This class of work has developed within the last few years, and, beginning with the heavier parts of marine and engine construction, it is now crowding the field of drop forgings. Steel castings are malleable, and are very much stronger than iron ones. The principles of molding involved are similar to those in other classes of molding, but practice is varied to meet special conditions.
The art of making steel castings may be divided into three heads: (1) preparation and melting of the metal; (2) making and pouring the molds; (3) heat-treatment of finished castings. As the first and third heads come more properly under other departments, we shall here simply outline these processes, dealing in detail with the second heading only.
This branch of foundry work has developed to a great extent since the early nineties. The metal is similar in mixture, method of melting, and physical properties to the steel which is poured into ingot molds for forging purposes. The graphitic carbon is entirely burned out; the strength of the metal is therefore very much greater than that of cast iron. Combined carbon, manganese, and silicon are the elements depended upon for this strength; sulphur and phosphorus are kept very low. A typical analysis shows these elements in the following proportions:
Owing to the purity of this form of iron, about 50 per cent more heat is required to melt it than is necessary in the case of pig iron - or about 3,300 degrees Fahrenheit. When melted, the metal runs much more sluggishly than cast iron, and, on account of the absence of graphitic carbon, it does not expand at the moment of solidifying, and therefore does not take as sharp an impression. To insure as perfect an impression as possible, the molds are constructed with a good head of metal in the risers, and they are poured under pressure. The shrinkage is double that of iron; the risers are made very large, and are placed directly on the casting to insure feeding well. Great care must be exercised that neither mold faces nor cores bind during cooling, as such binding might cause a flaw.
When two surfaces meet at right angles, the corner remains hot longest, and the sides shrink away, tending to cause a fracture at a, Fig. 143. To overcome this, thin webs are cut by the molder about every 4 inches or 6 inches - shown at b. These cool first, and hold the adjacent sides in position, preventing them from pulling away from each other. The internal strain due to this cooling is relieved by the annealing. After the casting is annealed, the webs are cut away.