It will be seen from the preceding paragraph that all objects made of iron and steel may be divided into two general classes in regard to the methods of shaping them. These are:

(1) Objects poured from molten cast iron or molten steel, and known as castings.

(2) Objects shaped by mechanical pressure from hot-steel ingots and billets, or from wrought-iron "piles" bundled and heated for welding together.

Objects of the first-named class are cast usually in sand moulds made in more or less complicated shapes to produce a piece of metal for a particular purpose, as a steam-engine cylinder in cast iron or stern and stem posts of a ship in cast steel. Objects of this class are not hammered, rolled or otherwise changed in shape for final use except the finishing of them by more or less superficial cleaning, chipping, filing and machining.

As cast iron and steel castings are made in the iron and steel foundries respectively, which are subjects of another chapter, it is not intended to give them more than general mention here. It may be stated, however, that small steel castings are frequently made in steel works of steel directly from the converter, open-hearth or crucible furnaces; and that very large steel castings are necessarily made at steel works, where molten steel is produced in large quantities, rather than in the steel foundry, which is usually equipped for producing steel only in quantities of a few tons at a time. Large steel castings include such objects as rotor drums for steam turbines, rudder frames, stems and stem-posts for ships, hydraulic cylinders, gun carriages, etc., which require a strength and elasticity not possessed by cast iron. These castings are made from low-carbon Bessemer or open-hearth steel.

The object of this chapter is mainly to outline the mechanical operations of the rolling mill and of the large forging press, both of which handle ingots of steel in their primary forms. The rolling mills converts ingots and billets into such well-known forms of metal as rods, bars, plates, railroad rails, and structural shapes used in bridge, ship and architectural construction. In these forms metals are supplied as stock to be re-manufactured into articles for mechanical, agricultural, domestic and many other uses.

Copper, brass, the bronzes and other metals are shaped into bars and sheets by rolling, and these metals, including the ductile grades of brass and bronze, are also shaped from their primary forms by the process of extruding, by which a solid mass of metal is forced hot or cold through a die of definite shape placed at the end of a steel cylinder which contains the metal.