Aside from copper and copper alloys, steel is the most popular pipe metal used today. An alloy of iron and carbon, steel has excellent corrosion resistance, a high melting point and a malleability that is attractive to manufacturers. As a result, pipes made of steel serve as reliable conduits for contents under enormous amounts of pressure. Recognizing this broad utility, engineers have developed several techniques for making steel pipe.
Seamless Pipe
"Seamless" steel pipes are those whose manufacture does not involve welding of any kind. The process starts with a long, solid cylinder of steel known as a "billet." The billet is loaded into a special machine that uses electrical resistance to heat the billet to a malleable temperature. A pair of rollers with perpendicular axes (Figure 1) rotate the heated billet rapidly around its own axis while slowly drawing it into special instrument known as a "piercer." The piercer slowly enters the rotating billet as a near perfect tube forms around it. To better illustrate, think of the process like a potter's wheel; when you put your finger into the top of the spinning clay, a perfectly-symmetrical hole forms around the digit. Formally, this phenomenon is known as the "Mannesmann Effect."
To get the desired wall thickness, the hollow billet enters an "elongator," which is a new set of perpendicular rollers with a specifically-shaped piercer to squeeze it into a longer, thinner tube.
Hot Extruded Pipe
The hot extruded method is a variation on seamless pipe. Taking the first-stage hollowed billet, manufacturers load it into heating press known as an "extruder" (Figure 2). A long metal bar meant to set the finished pipe diameter (i.e. the "mandrel") runs from the base of the press up through the middle of the billet. The other end of the press is covered with a patterned opening known as a "die," designed to set the outer diameter of the finished pipe. Electrical resistance heats the billet and the powerful hydraulic press pushes the malleable billet up through the die, resulting in a long stretch of steel pipe.
Butt Welded Pipe
A long, rectangular of steel is fed into a set of parallel rollers (Figure 3). These rollers help smooth imperfections and squeeze the material into the desired final thickness. These flattened sheets then enter a blast furnace, which heats them for greater malleability. Once heated, the sheets feed into a series of parallel rollers, which gradually bend the sheets into a tight spiral. This process is formally known as "forming."
Just as the edges of the sheets touch, an automatic arc welding torch fires along the seam, melting the metal and causing them to fuse together. The "butt" qualifier is short for "butt joint"--a welding joint consisting two non-overlapping pieces with non-beveled edges.
Electric Resistance Welded (ERW) Pipe
Designed for especially thin thicknesses of steel, ERW pipes start the same as butt welded pipes, except that extreme heating before the forming stage usually is not necessary. Once the edges of the coiled sheet touch, the seam passes between a pair of special electrodes set just millimeters apart. The terminal of each electrode touch the pipe on either side of the seam and pass a powerful current between the touching edges. The strength of the voltage generates massive electrical resistance across the steel. However, because the metal is so thin, it cannot displace the heat quickly enough and soon reaches its melting point, causing the two sides to fuse together.
U-Press, O-Press (UO) Pipe
Used mainly on pipes with a larger diameter, the process starts with a long, rectangular sheet of steel whose thickness is the same at the final pipe. Most importantly, the width of the sheet must be the same as the circumference of the final pipe.
First, a press with a rounded surface forces the sheet through a long opening, bending it into a deep, elliptical "U" (Figure 5). Then, the sheet is placed in a cylindrical "O-Press," which pushes down on its two upward-facing edges and bends the sheet into a circular cross section. The two edges are fused closed through a submerged arc welding (SAW) apparatus. In SAW, a powerful current passes from an electrode filled with a highly-conductive, anti-oxidant material called "flux" through the steel pipe and into a regulator and power source. The electrical resistance heats up the flux enough to melt, which in turn melts the electrode. Therefore, an endless length of new electrode is constantly fed into the fusing zone.
Spiral Pipe
Designed for wider pipes of a relatively thin thickness, the spiral pipe starts with long, rectangular sheets of steel that are twisted into a stack-like spiral by a series of rollers set along the edges (Figure 6). The edges are then fused using a SAW apparatus.