STEEL TUBING. This is the most mod ern and one of the widest used products of steel. For many years steel tubing was not con sidered entirely satisfactory by manufacturers because the welded parts were not as strong as the rest of the tubing. This difficulty has been overcome and to-day steel tubing finds a re markable market in the manufacturing of auto mobiles, engines, furniture, baby carriages, beds, boilers, etc. Steel tubing is made from solid or hollow billets or from steel ribbon.
Technology.— There are four methods of manufacturing steel tubing, i.e., lap welding, butt welding, the piercing method and the Lloyd oxy-acetylene method. The lap method has been in vogue for many years, dating back to the period when steel first began to be used. The early manner of lap welding was known as spiral welding. Steel sheets of a desired width were wound spirally to form a tube. The spiral joints were heated and welded by a hammer, although in many cases these joints were cold riveted. An even earlier method of making steel piping was to take two semi-circular pieces of steel and insert the edges into steel bars with grooves in either side. These were held in place by pressure applied on all sides. There was no welding or joining of the semi-circular pieces and the piping thus made was useless except for underground purposes or in such places where the pressure could be applied at all times. Later came the straight lap welding, where flat steel of a desired width was passed through rolls, drawn over dies and formed With the edges overlapping. Heat was applied and the overlapping edges were hammered or squeezed together. This left the welded part thicker and stronger than the rest of the tube and was never satisfactory for good manufac turing work. In a later method— the flat steel ribbon was heated and the edges were scarfed or bevelled by passing through rolls, drawn through dies and so formed that the scarfed or bevelled edges of the one side of the flat steel fitted into the same edges of the other side. This partly-made tubing was then re heated, passed through rolls with the inner lap resting on a stationary mandrel. The rolls and the mandrel pressed the heated steel to gether and welded it. The rough tubing was thenpassed through sizing rolls, straightened, rolled on cooling tables and finally forced through straightening machines by hydraulic pressure. Butt welding then came into use, the first process requiring that the entire piece of flat steel be heated to a welding heat and drawn through a conical shaped die, so that square edges were pressed together. Then came the
brazing process of butt welding, in which a blow-torch like that of a tinsmith, only with far greater heat, was used to centralize the heat at the square edges where the brazing is to take place. The brazing method is used when very thin tubing is to be made because the heat of the blow-torch is not sufficient to make thicker steel tubing. It is largely used in brass or copper welding, where a heat of from 1,200° to 1,400° is required, while steel needs from two and one-half to several times that heat. The electric process consisted of heating the edges and applying pressure so that the steel is pushed together and welded at the same time. Flat steel is put through conical shaped rollers, bringing the edges of the steel together at the point where electric heat is applied. As the steel is heated the rollers press the edges closer together. Phis was a decided improvement over other processes of butt welding, hut the greater strength and thickness of the welded part made several other processes of rolling, etc., necessary before the tubing was ideal.
The piercing seamless method is the latest and most efficient for producing heavy steel tubing. Solid or hollow billets are heated to about 2,300° and passed through diagonal rolls traveling at a tremendous speed. These rolls work the steel into desired size. While the billet is passing over the rolls it is pushed against a mandrel which is stationary but re volving 2,400 times per minute. The billet is thus worked into a desired size with a hole through the centre and comes through a tube after many operations. For example, if a o twinch tubing 46 inches long is desired, a billet three inches round and 26 inches long is heated, passed through the diagonal rolls and at the same time has the mandrel passed through it. When this operation is completed the tubing measures three and one-half inches on the outside with a seven-sixteenths shelL It is then taken to the round rolls where it is elongated and the shell reduced. Then it is cold drawn, hot rolled, pickled, cold drawn, annealed and so on. The lighter the shell de sired the more operations necessary. The piercing method found itself very applicable to the hollow steel billet made first in Sweden, where the molten steel was poured over a core, the latter removed when the steel shell had cooled, thus leaving a round billet with a rough hole through the centre. This made it possible for the piercing method to become much speedier in production.