly including core outer pipe, core inner pipe, casing, drill pipe, etc. As the drilling pipe is required to work at the depth of several thousand meters, the working conditions are extremely complex. The drill pipe is subject to tensile, compressive, bending, torsional and unbalanced impact loads, and is also subject to mud and rock wear. Therefore, the pipe must have sufficient strength, hardness, wear resistance and impact toughness. The steel pipe is represented by "DZ" (Geological Chinese phonetic prefix) plus the number one representing the steel yield point, The commonly used steel grades are dz45 (45mnb, 50Mn); Dz50 (40Mn2, 40mn2si); Dz55 (40mn2mo, 40mnvb); DZ60 (40mnmob); Dz65 (27mnmovb). Steel pipes are delivered in heat-treated condition.

Petroleum cracking pipe: seamless pipe for furnace pipe, heat exchanger pipe and pipeline of petroleum refinery. It is commonly made of high-quality carbon steel (10, 20), alloy steel (12CrMo, 15CrMo), heat-resistant steel (12cr2mo, 15cr5mo) and stainless steel (1Cr18Ni9, 1Cr18Ni9Ti). In addition to obtaining the chemical composition and various mechanical properties, the steel pipe shall also ensure the hydrostatic, flattening, flaring and other tests, as well as the surface quality and nondestructive testing. Steel pipes shall be delivered under heat treatment.

Stainless steel pipe: all kinds of hot-rolled and cold-rolled stainless steel pipes are widely used in petroleum and chemical equipment pipes and stainless steel structural parts for various purposes. In addition to ensuring the chemical composition and mechanical properties, all steel pipes used to withstand fluid pressure shall be qualified in the hydrostatic test. Various special steel pipes shall be guaranteed according to the specified conditions.

Welded steel pipe: also called welded pipe, it is made of steel plate or steel strip by bending and then welding.

According to the manufacturing process, it is divided into longitudinal resistance welded pipe ERW, spiral submerged arc welded pipe SSAW and longitudinal submerged arc welded pipe UOE.

According to the purpose, it is divided into general welded pipe, galvanized welded pipe, oxygen blowing welded pipe, wire sleeve, metric welded pipe, idler pipe, deep well pump pipe, automobile pipe, transformer pipe, electric welding thin-wall pipe, electric welding special-shaped pipe and spiral welded pipe.

General welded pipe: General welded pipe is used to transport low-pressure fluid. It is made of (q195a, q215a, Q235A) steel.

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