Forging in general treats of the hammering, working, or forming of heated metals. The materials upon which forging or blacksmithing is done, are wrought iron and steel. As explained in "Metallurgy", wrought iron is an iron from which the silicon, phosphorus, and most of the carbon has been removed. Steel usually contains some of the impurities that are characteristic of cast iron with the marked peculiarity of holding a varying percentage of carbon. Mild steels are so called on account of the small amount of carbon which they contain. As the percentage of carbon increases, it becomes more difficult to weld the metal. Greater care must also be used in heating lest the metal be burned and its strength destroyed. Until recently all heavy forgings involving welding were made of wrought iron, but now it is customary to make most forgings of mild steel, particularly large ones, although wrought iron is somewhat more satisfactory where a great amount of welding is required.
These metals may be roughly divided into three general classes, although the division line may not be sharply drawn between any two classes, as follows: (1) wrought iron; (2) machine steel; and (3) tool steel. The characteristics and method of manufacture of the metals are described in "Metallurgy." A rough distinction such as a blacksmith would use is about as follows: Wrought iron has a fibrous structure with stringy streaks of slag running lengthwise of the bar, giving it a decided fiber similar to wood. Machine steel, more properly described as mild steel, or sometimes called soft steel, has much the same properties as wrought iron excepting that it lacks the fiber and is somewhat stronger. Tool steel differs from the other two materials in the fact that by suddenly cooling from a high heat it may be made very hard, or hardens, to use the technical term. Wrought iron or machine steel are not hardened by the same treatment. Tool steel is practically the same thing as wrought iron or machine steel with a small percentage of carbon added. In fact, either of the two metals may be turned into tool steel by the addition of carbon. This principle is used in casehardening. Norway iron or Swedish iron is a grade of very pure wrought iron containing little slag. It is more expensive than ordinary wrought iron. Refined iron is a grade of wrought iron not as good as Norway iron but better than ordinary iron. Norway iron costs about twice as much as machine steel, which is somewhat cheaper than wrought iron of almost any grade. Machine steel, made by both the open-hearth and Bessemer processes, is used for forging.
Material from which forgings are ordinarily made comes to the forge shop in the shape of bars having uniform sections throughout; generally round, square, or rectangular in section, and varying from 1/8 inch thick to 18 inches square. Heavier sizes may be had to order. Bars are ordinarily 12 to 20 feet in length. Thin stuff, 1/8 inch or less in thickness, usually comes in strips of about 40 feet. This may be had from stock up to 6 or 8 inches wide. Tool steel also comes in bars generally about 6 or 8 feet long. The ordinary sizes of tool-steel stock are known as base sizes and the price is fixed on these base sizes. Stock of a larger or smaller size than the base sizes is generally charged for at an increase in price. Thus inch-square tool steel, which is a base size, is worth in certain grades about 14 cents a pound. Steel of exactly the same grade and character, 3/16 of an inch square, costs about 18 cents.
The outfit of a forge shop consists in general of the heating apparatus - the forge, furnaces, etc.; and the handling equipment - the anvil, the various tools, and the machines for shaping and working.