It is more than likely that the popularity of the bicycle, which created the recent great demand for strong, light and perfect tubes, was largely instrumental in developing the seamless tube industry, which may be said to have “sprung up” within the last twenty-five years. Previously all of the iron and steel pipes or tubes obtainable were either of the “butt” or “lap-weld” variety with the exception of those which were made from long pieces of metal by boring holes lengthwise through them. Tubes by this boring method are, of course, quite difficult and expensive to make.

Shorter and thicker billets of steel can more easily be bored. When the holes have been enlarged by pushing larger and larger-nosed rams through them in a hydraulic press, they can be rolled down to size over a mandrel, just as lap-welded pipe is rolled, only in this case they are put through several times and considerably reduced in diameter. In this way some of the seamless tubes are made.

The important starting point in all processes for seamless steel tubes is the piercing operation and it is mostly in the method of getting the first hole through the billet that they differ, since the hot-rolling and the cold-drawing processes by which they are finished have long been known.

One of the most important modern processes for seamless tubes, the Mannesmann, is based upon the principle that if a white-hot round steel bar is rapidly rotated between “cross rolls,” a longitudinal rupture which is almost a hole forms along its center. We may liken the motion of the bar in the rolls to the whirling of a lead pencil between the palms of our hands, except, of course, that the bar is kept rotating in one direction only. Though something like this tendency of a steel bar to open along the center through pressure applied at two opposite points on the outside, seems to have been known to forgemen, the Brothers Mannesmann came upon the similar tendency under action of the rolls, by accident.

They were German tool steel manufacturers. A critical customer wanted perfectly round and surface-polished bars of steel. They attempted to give his bars this perfect shape and smoothness by finishing them between cross rolls which spun the bars rapidly around while they were slowly passing along through the machine. The pieces were perfect outwardly, but, much to the steel makers’ chagrin, the customer reported that the quality of the steel was not as satisfactory as that which he previously had been receiving. Upon investigation it was found that this cross-rolling under pressure tended to form a small hole along the center of the bar with slight cracks in the metal all around it.

Upon this happenstance discovery is based the Mannesmann process for piercing the bar, which consists in pushing over a piercing head such a center-weakened piece as it comes through the cross rolls.

One or two modifications of the Mannesmann piercing method are also in use.

The material generally used for seamless steel tubes is medium soft open-hearth steel of .15% to .25% carbon. It is received as billets which are rolled down and cut at the mill or they are purchased as 3″ to 6″ “rounds” and cut into such lengths as give proper amounts of steel for the tubes which are to be formed. Usually the bars cut for tubes are from three to five feet in length.

They are heated in a furnace and after the end has been dented at the center, they go into the rolls.

The rolls seize the forward end of the bar and swiftly whirl it as it is slowly pulled in. A piercing head of high-speed steel at the end of a stiff mandrel extends between the rolls just as we saw it in the pipe-rolling process. As the forward end of the rapidly whirling white-hot steel bar pushes against this piercing head, the piece, weakened along its center line is pierced. As neither the rolls nor the piercing head can be resisted, it is forced through the rolls and grinds its way over the piercing head with the supporting bar, the walls of the white-hot tube being thinned down and the piece very materially lengthened.

It comes out a rough tube with thick and irregular walls. After its removal from the rolling mill bar upon which another and colder piercing head is placed in readiness for the next tube, it goes to other rolls through which it is passed, first without a mandrel inside, and later, with one, until it has become somewhere near the desired size and the walls have been pulled down to the proper thickness. The mandrel, of course, determines the size of the interior of the tube, and the rolls, its outside diameter.

Some are sold in this form as hot-finished tubes after having been straightened and cut to length by removal of the ends.

A great deal of the seamless tubing made is given the cold finish, i.e., it is drawn through dies much as rods are drawn in the making of wire.

For cold-drawing, one end of each tube is reduced in size over a length of a few inches, by forging or by other means. This is where the “pliers” are to take hold.

Now we can never heat steel without forming upon it a brittle oxide or scale which is much harder and harsher than the metal itself. During its sojourn in the heating furnace and its journey through the rolls, therefore, each of the tubes acquired a hard brittle surface which must be removed before the tube can be “drawn.” The most practical way of removing this scale is by “pickling” the tube in some weak acid, usually sulphuric (oil of vitriol). The acid dissolves some of the scale and loosens the remainder so that it can be washed off. To neutralize any excess acid which clings to the tube and to aid lubrication, it is dipped into lime-water and then dried.

The tube now goes to the drawing benches which are long steel frames along which a heavy steel draw chain is continuously traveling from the center toward one end. Anchored at the opposite end of the bench is a long bar upon which is fastened the mandrel or ball which is to determine the inside diameter of the tube in the drawing as did the mandrel between the rolls in the rolling.

The tube to be drawn is threaded over the long rod which is anchored in place, and the forged-down small end is pushed through the “die,” very firmly fastened to the bench near its center. The pliers take hold of the forged-down end of the tube with a vise-like grip, and are then hooked into the draw chain. The tube is thus slowly drawn through the hole in the die. As these dies are of very hard material, either hard cast iron or hardened steel with hole a little smaller than outside of the tube, they compress the tube upon the mandrel inside and the thickness of wall is thus regulated, the excess metal being squeezed out so that the tube is very materially lengthened. Tallow or grease with the lime-coat lubricate the tube, a little being continually drawn into and through the somewhat funnel-shaped die.

As was the case with the “cold finishing” of plate and the drawing of wire, this cold working increases the elastic limit and tensile strength of the steel. So cold-finished tubes are stronger than hot-finished. For many purposes such increase in strength is highly desirable. The exterior of the tube is also made very smooth and uniform in diameter by the drawing.

The cold-drawing has a disadvantage, however. It somewhat embrittles the steel, as may be inferred from the increase in strength. This is not a serious matter, however, unless the cold-drawing has been overdone.

But for smaller sizes of tubing many drawings have to be resorted to, to reduce the steel to the size required. Sometimes ten or even fifteen passes are required before the tubes reach their final size. In such case the tubes have to be annealed and repickled, limed and dried after each pass or two in order to restore to the steel its ductility. If this were not done the tube would eventually break in the die.

The last pass is through an accurate “sizing” die which corrects any variation in inside or outside diameter.

As the pulling strain which the steel will stand is limited, too much of a reduction in size in any single pass must not be attempted.

As annealing, pickling, drying, etc., have to be done after every pass or two, a considerable period of time elapses between the piercing of the billet and its final pass as a small tube. For economy in handling, tubes cannot be considered or handled singly, but must be treated in quantity, so this period between billet piercing and the final pass may be as much as two weeks, possibly more.

The tubes must next be straightened. This is done in cross rolls as has been mentioned under the manufacture of lap-welded pipe, or in various other types of machines.

Much seamless tubing goes into automobile, bicycle, and various other products for which very high grade and perfect material is desirable. One of the many interesting applications of seamless tubing is its use in very fine sizes for hypodermic needles.

Seamless tubes are easily bent, swaged, upset, spun or otherwise changed in form, as the material is ductile and there are no welds to open.

Very large tubes are not made in the way just described. They are rather made by “cupping” flat, round steel plates through a die. A cup is then in several successive drawings put through smaller dies, under which treatment it grows longer each time and gets a thinner wall until it has become a long tube with the one end still closed. For open ended tubes this and the upper, open end are cut or trimmed off.

Cold-drawing here necessitates annealing to restore ductility just as it does elsewhere and each annealing operation is necessarily followed by pickling for removal of the scale formed.

By rapidly spinning large tubes in lathes or other machines and the application of pressure with the proper tools and lubrication, the walls of the tubes may be deformed. In this way the ends may be expanded, made smaller, or completely closed. By such “spinning” operations large tubes are made into articles of various shapes.

By this same “cupping” or hydraulic drawing of flat, well lubricated sheets of soft steel, seamless high-pressure gas cylinders, steel drums, barrels, and the like are made.