Basic Knowledge of Steel Pipes

　　All steel pipes designed to withstand fluid pressure must undergo hydraulic testing to verify their pressure resistance and quality. They are considered qualified if no leakage, wetting, or expansion occurs under the specified test pressure. Additionally, certain steel pipes may also be subjected to edge curling tests, flaring tests, flattening tests, and other specialized inspections—as required by standards or the customer’s specifications.

　　Steel pipes are classified into two main categories: seamless steel pipes and welded steel pipes (also known as seam pipes). Depending on their cross-sectional shape, they can further be divided into round pipes and special-shaped pipes. While circular steel pipes are widely used, there are also various other shapes available, such as square, rectangular, half-round, hexagonal, equilateral triangular, and octagonal steel pipes.

　　All steel pipes designed to withstand fluid pressure must undergo hydraulic testing to verify their pressure resistance and quality. They are considered合格 (qualified) if no leakage, wetting, or expansion occurs under the specified test pressure. Additionally, certain steel pipes may also be subjected to edge curling tests, flaring tests, flattening tests, and other specialized inspections—as required by standards or the customer's specifications.

　　Seamless steel pipes are manufactured by first piercing a steel ingot or solid tube blank to create a hollow billet, which is then hot-rolled, cold-rolled, or cold-drawn into the final product. The dimensions of seamless steel pipes are typically specified as outer diameter multiplied by wall thickness, in millimeters. These pipes are categorized into two types: hot-rolled and cold-rolled (or cold-drawn) seamless steel pipes.

　　Hot-rolled seamless steel pipes are categorized into general-purpose pipes, low- and medium-pressure boiler tubes, high-pressure boiler tubes, alloy steel pipes, stainless steel pipes, petroleum cracking tubes, geological steel pipes, and other types of steel pipes.

　　Cold-rolled (drawn) seamless steel tubes include not only general-purpose tubes, low- and medium-pressure boiler tubes, high-pressure boiler tubes, alloy tubes, stainless steel tubes, petroleum cracking tubes, and other specialized steel tubes, but also carbon-based thin-walled tubes, alloy thin-walled tubes, stainless thin-walled tubes, and special-shaped tubes.

　　Hot-rolled seamless pipes typically have an outer diameter greater than 32 mm and wall thickness ranging from 2.5 to 75 mm; cold-rolled seamless steel tubes can achieve outer diameters as small as 6 mm, with wall thicknesses down to 0.25 mm. Thin-walled tubes may have outer diameters as low as 5 mm and wall thicknesses thinner than 0.25 mm. Notably, cold rolling offers higher dimensional accuracy compared to hot rolling.

　　Seamless steel pipes are typically manufactured from high-quality carbon steels such as (10, 20, 35, 45), as well as low-alloy structural steels like 16Mn and 5MnV, or alloy steels like (40Cr, 30CrMnSi, 45Mn2, 40MnB) through hot-rolling or cold-rolling processes. Seamless tubes made from low-carbon steels like (10 and 20) are primarily used for fluid transport pipelines. On the other hand, seamless pipes produced from medium-carbon steels such as (45 and 40Cr) are commonly employed in manufacturing mechanical components—such as load-bearing parts for automobiles and tractors.

　　Generally, seamless steel pipes must meet strength and flattening test requirements. Hot-rolled steel pipes are delivered either in the hot-rolled condition or after heat treatment, while cold-rolled pipes are supplied in the heat-treated condition.

　　Seamless steel tubes for low- and medium-pressure boilers: Used in the manufacturing of various low- and medium-pressure boilers, superheated steam pipes, boiling water tubes, water-cooled wall tubes, as well as superheated steam pipes, large flue tubes, small flue tubes, and arch brick tubes for locomotive boilers. These high-quality carbon structural steel seamless tubes—hot-rolled or cold-rolled (drawn)—are primarily made from steels such as (10 and 20). In addition to meeting stringent chemical composition and mechanical property requirements, they undergo mandatory hydrostatic pressure tests, along with additional inspections like edge rolling, flaring, and flattening tests. Hot-rolled tubes are delivered in the hot-rolled condition, while cold-rolled (drawn) tubes are supplied after heat treatment.

　　High-pressure boiler steel tubes—primarily used for manufacturing high-pressure and ultra-high-pressure steam boiler pipelines—are made from high-quality carbon structural steels, alloy structural steels, and stainless heat-resistant steels in seamless forms. These boiler tubes are routinely subjected to extreme conditions of high temperature and pressure, often exposed to hot flue gases and water vapor, which can lead to oxidation and corrosion over time. Therefore, the tubes must exhibit exceptional long-term strength, outstanding resistance to oxidation at elevated temperatures, and excellent microstructural stability. Commonly used steel grades include: - High-quality carbon structural steels such as 20G, 20MnG, and 25MnG; - Alloy structural steels like 15MoG, 20MoG, 12CrMoG, 15CrMoG, 12Cr2MoG, 12CrMoVG, and 12Cr3MoVSiTiB; - Stainless heat-resistant steels typically featuring 1Cr18Ni9 and 1Cr18Ni11Nb. In addition to meeting stringent chemical composition and mechanical property requirements, each high-pressure boiler tube undergoes individual hydrostatic testing, as well as flaring and flattening tests. The tubes are delivered in a heat-treated condition. Furthermore, the final product must also satisfy specific criteria regarding its microstructure, grain size, and decarburization layer depth.

　　Seamless steel pipes for geological and oil drilling: Drilling rigs are used to explore underground rock structures, as well as groundwater, oil, natural gas, and mineral resources. Oil and natural gas extraction heavily rely on well-drilling operations, making seamless steel pipes—specifically those designed for geological and petroleum applications—their primary equipment. These pipes include key components such as core outer tubes, core inner tubes, casing, and drill rods. Since drilling pipes must operate at depths of several kilometers under extremely challenging conditions, drill rods endure complex stresses, including tension, compression, bending, torsion, and uneven impact loads. Additionally, they face constant exposure to abrasive mud and rock formations. Therefore, the materials used must exhibit exceptional strength, hardness, wear resistance, and impact toughness. Steel grades for these pipes are designated with the prefix "DZ" (the pinyin initials for "geological"), followed by a number indicating the steel's yield strength. Commonly used steel grades include DZ45 (containing 45MnB or 50Mn), DZ50 (featuring 40Mn2 or 40Mn2Si), DZ55 (including 40Mn2Mo or 40MnVB), DZ60 (such as 40MnMoB), and DZ65 (comprising 27MnMoVB). All steel pipes are delivered in a heat-treated condition.

　　Petroleum cracking tubes: Seamless pipes used for furnace tubes, heat exchanger tubes, and pipelines in oil refineries. Commonly manufactured from high-quality carbon steels (10, 20), alloy steels (12CrMo, 15CrMo), heat-resistant steels (12Cr2Mo, 15Cr5Mo), and stainless steels (1Cr18Ni9, 1Cr18Ni9Ti). In addition to certified chemical compositions and comprehensive mechanical properties, these steel tubes must also undergo rigorous testing—including hydrostatic pressure, flattening, and flaring—as well as stringent surface quality checks and non-destructive inspections. The steel tubes are delivered in a heat-treated condition.

　　Stainless steel tubes: Manufactured from a variety of hot-rolled and cold-rolled stainless steel grades, these tubes are widely used in pipelines for petroleum and chemical equipment, as well as in stainless steel structural components designed for diverse applications. In addition to meeting stringent requirements for chemical composition and mechanical properties, any stainless steel tube intended for withstanding fluid pressure must pass a successful hydrostatic test. Furthermore, all specialized stainless steel tubes must adhere to the specified performance criteria.