The difference between seamless steel pipe and welded pipe

Steel pipes are divided into two categories: seamless steel pipes and welded steel pipes (seamed pipes). According to the cross-sectional shape, it can be divided into round tubes and special-shaped tubes. Round steel tubes are widely used, but there are also some square, rectangular, semicircular, hexagonal, equilateral triangle, octagonal, and other special-shaped steel tubes. For steel pipes under fluid pressure, a hydraulic test is required to check their pressure resistance and quality. It is qualified if no leakage, soaking, or expansion occurs under the specified pressure. Some steel pipes also need to be crimped according to the standards or requirements of the buyer, Flaring test, flattening test, etc.

Seamless steel pipe: seamless steel pipe is made of steel ingot or solid tube billet through the perforation into a capillary tube, and then made by hot rolling, cold rolling, or cold drawing. The specifications of seamless steel pipes are expressed in millimeters of outer diameter * wall thickness. There are two types of seamless steel pipes: hot-rolled and cold-rolled (dial) seamless steel pipes.
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. In addition to general steel pipes, low and medium-pressure boiler steel pipes, high-pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, and other steel pipes, cold-rolled (dial) seamless steel pipes also include carbon thin-walled steel pipes, alloy thin-walled steel pipes, Stainless steel thin-walled steel pipes, special-shaped steel pipes. The outer diameter of the hot-rolled seamless pipe is generally greater than 32mm, and the wall thickness is 2.5-75mm. The diameter of cold-rolled seamless steel pipe can reach 6mm, and the wall thickness can reach 0.25mm. Rolling has higher dimensional accuracy than hot rolling. Generally, seamless steel pipes are made of high-quality carbon structural steels such as 10, 20, 30, 35, 45, 16Mn, 5MnV, and other low-alloy structural steels, or 40Cr, 30CrMnSi, 45Mn2, 40MnB and other combined steels that are hot-rolled or cold-rolled. 10, 20, and other low-carbon steel seamless pipes are mainly used for fluid-conveying pipelines. Seamless tubes made of medium carbon steel such as 45 and 40Cr are used to manufacture mechanical parts, such as stressed parts of automobiles and tractors. Generally, seamless steel pipes are used to ensure strength and flattening tests. Hot-rolled steel pipes are delivered in a hot-rolled or heat-treated state; cold-rolled steel pipes are delivered in a heat-treated state. Seamless steel pipes for low and medium-pressure boilers: used to manufacture various low and medium-pressure boilers, superheated steam pipes, boiling water pipes, water wall pipes, and superheated steam pipes for locomotive boilers, large smoke pipes, small smoke pipes, and arch brick pipes, etc. Hot-rolled or cold-rolled (dial) seamless steel pipes are made of high-quality carbon structural steel. It is mainly made of No. 10 and No. 20 steel. In addition to ensuring the chemical composition and mechanical properties, hydrostatic tests, crimping, flaring, and flattening tests are required. Hot rolling is delivered in a hot-rolled state, and cold rolling (dial) is delivered in a heat-treated state. High-pressure boiler steel pipe: mainly used to manufacture high-quality carbon structural steel, alloy structural steel, and stainless heat-resistant steel seamless steel pipe for steam boiler pipes with high pressure and above pressure. These boiler pipes work under high temperatures and high pressure 1. The pipe will also be oxidized and corroded under the action of high-temperature flue gas and water vapor. Therefore, the steel pipe is required to have high durable strength, high oxidation resistance, and good structural stability. The steel grades used are: high-quality carbon Structural steel grades are 20G, 20MnG, 25MnG; alloy structural steel grades are 15MoG, 20MoG, 12CrMoG, 15CrMoG, 12Cr2MoG, 12CrMoVG, 12Cr3MoVSiTiB, etc.; rusty heat-resistant steel is commonly used in 1Cr18Ni9, 1Cr18Ni11Nb high-pressure boiler tubes to ensure chemical composition and mechanical properties In addition, hydrostatic tests should be done one by one, and flaring and flattening tests should be done. Steel pipes are delivered in a heat-treated state. 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 control; to prove the underground rock structure, groundwater, oil, natural gas, and mineral resources, use drilling rigs to drill wells. The exploitation of oil and natural gas is inseparable from well drilling. Seamless steel pipes for oil drilling for geological drilling control are the main equipment for drilling, mainly including core outer pipes, core inner pipes, casings, drill pipes, etc. Since the drilling pipe has to work deep into the formation depth of several thousand meters, the working conditions are extremely complicated. The drill pipe is subjected to stresses such as tension, compression, bending, torsion, and unbalanced impact loads, and is also subject to mud and rock wear. Therefore, pipes are required must have sufficient strength, hardness, wear resistance, and impact toughness. The steel used for steel pipes is represented by "DZ" (geological Chinese pinyin prefix) plus a number one to represent the yield point of the steel. The commonly used steel grades are 45MnB and 50Mn of DZ45; 40Mn2, 40Mn2Si of DZ50; 40Mn2Mo, 40MnVB of DZ55; 40MnMoB of DZ60, 27MnMoVB of DZ65. Steel pipes are delivered in a heat-treated state. Petroleum cracking tubes: seamless tubes for furnace tubes, heat exchanger tubes, and pipelines used in petroleum refineries. Commonly used high-quality carbon steel (10, 20), alloy steel (12CrMo, 15CrMo), heat-resistant steel (12Cr2Mo, 15Cr5Mo), stainless steel (1Cr18Ni9, 1Cr18Ni9Ti) manufacturing. In addition to the chemical composition and various mechanical properties of the steel pipe, it is also necessary to ensure the water pressure, flattening, flaring, and other tests, as well as surface quality and non-destructive testing. Steel pipes are delivered in the heat-treated condition. Stainless steel pipe: all kinds of stainless steel hot-rolled and cold-rolled stainless steel pipes are widely used in petroleum, chemical equipment pipelines, and stainless steel structural parts for various purposes. The steel pipe shall be guaranteed to pass the hydrostatic test. All kinds of special steel pipes shall be guaranteed according to the regulations.

Welded steel pipe: also called welded pipe, is made of steel plate or steel strip after bending and then welding. According to the weld form, it is divided into straight seam welded pipe and spiral welded pipe. According to the purpose, it is divided into general welded pipe, galvanized welded pipe, oxygen blown welded pipe, wire casing, metric welded pipe, idler pipe, deep well pump pipe, automobile pipe, transformer pipe, electric welded thin-walled pipe, electric welded special-shaped pipe and spiral welded pipe. ; General welded pipe: general welded pipe is used to convey low-pressure fluid. Made of Q195A, Q215A, Q235A steel;. It can also be manufactured from other mild steels that are easy to weld. Steel pipes are subject to water pressure, bending, flattening, and other tests, and have certain requirements for surface quality. Usually, the delivery length is 4-10m, and it is often required to deliver in fixed length (or double length). The specification of the welded pipe is represented by the nominal diameter (millimeter or inch). The nominal diameter is different from the actual one. The welded pipe has two types of ordinary steel pipe and thickened steel pipe according to the specified wall thickness. With the improvement of welded steel pipe technology and the ease of processing, now welded pipes replace some seamless pipes, and welded steel pipes have been able to produce more than 219mm (8 inches).