Practical forging and art smithing

by Thomas F. Googerty

Wrought iron is made by melting the pig iron in a puddling furnace; about one-half ton is charged at a time. After it is softened, it is stirred with large iron hooks by the puddler and his helper. It is kept kneaded to expose every part to the action of the flame, so as to burn out all of the carbon. All the other impurities separate from the iron and form what is known as the puddle clinker.

Pig iron melts at about 2100° F., steel at 2500° F., and wrought iron at 2800° F., so the temperature of the puddling furnace is kept high enough to melt pig iron but not hot enough to keep wrought iron in a liquid state. Consequently, as soon as the iron becomes pure it forms a spongy mass. This mass of sponge is divided into lumps of about 100 or 150 pounds which are taken to a squeezer and formed into blocks. In the operation of squeezing the greater proportion of impurities left in the iron after the puddling, are removed. While these blocks are still hot they are rolled into flat musk bars. The bars are now cut and heated to white heat in a furnace, taken to the rolls, welded and rolled into merchant bars. In the welding and rolling the cinder coated globules of iron are forced close together as the iron is welded. This gives the iron a fibrous structure increasing its strength.

Bessemer Process.

In making steel by the Bessemer process, the pig iron is put into a large pear shaped vessel called the converter. The bottom is double; the inner casing is perforated with holes called tuyeres, to admit air forced under pressure. From ten to fifteen tons of molten iron at one time are poured into the converter while it is lying on its side. The compressed air is now turned into the double bottom as the converter rises to a vertical position. The air has sufficient pressure to prevent the metal from entering the tuyeres, and it passes up and thru the metal, burning out the carbon. After the blast which lasts about ten minutes, the metal is practically liquid wrought iron. The converter is now laid on its side and the blast is shut off. A certain amount of molten spiegeleisen (white cast iron containing much carbon or ferromanganese) is added so as to give the steel the proper amount of carbon and manganese to make it suitable for its purpose. The steel is then poured into ingots and rolled into rails, girders, etc. Carbon is pure charcoal; manganese is a chemical element very difficult to fuse, but easily oxidized.

Open Hearth Process.

The open hearth process of steel manufacturing is similar to the puddling process. The carbon is removed by the action of an oxidizing flame of burning gas. The furnace has a capacity of forty or fifty tons and is heated with gas or oil. The gas and air needed for its combustion are heated to a temperature of over 1000° F. before entering the combustion chamber, by passing thru so-called regenerative chambers. Owing to the preheating of the gas and air a very high temperature can be maintained in the furnace so as to keep the iron liquid after it has parted with the carbon. The stirring up of the metal is not done with hooks as in puddling furnace but by adding certain proportions of iron scales or other oxides the chemical reaction of which keeps the metal in a state of agitation. With the open hearth process the metal can be tested from time to time. When it contains the proper amount of carbon it is drawn off thru the tapping hole at the bottom of the hearth, leaving the slag at the top. As steel is produced in a liquid form, from which impurities have been removed in the form of slag that rises and floats at the top, the metal is homogeneous and practically without grain. Wrought iron will outlast steel when exposed to the weather.

Crucible steel, or tool steel, also called cast steel, is made by using high grade, Swedish, wrought iron and adding carbon which is low in phosphorus content. The oldest method is called the “Cementation Process.” The iron bars were packed in air-tight retorts with powdered charcoal between them. They were put in a cementation furnace, heated red and kept at this temperature for several days. The bars, in this way, absorbed the carbon from the charcoal. The carbonized bars (called “blister steel”) were then cut into small pieces, remelted in a crucible, poured in ingots and rolled into bars.

The newer method is to melt small pieces of Norway or Swedish iron base with charcoal in a graphite or clay crucible. It is then poured into moulds and made into ingots, after which it is forged or rolled into bars.

The crucible process enables the manufacture of steel to almost exact analysis and insures a clean and pure material. It also absorbs the carbon much faster than steel made the old way.