Tubes
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Tubular products are manufactured according to two basic technologies. One is the welding of tubes from strip, and the other is the production of seamless tube from rounds or blooms.
Welded tubes
The most widely used welding system, the electric-resistance welding (ERW) line, starts with a descaled hot-rolled strip that is first slit into coils of a specific width to fit a desired tube diameter. In the entry section is an uncoiler, a welder that joins the ends of coils for continuous operation, and a looping pit, which permits constant welding rates of, typically, three metres per minute. Several consecutive forming rolls then shape the strip into a tube with a longitudinal seam on top, as shown schematically in A in the . Two squeeze rolls press the seam together, while two electrode rolls or sliding contacts feed the electric power to the seam for resistance heating and welding. A cutting tool removes the flash created during welding, and, after a preliminary inspection, the tube is cut into cooling-bed length by a saw that moves with the tube.
Tubes up to 500 millimetres in diameter with walls 10 millimetres thick are produced on ERW lines. Larger-diameter pipes are often produced by forming the strip into an endless spiral, as shown schematically in B in the . Forming is followed by continuous welding of the seam, often by automatic arc welding. Pipes up to 1.5 metres in diameter and with a 12-millimetre wall thickness are sometimes produced by this spiral welding process. Still larger pipes are produced from plates by a U-ing and O-ing process, which applies heavy presses to form plates into a U and then an O. The longitudinal seam (or seams) are then welded by automatic arc-welding equipment.
Seamless tubes
Seamless tube rolling always begins by piercing a round or bloom to generate a hollow. In roll piercing, an oval round is preheated to about 1,200° C and is cross-rolled slowly between two short, large-diameter rolls that rotate in the same direction (shown schematically in C in the ). The round also revolves and is pulled into the roll gap in a spiraling motion, because the rolls have a converging-diverging shape and are installed relative to each other at an angle of about 20°. This revolving, continuous plastic working of an oval cross section between the two rolls creates tensile stresses in the long axes of the oval, which rupture the centre and create a cavity. At this point the cavity meets the piercer, which is a projectile-shaped rotating cone held in place by a bar and a thrust bearing. The piercer acts like a third roll in the centre and produces the inside of the tube.
The cross or helical rolling action of roll piercing demands excellent hot formability of the prerolled round. Another process, push piercing, does not have such exacting requirements. This usually takes continuously cast square blooms and forms them into hollow rounds by the action of a heavy hydraulic pusher, which pushes them into the gap of two large-diameter contoured rolls that form together a circular pass line. In the roll gap the bloom is met by a heavy piercer, which forms the hollow, as shown in D in the . This mill can form a 250-millimetre-square, 3-metre-long bloom into a tube with an outside diameter of 300 millimetres and an inside diameter of 150 millimetres. Since there are only compression forces acting on the steel in this process, the workpiece is practically not elongated at all.
A number of rolling technologies are used to form the pierced hollows into tubes with specific dimensions and tolerances. Often, the hollow is reheated and then sent through another cross-roll piercer mill, called the elongator; this reduces the wall thickness by 30 to 60 percent. In a subsequent step, a long, preheated, lubricated cylinder called a mandrel may be inserted into the tube. The tube would then be rolled, with the mandrel inside, in a continuous close-coupled, seven-stand, two-high mill, usually with the rolls arranged at a 45° angle and in an alternating pattern like the horizontal and vertical rolls. A very uniform wall thickness can be formed by this process. Smaller diameter tubes are often formed from larger tubes in a continuous three-roll, close-coupled stretch-reduction mill (E in the ). These mills sometimes have 20 sets of rolls arranged in tandem.
Open-die forging
Heavy ingots, some weighing up to 300 tons, are sometimes formed at steel plants by huge hydraulic presses with a forging force of up to 10,000 tons. These make such large products as rotors for power-generating units or large sleeves for rolls or pressure vessels. Careful, uniform heating of the ingots to forging temperature may take 60 hours, and, before completion of the forging process, the workpiece may be reheated six times. The forging is accomplished by flat-, vee-, or swage-shaped dies, depending on the shape of the final product. Saddles and mandrels are used for forging rings and sleeves. The workpiece is connected to a long bar, which helps to move and turn it by a crane or manipulator. Large heat-treating furnaces are available in these forging shops to improve microstructure and to release internal stresses caused by the forging operation.
Wire
The cold drawing of wire is an important and special sector of steelmaking. It produces wire in hundreds of sizes and shapes and within a spectrum of physical properties unmatched by other steel products. Wire is also produced with many types of surface finish.
Treating of steel
Heat-treating
In principle, heat-treating already takes place when steel is hot-rolled at a particular temperature and cooled afterward at a certain rate, but there are also many heat-treating process facilities specifically designed to produce particular microstructures and properties. The simplest heat-treating process is normalizing. This consists of holding steel for a short time at a temperature 20° to 40° C above the G-S-K line (shown in the iron-carbon diagram in the ) and then cooling it afterward in still air. Holding the steel in the gamma zone transforms the as-rolled or as-cast microstructure into austenite, which dissolves carbides. Then, during cooling, a very uniform grain is formed, consisting of either pearlite and ferrite or pearlite and cementite, depending on carbon content.
In all heat-treatment operations, the temperatures, holding times, and heating and cooling rates are varied according to the chemical composition, size, and shape of the steel. In general, alloy steels, which have a lower heat conductivity than carbon steels, are heated more slowly to avoid internal stresses.
Annealing
To make steel ductile for subsequent forming operations, an annealing treatment is applied. In annealing, the steel is usually held for several hours at several degrees below Ar1 (shown by the P-S-K line in the ) and then slowly cooled. This precipitates and coagulates the carbides and results in large ferrite crystals. Cold-formed steel is usually annealed and recrystallized in this manner, holding it for several hours at about 680° C (1,260° F).
Annealing is performed in an inert or reducing atmosphere to prevent any oxidation of the steel surface. In batch annealing of cold-rolled strip, for example, several coils are set on a base and on top of one another. Then they are covered with a shell made of heat-resistant steel, which is sealed on the bottom and holds the inert gas during annealing. A gas-fired bell furnace is then lowered by a crane over this cover for heating. The total processing time, including cooling, may be 50 to 120 hours, depending on furnace load and steel grade.
In a different system, the cold-rolled strip is pulled through an 80-metre-high furnace with the strip moving up and down between many top and bottom rolls. These continuous-annealing furnaces are usually heated by gas-fired radiation tubes in order to separate combustion gases from the inert atmosphere surrounding the strip. In this dynamic annealing process, the strip is heated to higher temperatures (for example, 780° C, or 1,440° F), held for only a few seconds, and immediately cooled by fast-circulating inert gas. The entry and exit sections of continuous-annealing lines are built, as on other strip-processing lines, to allow an uninterrupted and constant travel (at, say, 500 metres per minute) of the strip through the process section—in this case, the heating and cooling zones. The entry group has two uncoiling reels, a cross-shear, welding equipment for joining two strips, and a strip accumulator. The latter is often a looping tower, which supplies the process section above with strip at constant speed while welding is done at the entry section. The exit group works in a similar fashion, with a looping tower and two reels; it also cuts samples and substandard portions out of the strip.
Continuous-annealing lines are often 200 metres long, and the strip between uncoiler and recoiler is more than one kilometre in length. Strip annealed this way is not as soft as batch-annealed steel—a disadvantage compensated for by using ultralow-carbon steels—but it does have operating advantages in that annealing of one coil may take only one hour and the mechanical and surface properties of the strip are very uniform.
Quenching and tempering
The most common heat treatment for plates, tubular products, and rails is the quench-and-temper process. Large plates are heated in roller-type or walking-beam furnaces, quenched in special chambers, and then tempered in a separate low-temperature furnace. Uniform heating and quenching is crucial; otherwise, residual stresses will distort and warp the plate. Tubes made for very demanding services, such as oil drilling, are usually heat-treated in walking-beam furnaces and special quench-and-temper systems.
The heads of rails are sometimes heat-treated in-line by induction heating coils, air quenching, and tempering by a controlled use of the heat retained in the rail after quenching. Heavy-walled structural shapes are sometimes water-quenched directly after the last pass at the rolling mill and also tempered by the heat retained in the steel. In-line heat-treating results in cost savings because it eliminates extra heat-treating processes and facilities.
The quenching media and the type of agitation during quenching are carefully selected to obtain specified physical properties with minimum internal stresses and distortions. Oil is the mildest medium, and salt brine has the strongest quenching effect; water is between the two. In special cases, steel is cooled and held for some time in a molten salt bath, which is kept at a temperature either just above or just below the temperature where martensite begins to form. These two heat treatments are called martempering and austempering, and both result in even less distortion of the metal.
