Metal casting and foundry materials
Foundries work with a large number of materials. These are classified under three main headings: casting metals, molding sands, and foundry refractory.
There are two large groups of casting metals, the ferrous ones and the non-ferrous varieties. A metal of the ferrous group always carries other elements, and is, therefore, more of an alloy than a pure metal. In the non-ferrous group, the metals are usually sold as pure, as it is practical to make them in a pure state. The foundries mix the relatively pure metals to produce the desired alloys. The ferrous or iron alloys include all the cast irons, carbon steels, and alloy steels. The non-ferrous metals include copper-base alloys, aluminium-base alloys, magnesium-base alloys, zinc-base alloys, and other less used alloys.
Casting metals
Cast Iron. — The term cast iron is applied to ferrous alloys, which, though chiefly composed of iron, carbon, and silicon, also contain manganese, phosphorus, and sulphur as normal elements. The modification of some properties existing in cast iron may be obtained by the addition of other elements, such as chromium, molybdenum, nickel, vanadium, copper, titanium, etc.
Among the ferrous metals, cast iron occupies first position and is recognized as one of the most indispensable materials used in the manufacture of products necessary in everyday life. Cast iron is not considered a very strong or tough structural material, but it is the most economical: Its low melting point, low shrinkage, good fluidity, and machinability are properties that recommend its use.
Among the disadvantages of cast iron, the most important are its lack of ductility and its low strength, although some progress has been made in recent years in controlling the melting technique and composition of the charge in the plain or unalloyed irons. Considerable progress has been made in improving the properties of alloyed cast irons and malleable cast irons, which fact is greatly responsible for their wide application in industry at the present time. The diversity of properties existing in various grades and classes of cast iron makes it possible to choose irons possessing tensile strengths of from 20,000 to 80,000 pounds per square inch.
The general classification called cast irons include pig iron, gray cast iron, white cast iron, and mottled, chilled, malleable, and alloy cast irons. None of these various forms of cast iron is as forgeable into desired shapes as steel or wrought iron, although malleable cast iron is much more workable (ductile) than either white or gray cast iron. For the most part, therefore, they must be melted and cast into molds which form the products. Their lack of plasticity is due chiefly to the presence of a large percentage of carbon in their make-up. Most commercial cast irons contain betwen 2.50 per cent and 3.75 per cent of total carbon, which is well in excess of 2 per cent—a value used to differentiate cast irons from steels. However, silicon and other elements present in iron-carbon alloys may reduce the limiting figure of 2 per cent. Iron-carbon alloys containing about 2 per cent silicon are classed as cast irons if their carbon content is 1.5 per cent or over.
Pig Iron. — The chief raw material for cast iron is pig iron, which is produced in a blast furnace by the process of smelting iron ore with coke and a flux (substance promoting fusion) such as limestone. The final analysis of the pig iron is substantially determined by the kind of iron ore used in the smelting process.
Modern blast furnaces are built to produce 500 to 1,200 tons of pig iron per day, and must be kept in continuous operation because of the nature of the smelting process. When some portion fails, the furnace is shut down and reconstructed.
Pig iron got its name from the shape of the molds in which metal from the blast furnace was cast. Originally, the pigs were cast in sand molds. A runner was built with a slight incline from the tapping hole of the blast furnace. From this main runner, side runners were built. From these side runners, the pig molds were constructed.
Modern large-volume production of pig iron is carried out by casting blast-furnace metal by means of a large machine, which is in principle an endless conveyer chain of pig molds. These molds are moved by an electric motor, and the length of the chain conveyer is determined by the time required for the pig iron to solidify before being dumped from the machine. The pigs, when dumped, fall into a car on a railroad siding, thus reducing the time required to handle the pigs before they are shipped out. The cooling of the hot pigs is done by a spray of water. The pig molds are sprayed with lime water to keep the hot metal from sticking to the mold. The machine-cast pig iron has one disadvantage: it is not аь easy to melt as the sand-cast pig iron because of its greater bulk.
Chemical analysis furnishes the most important classification of pig iron. Different chemical analyses are required for the several types of pig iron in use, depending upon their application. Some pig irons are used in gray-iron foundries, and are called foundry pig irons. Pig iron used for making steel by the acid Bessemer process or the acid open-hearth process is known as Bessemer pig iron. Malleable Bessemer pig iron is used for malleable cast iron. Basic pig iron is used for the basic open-hearth process.
Iron and Steel Scrap. — Scrap iron is graded in several ways and by different standards. The several gradings, when all are considered, do not result in a very definite classification.
The order of classification depends upon the type of steel scrap and the remelt furnace. Open-hearth steel scrap is divided into two grades, heavy and light. Heavy steel scrap is divided into thirteen classifications, each bearing a number; light steel scrap is divided into six classifications. The number of these classifications are arranged in the order of the quality, size, and cleanness of the scrap. Almost all iron and steel castings are made from some new material for remelt, there would still be scrap from gates, risers, and rejected castings produced in the foundry. Naturally, this scrap is not allowed to accumulate and must be used up. Of course, this kind of foundry scrap is only a small portion of the total scrap, used, but its chemical composition is generally known.
In the case of iron castings, most foundries seldom use more than 50 per cent pig iron; the remainder is scrap iron in various forms, even for high-grade castings. A number of foundries operate on practically 100 per cent scrap. For example, car-wheel foundries remelt old car wheels and add to this a small amount of rich alloys of ferro-silicon and ferro-manganese. This combination results in a high-grade product.
Similarly, in the production of steel castings, the scrap in the metal charge delivered to the remelt furnace far exceeds the new metal. In numerous cases the charge consists almost wholly of steel scrap. High-quality steel castings are produced by remelting practically all-steel scrap in an electric furnace. Everything that is remelted in a furnace is greatly responsible for the quality of the casting produced, and since the selection of new materials is so easy compared to the selection of scrap, the quality of the latter is most important in producing a high-quality casting.
Non-Ferrous Metals.— The non-ferrous metals used in the foundry are usually alloys of two or more metals. Non-ferrous castings include those composed of copper-base alloys (brass and bronze), aluminium-base alloys, zinc-base alloys, tin-base alloys, lead-base alloys, bearing metals, and some special alloys composed of magnesium or nickel and other metals. All these metals except nickel have a lower melting temperature than iron alloys. These metals are usually purchased as pure metals, and then alloyed as desired, or they may be purchased as alloys.
Aluminium for foundry remelt is obtainable in several grades. The usual grade of aluminium for foundry use runs about 98 to 99 per cent pure. When high-grade aluminium castings are required, the highest grade of stock metal is specified, which is 99.4 per cent pure.
Zinc is purchased in its pure state and should contain no aluminium. Zinc is classified in five groups, and all except one contain lead, iron, and cadmium. The iron content is detrimental, and only the minimum amount should be present. Lead and cadmium are not as detrimental as iron. In a good grade of zinc, the sum of lead, iron, and cadmium should not exceed 1 per cent.
Lead is usually found with silver. Upon removal of the silver, the lead is procured in its pure state for foundry use. The purity of common lead is 99.726 per cent. It contains less than 0.25 per cent bismuth. The better grades of lead are at least 99.9 per cent pure.
When copper is alloyed with aluminium for foundry use, a combination consisting of 55 per cent of each metal is prepared; variation is not to exceed 1 per cent either way. For brass castings, copper is alloyed with zinc, and for bronze castings, copper is alloyed with tin. There are a number of grades of brass and bronze castings, depending upon the zinc and tin content in each case.
Non-Ferrous Scrap. — Contrary to remelt practice in iron foundries, scrap in non-ferrous casting practice is either avoided as much as possible, or is used in minimum amounts.
Aluminium castings are used extensively for automobile and aircraft parts, and are made from very good and mostly pure component materials constituting the desired alloy. Aluminium scrap may be used if it is carefully cleaned and care is taken to see that no harmful impurities remain in the cleaned scrap. Numerous castings are now made from aluminium alloys containing silicon—a circumstance making it more difficult to judge the quality of scrap.