This first operation in making seamless steel tubes begins with small ingots supplied by the rolling mill, free from surface flaws. These ingots are heated and rolled into cylindrical form, and are sawed hot into lengths required. Each of these lengths, called a blank, may range in weight from 40 to 1000 lbs., according to the size of tube to be made.

When cold, a hole about 5/8-inch diameter and 3/4-inch deep is drilled in the center of one end of each blank, and the blanks are then reheated to about 2200° F. for piercing.

The reheating furnace may be any of the various types used for heating billets. The bed of the furnace is usually inclined so that blanks may gradually roll in a continuous line one against another through the furnace.

From the furnace, each hot blank is taken directly to the piercing mill, the principle of which is shown in Fig. 67. This is the Stiefel mill, and is used more than other mills in this work. The principle of operation is the same in all piercing mills, and they vary only in their mechanism.

The discs A and B, beveled alike on the faces, revolve in the same direction, and their axes X and Y are parallel. The blank K is supported in a trough so placed that its axis, DD, makes equal angles with the face C and with the bevel F of the two discs; also the axis DD is in the same plane which contains the axes X and Y of the discs.

Fig. 67. - Making Seamless Tubes. Piercing Rolls.

The blank is pushed by a bar in the hands of workmen until gripped by the discs, which take hold of it, giving it a rotary motion and forcing it against the piercing mandrel P, a conical piece of cast steel held in place by the piercing bar R. The piercing mandrel is made to start centrally by the small hole drilled in one end of the blank, and the piercing bar is held to its place by a thrust bearing which allows it to revolve freely. The motion of the discs gives the blank a motion of translation in the direction of its axis, besides the rotary motion. This forces the blank entirely over the piercing mandrel in a few seconds, and the operation of piercing lengthens the blank into a tube from two to four times the length of the blank. 12

This tube is in a rough state after piercing, not uniform in diameter and with a surface somewhat wavy. The original process of piercing tubes, invented by the Messrs. Mannesmann gave the pierced blank a spiral twist. This was found to be a disadvantage, giving an unnecessary strain to the metal of the tube walls, and the Steifel process is among those devised to reduce this fault.

To smooth the tube up and give it uniform diameter, it is subjected to five finishing operations, with or without reheating, according to the size of the tube. These operations are:

Fig. 68. - Making Seamless Tubes. Tube Rolls.

(1) Rolling the pierced blank to give uniform diameters and thickness of wall.

(2) Cross rolling, to remove scratches and smooth the tube surface.

(3) Sizing, to reduce the tube to a specified diameter.

(4) Straightening.

(5) Cutting to length.