By the term cupola mixture is meant the proportioning of the various pig irons and the scrap that make up cupola charges, with the object of obtaining definite physical and chemical properties in the resulting castings.
The requirements of castings vary; and metal that would be good if run into thin stove plate, would be entirely too soft for heavy machine castings. Again, iron that might answer all requirements of a bed plate would not be strong and tough enough for steam-cylinder work. The one in charge of this work, therefore, must so mix the different irons that his castings shall be soft enough to machine well if necessary, and at the same time be hard enough to stand the wear and tear of use.
Formerly the appearance of the fracture of a pig or of scrap was the sole guide in determining mixtures. Unquestionably the fracture of iron indicates to the experienced eye much as to its physical properties, but this method of mixing has repeatedly proved misleading.
Representative practice today recognizes chemical analysis of the various irons as most essential to the proper mixing. Many firms now buy their pig iron and many other allied supplies by specification; and the chemical analysis of the iron must show that its various metalloids come within certain limited per cents.
To understand, then, these modern methods, we must consider the subject of the chemistry of iron.
By the chemical definition, an element is a form of matter which cannot be decomposed, or, in other words, cannot be broken up into other forms by any means known to science. Iron is such an element; but absolutely pure iron is of no commercial value; it is only when it is combined with impurities - or, as we must recognize them, other chemical elements - that mankind is interested in it.
In the forms of iron with which we are dealing - pig iron, and cast iron - five elements are considered as affecting their physical properties. These elements are carbon, silicon, sulphur, phosphorus, and manganese.
Carbon is the most important and most abundant of all the chemical elements. It forms the principal part of many substances in daily use about us, such as coal, coke, lead pencils, graphite facings, etc.
In its relation to iron, carbon is peculiar in that it occurs in iron in two forms. One is in a chemical combination forming a hard substance with a fine grain, of which tool steel is the purest type. The other is simply a mechanical mixture forming minute facets of free carbon interposed between the crystals of the combined form. It softens cast iron, but weakens it by causing larger crystals to form. In drawing the finger across a freshly cut surface or fracture of cast iron, some of this free carbon may be rubbed off, and shows as dirt on the finger. We shall use the term graphite in referring to this form of free carbon, and the term combined carbon in referring to the element in its combined state.
Silicon, of itself, is a hardening element in cast iron, but on account of its marked influence upon carbon formations, it is usually considered a softener. During the cooling process, silicon retards the formation of combined carbon, thus increasing the formation of graphite in proportion to the increase of silicon. At the same time, through its own influence on iron, it preserves the fine character of the grain, and so maintains the strength of the casting. In other words, within certain limits, the addition of silicon softens castings without impairing their strength. It makes iron run more fluid, and reduces shrinkage. Silicon varies in castings from 1.50 to 2.50 per cent.
Sulphur is the most injurious element in iron. It makes castings hard, red-short, and tends to the formation of blowholes. At the melting temperature, iron absorbs sulphur from the fuel - a decided reason why foundry coke should be as free as possible from this element. Sulphur in castings should not exceed 0.07 per cent.
Phosphorus tends to make iron run very fluid when melted. It is a hardener. For machine castings it should not exceed 1 per cent.
Manganese strengthens, and, of itself, hardens iron. Chemists are beginning to consider its proportions more carefully, in the belief that under certain conditions it acts as does silicon, softening the castings while retaining their strength. It is usual to keep it below 0.50 per cent.
The strength of a casting and the finish which it is capable of taking are largely dependent upon its having a fine even grain. We have seen that the porportions between the combined carbon, the graphite, and the silicon have decided influence upon this condition. But the rate of cooling must also be taken into account. A thin casting cools rapidly, tends to increase the combined carbon, and, without the influence of silicon, would be hard and brittle. In a heavy casting, the metal stays liquid longer, more graphite is thrown off, and the casting is naturally softer. Therefore light work requires a larger proportion of silicon to counteract the effect of the rapid cooling than does larger work.
Modern practice makes daily analysis for the two carbons, for the silicon, and the sulphur, occasionally testing for the other elements to see that they are kept within their safe limits. Silicon, however, is used as the guide for regulating mixtures.
The following shows good proportions of silicon for different classes of work:
Silicon (per cent)
Medium heavy work (1/2-inch to 2-inch thickness)
Light work (less than 1/2-inch thickness)
A more complete analysis of results to be aimed for is:
Elements (per cent)
Corliss engine cylinders (1 1/4- to 1 1/2-inch thickness)
1.20 to 1.70