The great industry which annually melts some 61000,000 tons of iron into a bewildering variety of objects, large and small, hard and soft, some for strength and some for beauty, is all too diversified for summarizing.
Foundry practice includes the blowing of pig iron to steel in small converters; the casting of the steel; and the annealing of the steel. Much ordinary carbon steel is made and, also, considerable special material, notably manganese steel.
Cast iron of course is the main material for the foundry. Brass founding is much more restricted in tonnage and scope, and the metal also is melted usually in crucibles. The latter product cannot compare in structural diversity with that made in the former and more important branch of founding.
Most foundries melt their iron in shaft furnaces, called cupolas, with the aid of coke and a little flux. Cupolas range in internal diameter from 20 inches to 10 feet. The proper mixture is charged into the cupola as it melts down, while ladleful after ladleful is tapped out of the crucible as the proper amount collects inside. If the cupola is large enough to run continuously, the metal will be poured from a large receiving ladle into smaller ones for transfer to the molds.
The best cast iron is made by melting the pig in small reverberatories or in open-hearth steel furnaces. This keeps the phosphorus and sulphur of the fuel from getting into the iron, and admits of making large melts of uniform and precise specifications.
For some purposes the molds to receive the metal are made of iron or steel; usually they are of a porous, tenacious, and highly siliceous sand. The molds may be made up of a damp mixture inside proper supports; and they then will be termed green-sand molds. If the mold is baked hard after the shaping, it will be known as a dry-sand mold.
While White cast iron has all the carbon in the combined condition as produced by proper composition and quick cooling. This white, or chilled, iron is used for carwheel runs out* side a spider of gray iron; it is used for crusher jaws for roll shells, for the rolls of rolling mills, or wherever a hard resistant surface is demanded.
Gray cast iron should contain little or no free carbide but should have the carbon mainly as graphite; it is used for number-leas objects, from kitchen utensils to engine beds weighing many tons. Often gray-iron castings are made soft enough to machine with steel tools and so to be of precise dimensions.
Fig. 21. Medium-Grain Cut Iron, Showing Graphite. Pearlite, and Ccmentite.
For making malleable iron the castings are first made chilled; they are then taken from the molds and assembled in rooms, to be heated for some days to a bright red heat while the carbon separates in the amorphous state and the metal assumes the properties of quite pure ferrite.
Fig. 21 shows what a rather hard cast iron looks like when polished, etched, and magnified 200 diameters. The black spots are graphite; they make cast iron soft and lubricate the cutting steel. The white spots are free carbide; this substance makes iron hard and brittle and dulls the cutting tool; soft iron should have no such white spots, for all this carbide (Fe3C) should have decomposed to ferrite and graphite while the iron was hot. The intermediate gray areas are pearlite; it is this substance which makes iron strong. An iron composed entirely of this structure would be a steel, and could be treated and used just as all steels are; one sees that irons and steels really are related closely. The high carbon content of irons, usually present as graphite, is the main difference between irons and steels.
In the foundry there is much about the cupola requiring extensive scientific and technical knowledge. Conditions of mixing and melting cannot be studied too thoroughly.
The preparation of the molds is another department demanding expert knowledge. The production of the shapes, the composition and character of the sands, the manipulation of the patterns, and the finishing of the objects all call for scientific as well as for practical attainment. In the more numerous small plants one often sees founding unfortunately botched by ignorant management and labor.