CHAPTER IX
The Manufacture of Iron and Steel
178. Refining Pig Iron.—Two distinct methods have been adopted for the conversion of pig iron into wrought iron, each depending upon the kind of furnace used. They are called the open-hearth or finery process, and the puddling process. The chemical reactions are similar in both processes, being based on the oxidation of the impurities in the metal. This is accomplished both by means of the oxygen in the air supplied and by the oxide of iron in the fluxes that are added to assist the operation.
179. The Open-hearth or Finery Process.—This is carried on in what is sometimes termed a “bloomery” from the product which is called a bloom. The pig iron is placed in direct contact with the fuel on the hearth which is formed of cast-iron plates exposed to a current of air to keep them cool. This mixture of the iron with the fuel is objectionable, because while the fuel acts as a reducer the excess air decarbonizes the product only partly, besides prolonging the process considerably; by the addition of hammer scale and rich slag, the operation is hastened greatly. However, if some carbon is supposed to be contained in the product, making it of a steely nature, then the open-hearth process is considered a good method of refining.
Fusion is allowed to take place gradually, so as to expose the metal for a long period to the oxygen of the blast. At the moment of fusion the foreign elements are rapidly oxidized and form a fusible slag. After the slag becomes neutral and has been partly removed, fresh basic slag and hammer scale are added, to hasten the operation. Then the mass of iron, which is now of a white spongy texture, is lifted up in the furnace to a level with the tuyère, in order that the combined carbon may be completely oxidized. It is then formed into balls of about 60 to 80 pounds each, after which it is removed and formed into a bloom by means of a squeezer or hammer. This furnace is not illustrated, because most of the wrought iron is produced by the puddling process. An open-hearth furnace, such as is used in producing steel, is somewhat similar to the one here described and is shown in [Fig. 163].
180. The Puddling Process.—The greatest amount of wrought iron is produced from pig iron by this process, owing to the superior quality of the product. The term “puddling” was originally applied to the process of working iron that had never been completely melted, but had only reached a puddled or pasty state. But later, when refined or pig iron was similarly treated, it was discovered that it would melt perfectly and boil up freely. The process was then termed “pig boiling.”
The furnace used in this process is of the reverberatory type; the fuel does not come in contact with the iron. (See [Fig. 150].) It is built in a rectangular form; the fireplace A is located at one end, next to it is the hearth C where the metal is placed, and beyond are the flue B and the chimney D.
From the fireplace the heat is supplied and directed upon the metal by the top or roof, which is curved downward from the fireplace toward the flue and chimney. The fireplace also is separated from the hearth by a partial partition wall E, called the fire bridge, which prevents the fuel from coming in contact with the metal. Another similar partition F, located between the hearth and the flue, prevents the metal from going into the latter and is called the flue bridge.
Fig. 150.—Puddling Furnace of a Reverberatory Type.