The only objection to this method is that the metal is chilled rather suddenly by the water through which the molds are led. This sudden chilling causes a structure different from that found in the same quality of metal molded in the sand and allowed to cool off gradually, and most foundrymen as well as other users of iron judge the quality by the appearance of a fracture. On this account machine-molded pigs are objectionable. It is claimed, however, that some machines in use at present have overcome this difficulty.
The approximate dimensions of a modern coke-burning furnace are as follows (see [Fig. 146]): The hearth K is about 13 feet in diameter and about 9 feet high. The diameter of the portion above the hearth increases for about 15 feet to approximately 21 feet in diameter at the boshes E. From the top of the boshes the diameter gradually decreases until it is about 14 feet in diameter at the stock line. The throat, or top, where the fuel and ore are charged in through the bell and hopper, is about 70 feet above the hearth. On the brackets which are connected to the pillars, the blast main rests, completely surrounding the furnace, and at numerous places terminal pipes convey the blast to the tuyères. After the furnace has been charged, or “blown in,” as it is commonly called, it is kept going continually night and day, or until it becomes necessary to shut down for repairs.
A general view of a smelting plant is shown in [Fig. 149]. The four circular structures to the left with a tall stack between them are the stoves for heating the blast. Next to these in the center of the picture is the blast furnace, somewhat obstructed by the conveyor which carries the ore and fuel to the top for charging. The structural work to the right is the unloader, which takes the ore from the vessels and conveys it to the stock pile in the foreground, where the ore is allowed to drop.
Fig. 148.—Pig Molding Machine.
170. Classification of Pig Iron.—The pig iron produced by the blast furnace is graded as to quality, and is known by the following names: Bessemer, basic, mill, malleable, charcoal, and foundry iron. This classification indicates the purpose for which each kind is best suited.
171. Bessemer iron is that used for making Bessemer steel. In this grade the amounts of sulphur and phosphorus should be as low as possible. Bessemer iron is generally understood to contain less than 0.1 per cent of phosphorus and less than .05 per cent of sulphur.
172. Basic iron is that which is generally used in the basic process of steel manufacture. It should contain as little silicon as possible, because the silicon will attack the basic lining of the furnace; therefore the surface of the pig iron used for this purpose should, if possible, be free from sand. By the basic process of making steel, most of the phosphorus in the pig iron is removed, consequently basic iron may contain considerably more phosphorus than if it were to be used in the Bessemer process.
173. Mill iron is that which is used mostly in the puddling mill for the manufacture of wrought iron. It should contain a low percentage of silicon. Therefore pig iron that has been made when the furnace was working badly for foundry iron is sometimes used for this purpose.
174. Malleable iron is that used for making malleable castings. It usually contains more phosphorus than Bessemer iron and less than foundry iron. The percentage of silicon and graphitic carbon is also very low in this class.