### Definition and Classification of Steel Pipes (Part 1)

Steel pipes are hollow long steel bars that are widely used as fluid transmission pipes, such as for oil, natural gas, water, gas, steam, etc. Additionally, due to their similar bending and torsional strength but lighter weight, they are also extensively used in manufacturing mechanical parts and structural components. Steel pipes are commonly used in producing various conventional weapons, gun barrels, and ammunition.

Classification of Steel Pipes: Steel pipes are divided into two major categories: seamless steel pipes and welded steel pipes (with seams). According to their cross-sectional shapes, they can be classified as round pipes and irregular-shaped pipes. Round steel pipes are commonly used, but there are also square, rectangular, semi-circular, hexagonal, equilateral triangular, octagonal, and other irregular-shaped pipes. For steel pipes that need to withstand fluid pressure, hydraulic tests are conducted to check their pressure resistance and quality. A pipe is considered qualified if it does not leak, wet, or expand under specified pressure. Some steel pipes also require edge rolling tests, expansion tests, flattening tests, etc., according to standards or customer requirements.

Seamless Steel Pipes: Seamless steel pipes are made from steel ingots or solid tube blanks through piercing, followed by hot rolling, cold rolling, or cold drawing. The specifications of seamless steel pipes are indicated by the outer diameter * wall thickness (in millimeters). Seamless steel pipes are divided into hot-rolled and cold-rolled (drawn) types. Hot-rolled seamless steel pipes include general-purpose pipes, low-pressure boiler pipes, high-pressure boiler pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, geotechnical pipes, and other types of pipes. Cold-rolled (drawn) seamless steel pipes include carbon thin-walled pipes, alloy thin-walled pipes, stainless thin-walled pipes, and irregular-shaped pipes, in addition to the above categories. The outer diameter of hot-rolled seamless pipes is generally greater than 32 mm, with a wall thickness ranging from 2.5 to 75 mm. The outer diameter of cold-rolled seamless pipes can reach 6 mm, with a wall thickness down to 0.25 mm. The outer diameter of thin-walled pipes can reach 5 mm, with a wall thickness less than 0.25 mm. Cold-rolling provides higher dimensional accuracy than hot-rolling. Generally, seamless steel pipes are made from high-quality carbon structural steels such as 10, 20, 30, 35, 45, low-alloy structural steels like 16Mn, 5MnV, or alloy structural steels like 40Cr, 30CrMnSi, 45Mn2, 40MnB, through hot rolling or cold rolling. Seamless pipes made from low-carbon steels like 10 and 20 are primarily used for fluid transmission pipelines. Seamless pipes made from medium-carbon steels like 45 and 40Cr are used to manufacture mechanical parts, such as stressed components in automobiles and tractors. Generally, seamless steel pipes must pass strength and flattening tests. Hot-rolled steel pipes are delivered in the hot-rolled state or after heat treatment; cold-rolled steel pipes are delivered after heat treatment.

—

### Definition and Classification of Steel Pipes (Part 2)

Low-Pressure and Medium-Pressure Boiler Seamless Steel Pipes: Used for manufacturing various low-pressure and medium-pressure boilers, superheated steam pipes, boiling water pipes, water-cooled wall pipes, and superheated steam pipes, large flue pipes, small flue pipes, and arch brick pipes for locomotive boilers. Made from high-quality carbon structural steel through hot rolling or cold rolling (drawing). Primarily made from grades 10 and 20 steels. In addition to ensuring chemical composition and mechanical properties, hydraulic tests, edge rolling tests, expansion tests, and flattening tests are performed. Hot-rolled steel pipes are delivered in the hot-rolled state; cold-rolled (drawn) steel pipes are delivered after heat treatment.

High-Pressure Boiler Steel Pipes: Primarily used for manufacturing high-pressure and above steam boiler pipes using high-quality carbon structural steel, alloy structural steel, and stainless heat-resistant steel seamless pipes. These boiler pipes operate under high temperatures and pressures. The pipes undergo oxidation and corrosion due to exposure to high-temperature flue gases and steam. Therefore, they require high endurance strength, excellent oxidation resistance, and good organizational stability. Common grades include high-quality carbon structural steels like 20G, 20MnG, 25MnG; alloy structural steels like 15MoG, 20MoG, 12CrMoG, 15CrMoG, 12Cr2MoG, 12CrMoVG, 12Cr3MoVSiTiB; and stainless heat-resistant steels like 1Cr18Ni9, 1Cr18Ni11Nb. In addition to ensuring chemical composition and mechanical properties, hydraulic tests, expansion tests, and flattening tests are performed. Steel pipes are delivered after heat treatment. Additionally, certain requirements are placed on the microstructure, grain size, and decarburization layer of the finished steel pipes. Geotechnical drilling and oil drilling use seamless steel pipes. To explore underground rock formations, groundwater, oil, natural gas, and mineral resources, drilling machines are used to drill wells. Oil and natural gas exploration cannot be done without drilling. Geotechnical drilling uses seamless steel pipes, including core outer tubes, core inner tubes, casing tubes, and drill rods. Since drilling pipes must work at depths of several kilometers and face extremely complex conditions, they are subjected to tensile, compressive, bending, twisting, and uneven impact loads, as well as wear from mud and rocks. Therefore, the pipes must have sufficient strength, hardness, wear resistance, and impact toughness. The steel used for steel pipes is denoted by “DZ” (the first character of “geological” in Chinese) plus a number representing the yield point of the steel. Common grades include DZ45 (45MnB, 50Mn), DZ50 (40Mn2, 40Mn2Si), DZ55 (40Mn2Mo, 40MnVB), DZ60 (40MnMoB), and DZ65 (27MnMoVB). Steel pipes are delivered after heat treatment.

Petroleum Cracking Pipes: Used for furnace tubes, heat exchanger tubes, and pipeline seamless pipes in petroleum refineries. Made from high-quality carbon steels (10, 20), alloy steels (12CrMo, 15CrMo), heat-resistant steels (12Cr2Mo, 15Cr5Mo), and stainless steels (1Cr18Ni9, 1Cr18Ni9Ti). In addition to ensuring chemical composition and various mechanical properties, hydraulic tests, flattening tests, and expansion tests are performed. Steel pipes are delivered after heat treatment.

Stainless Steel Pipes: Made from various stainless steels through hot rolling and cold rolling. Widely used in petrochemical equipment pipes and various stainless steel structural parts. In addition to ensuring chemical composition and mechanical properties, hydraulic tests are required for pipes designed to withstand fluid pressure. Specialized steel pipes must meet specified conditions. Welded Steel Pipes: Also known as welded pipes, these are made from steel plates or strips that are bent and then welded. According to the weld seam form, they are divided into straight seam weld pipes and spiral weld pipes. According to their application, they are further classified into general weld pipes, galvanized weld pipes, oxygen-blowing weld pipes, electrical conduit pipes, metric weld pipes, roller pipes, deep well pump pipes, automotive pipes, transformer pipes, electric welding thin-walled pipes, electric welding irregular-shaped pipes, and spiral weld pipes. General Weld Pipes: General weld pipes are used for conveying low-pressure fluids. Made from Q195A, Q215A, Q235A steels. They can also be made from other easily weldable soft steels. Weld pipes must undergo hydraulic, bending, and flattening tests. Surface quality must meet certain requirements. Typically delivered in lengths of 4-10 meters, often with fixed-length (or double-length) delivery. Weld pipe specifications are indicated by nominal diameter (in millimeters or inches), which differs from the actual size. Weld pipes are available in regular and thick-walled versions, and they can be delivered with or without threads.

—

### Definition and Classification of Steel Pipes (Part 3)

Galvanized Steel Pipes: To improve the corrosion resistance of general steel pipes (black pipes), they are galvanized. Galvanized steel pipes are divided into hot-dip galvanized and electro-galvanized types. Hot-dip galvanized pipes have thicker zinc layers, while electro-galvanized pipes are cheaper.

Oxygen-Blowing Weld Pipes: Used for oxygen-blowing pipes in steelmaking. Generally made from small-diameter welded pipes, with diameters ranging from 3/8 inch to 2 inches. Made from 08, 10, 15, 20 steels or Q195-Q235 steel strips. To prevent corrosion, some are treated with aluminum infiltration.

Electrical Conduit Pipes: Also made from ordinary carbon steel welded pipes, used in concrete and various structural electrical distribution projects. Common nominal diameters range from 13 to 76 mm. Electrical conduit pipes have thinner walls and are often coated or galvanized. They require cold bending tests.

Metric Weld Pipes: Specifications are indicated by the outer diameter * wall thickness (in millimeters) of welded pipes. Made from ordinary carbon steel, high-quality carbon steel, or common low-alloy steel hot-rolled or cold-rolled strips. Metric weld pipes are divided into regular and thin-walled types. Regular types are used for structural components like shafts or fluid conveyance, while thin-walled types are used for furniture and lighting fixtures. They must pass strength and bending tests.

Roller Pipes: Used for idler rollers in belt conveyors, made from welded pipes using Q215, Q235A, Q235B steels, and 20 steels. Outer diameters range from 63.5 to 219.0 mm. Requirements include straightness, perpendicularity of the ends to the centerline, and ovality. Water pressure and flattening tests are typically performed.

Transformer Pipes: Used for manufacturing transformer cooling pipes and other heat exchangers. Made from ordinary carbon steel. Required to pass flattening, expansion, bending, and hydraulic tests. Transformer pipes are delivered in fixed or double lengths, with certain requirements for curvature.

Irregular-Shaped Pipes: Made from ordinary carbon structural steels and 16Mn steels, these include square pipes, rectangular pipes, hat-shaped pipes, and hollow steel windows. Primarily used for agricultural machinery components, steel window frames, etc.

Electric Welding Thin-Walled Pipes: Primarily used for furniture, toys, and lighting fixtures. In recent years, thin-walled pipes made from stainless steel strips have been widely used in high-end furniture, decorations, railings, etc.

Spiral Weld Pipes: Made by rolling low-carbon structural steel or low-alloy structural steel strips at a certain angle (called the forming angle) into a tube blank, then welding the seam. They can be produced using narrower strips to make larger diameter pipes. Spiral weld pipes are primarily used for oil and natural gas pipelines. Their specifications are indicated by the outer diameter * wall thickness. Spiral weld pipes can be single-sided or double-sided welded. They must pass hydraulic tests, ensure tensile strength of the weld seam, and meet cold bending performance requirements as specified.