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 figure. 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 figure. 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 figure). 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 figure. 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 figure). These mills sometimes have 20 sets of rolls arranged in tandem.