Steel tubes and pipe can be broadly categorized according to manufacturing method as seamless tubes and pipe made by hot rolling or hot extrusion, and welded pipe and butt-welded pipe made by bending and welding sheets or plates. When seamless pipe is made by rolling, the rolling method involves piercing the material while it is being rolled, and is suitable for mass production. The figure shows the manufacturing process used in the Mannesmann plug mill, which is a typical rolling process. The Mannesmann-type piercer reduces the material by rolls that are inclined obliquely to each other. When the round billet is rotated while being compressed in the diametric direction, the central part of the billet becomes loose, which makes it easy to pierce a hole through the center. This is called the Mannesmann effect. The pierced portion is expanded by the elongator, and the wall thickness is then thinned and elongated by the plug mill. The internal and external surfaces are smoothed by the reeler, and the final dimensional adjustments are made by the sizer. The hot-extrusion method involves working in the compressive-stress field. Therefore, it is characteristic of this method that high-alloy steel pipe of low deformability can be produced, as well as heavy-wall and large-diameter pipes. Seamless pipe has outstanding homogeneity in the circumferential direction and is thus highly resistant to internal pressure and torsion. Taking advantage of this feature, seamless pipe is widely used for drilling and pumping petroleum and natural gas. Welded pipe is divided into electric-resistance welded (denoted ERW hereinafter) pipe, spiral pipe, and UO pipe according to the forming and welding method. ERW pipe and butt-welded pipe are produced by continuously forming a hot-rolled coil into a tubular shape by forming mills. ERW pipe is produced by cold forming, and the seam is welded by electric-resistance welding. This type of steel pipe is used in large quantities as line pipe for transporting petroleum and gas. Butt-welded pipe is produced by hot forming after the whole material has been heated, and seams are then butt welded. This type of pipe is hot-dip galvanized and used for carrying water and gas. The outer diameter of ERW pipe and butt-welded pipe is determined by the width of the material coil. However, as shown in the figure, spiral pipe is made by forming the coil into a spiral shape, which makes it possible to obtain a large outer diameter regardless of the width of the material. UO pipe is usually large in diameter and produced one piece at a time by forming plates. The plate is first pressed into a U shape by the U-press, and then into an O shape by the O-press. Because relatively thick material is used for making spiral and UO pipes, submerged arc welding is used for joining. The principal application of spiral pipe is pipe piles. UO pipe, as mentioned above, is mainly used as line pipe for transporting petroleum and natural gas in large quantity over long distances.