Rolling Down the Pierced Tube
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.
