There are two types of steel tubing: welded and seamless. Either type may be made of any alloy of steel that is formable. Welded tube requires that the alloy can be welded. Welded tubing begins as a flat strip. Seamless tubing starts in a melting furnace.
A tube is a closed shape used to perform some structural function. The shape can be round, square, rectangular or whatever can be designed and manufactured for a particular application. Round tubes differ from pipes in the way they are dimensioned. A 1 1/2-inch round tube has a 1 1/2-inch outside diameter. A 1 1/2-inch round pipe has a 1 1/2-inch inside diameter. The reason being that the outside of tubes is important for structures, however the inside of pipes is used for fluid flow.
Welded Steel Tube
Welded steel tubing is made from either hot-rolled or cold-rolled steel that is provided to the tubing manufacturer in coils. The coil may be processed to remove scale, improve finish, amend heat treatment or in other ways to achieve the specified end product. The coil is then run through grooved rollers until the shape has been formed and the free edges are correctly spaced for welding.
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The edges of the tube are butt welded together, generally with electrical resistance welding. The outside, and sometimes the inside, welding residue is then removed. The tube, which is at this point somewhat oversized, is then put through two semicircular rollers that bring it to its final outside dimension.
Seamless Steel Tube
The manufacture of seamless steel tube typically begins in an electric arc furnace. The steel is then cast as an ingot or continuously cast as a bloom. The bloom is then rolled into a billet. The solid billet becomes a tube by being drawn over a piercing tool by two external rollers set at an angle to each other that move the billet. At this point a “rough” tube has been formed.
The rough tube is put through an elongator, in which three rollers and an internal mandrel shape the tube closer to its final outside diameter and wall thickness. The next step takes place at a reducing mill, where the outside diameter is brought still closer to its final size and the tube is made oval to ultimately improve its final finish. The rotary sizer is the last step in the basic seamless tube-making process. The rotary sizer restores the tube to a round shape and brings it to its final outside dimension.
There are additional processes possible in the manufacture of welded and seamless tubing. The most common mechanical process is cold drawing. This is done by drawing the tube over a mandrel. The product is known as DOM (drawn over mandrel) tube. Cold drawing improves the physical properties, the surface finish and can tighten dimensional tolerances.
Various heat treatments may be used in the manufacture of welded and seamless tubing. These include annealing that improves the "microstructure" of the metal; normalizing that makes the tube properties more uniform and tempering that makes the steel more ductile.