Seamless steel tubes are manufactured by first perforating solid billets. The production process involves several methods, including hot-rolling, cold-rolling, cold-drawing, and extrusion, each suited for different applications and quality requirements.
Hot-rolled seamless pipes are typically produced on automatic pipe mills. Before processing, the solid billet is inspected for surface flaws, then cut to the desired length. It is centered on the perforated end face and fed into a heating furnace. During the perforation process, the billet rotates and advances as rollers and a mandrel shape the internal cavity, forming what is known as a capillary tube. This is then sent through an automatic mill for further rolling and sizing to meet dimensional specifications. Continuous rolling mills are considered a more advanced method for producing high-quality hot-rolled seamless tubes.
For smaller diameter tubes with improved surface finish and precision, cold rolling or cold drawing is used. Cold rolling is commonly done on two-high mills, where the tube is rolled in a ring-shaped pass formed by a variable-section circular groove and a stationary conical head. Cold drawing, on the other hand, is performed using single or double chain draw benches with capacities ranging from 0.5 to 100 tons.
Extrusion is another technique used to produce small-diameter seamless tubes. In this process, a heated tube blank is placed inside a closed extrusion cylinder, and the perforation rod moves along with the extrusion rod to push the material through a smaller die hole, resulting in the formation of the tube.
Seamless steel tubes have a wide range of applications. General-purpose tubes are made from carbon or low-alloy structural steels and are primarily used for fluid conveyance or structural components. They can be supplied based on chemical composition and mechanical properties, mechanical performance alone, or hydrostatic testing. Pipes supplied under the first two categories must also undergo hydrostatic testing if they are intended for use under liquid pressure.
Specialized seamless tubes include those designed for boiler applications, geological drilling, and oil and gas industries.
In terms of types, seamless steel tubes are categorized based on production methods (hot-rolled, cold-rolled, cold-drawn, extruded), appearance (round, square, rectangular, elliptical, triangular, hexagonal), materials (carbon steel, alloy steel, stainless steel), and specific use (boiler tubes, oil tubes, etc.).
According to GB/T 8162-87 standards, the outer diameter of hot-rolled pipes ranges from 32 to 630 mm, with wall thicknesses between 2.5 and 75 mm. Cold-rolled or cold-drawn tubes have outer diameters from 5 to 200 mm and wall thicknesses from 2.5 to 12 mm. The inner and outer surfaces must be free from cracks, folds, seams, delamination, hairline cracks, and scarring. Any defects found must be removed without exceeding the allowable negative deviation in wall thickness or outer diameter.
The ends of the tubes should be cut at right angles, and burrs should be removed. For tubes with wall thickness over 20 mm, gas cutting or hot saw cutting is permitted, depending on agreement between the supplier and buyer.
For cold-drawn or cold-rolled precision seamless tubes, surface quality should follow the guidelines set in GB 3639-83.
Chemical composition testing is essential for domestic and imported seamless pipes. Steels such as 10, 15, 20, 25, 30, 35, 40, 45, and 50 must comply with GB/T 699-88. For steels like 09MnV, 16Mn, and 15MnV, GB 1591-79 applies. The analysis methods and allowable deviations are specified in GB 223-84 and GB 222-84, respectively.
This comprehensive overview provides a clear understanding of the production, classification, and quality standards of seamless steel tubes.
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