Seamless steel tubes are manufactured by rolling solid billets after they have been perforated. This process allows for the creation of high-quality, uniform tubing without any welds or seams. The production methods vary depending on the desired properties and application, and can be categorized into hot-rolled, cold-rolled, cold-drawn, and extruded tubes, among others.
Hot-rolled seamless pipes are typically produced using automatic pipe mills. Before processing, the solid billet is inspected to remove surface defects, then cut to the required length and centered on the perforated end. It is then heated in a furnace and fed into a perforator, where it is rotated and advanced to form a hollow cavity, known as a capillary tube. This capillary tube is further rolled in an automatic mill to achieve uniform wall thickness and size through a sizing machine. Continuous rolling mills are considered more advanced due to their efficiency and precision.
For smaller diameter tubes with better quality, cold rolling or cold drawing methods are preferred. Cold rolling is often performed on two-high mills, where the tube is rolled through a ring-shaped pass formed by variable-section circular grooves and a stationary conical head. Cold drawing, on the other hand, uses single or double chain machines with capacities ranging from 0.5 to 100 tons, offering greater dimensional accuracy.
Extrusion is another method used to produce small-diameter seamless tubes. In this process, a heated billet is placed inside a closed extrusion cylinder, and a perforation rod moves along with the extrusion rod to squeeze the material through a smaller die hole, forming the final shape.
Seamless steel tubes have a wide range of applications. General-purpose tubes are made from carbon structural steels, low-alloy steels, or alloy steels and are commonly used for fluid transport or structural components. They can be supplied based on chemical composition, mechanical properties, or hydrostatic testing. Pipes supplied under categories a and b must undergo hydrostatic testing if they are intended for use under liquid pressure.
Special-purpose seamless tubes include those designed for boilers, geological exploration, and petroleum industries, each tailored to specific performance requirements.
In terms of classification, seamless tubes can be divided by production method (hot-rolled, cold-rolled, etc.), by shape (round, square, rectangular, elliptical, triangular), by material (carbon steel, alloy steel, stainless steel), and by function (boiler tubes, oil tubes, etc.).
According to GB/T 8162-87 standards, the specifications for hot-rolled tubes range from an outer diameter of 32 to 630 mm, with wall thicknesses between 2.5 and 75 mm. Cold-rolled or cold-drawn tubes have an outer diameter of 5 to 200 mm and wall thicknesses of 2.5 to 12 mm. The surface of the tubes must be free from cracks, folds, scarring, or other defects, which 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 must be removed. For thick-walled tubes over 20 mm, gas cutting or hot saw cutting may be allowed upon 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 tests are essential for ensuring the quality of seamless pipes. Domestic pipes must comply with GB/T 699-88 for steels like 10, 15, 20, 25, 30, 35, 40, 45, and 50. Imported pipes are tested according to contract-specified standards, while steels such as 09MnV, 16Mn, and 15MnV must meet GB 1591-79 requirements. Analysis methods and permissible deviations are outlined in GB 223-84 and GB 222-84, respectively.
This detailed information provides a comprehensive overview of the manufacturing processes, classifications, specifications, and quality standards for seamless steel tubes.
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