Bronze is an alloy of copper and tin, and is used for hardware, coinage, bells, musical instruments, and the like. Brass is an alloy of copper and zinc, and has much the same use as bronze. Stainless steel is an alloy of iron, carbon, chromium, nickel, and copper. Babbitt is mainly tin with some copper and antimony;. its chief use is in lining the bearings of engines and machines which are subject to much friction. There is no limit to the possible number of alloys, for alloys differ both in the pure metals they contain and in the proportion in which they are mixed.
Metallic elements and their alloys can be treated in a great variety of ways. They can be molded, that is, melted and poured into forms. They can be stamped or pressed into shape.
They can be worked on lathes and milling machines which cut away metal to shape it. They can be heated and beaten as the blacksmith forms a horseshoe.
Metal is supplied to industry in a variety of forms, such as sheet, plate, bar, wire, and band. The form most often used by industry is sheet metal, such as sheet topper, brass, steel, and aluminum. To keep sheet steel from rusting, oxygen in the air must be kept from it. This is best accomplished by galvanizing, or dipping the steel sheets into melted zinc, which places a rustles layer of the zinc on the steel. A cheaper but less satisfactory method is coating the steel sheet with black pigment. Sheet metals vary in thickness, size, hardness, and springiness.
Plate metal comes as tin plate and steel plate. Tin plate is steel covered with a protective coat of tin, which not only prevents the steel from rusting but prevents most food acids (such as vinegar) from attacking it. Therefore it is excellent for manufacturing cans for food preservation.
Bands are narrow, heavy strips of steel plate, sometimes called band iron or strap iron. Angle iron is also made from the same material.
Bars may be round, half-round, square, flat, or many other shapes in cross section. Cold-rolled steel bars have harder texture and greater stiffness than bars of hot-rolled steel. Copper and brass bars may be solid, or hollow in the form of tubing.
Wire is generally made by pulling, or drawing, various metals in the form of round bars through small holes in thick, hard pieces of metal called draw plates. First the bar is drawn through a hole a little smaller than the diameter of the bar. The process is repeated, the size of the holes used being gradually reduced until the desired diameter is obtained.
Various metal forms are generally used according to certain ranges of width, length, and thickness.
Iron and steel
Cast iron belongs to the group of metals called the ferrous metals — metals that are made largely of iron. The cut surf ace of a piece of gray cast iron will blacken a finger much as a "lead" pencil does. In both cases the blackening of the finger would be produced by the same substance, graphite. It is the presence of graphite that distinguishes cast iron from the other ferrous metals and that gives cast iron its characteristic properties.
When a casting is made, the molten iron is poured into a mold which has been made of specially prepared sand. As the metal cools off and becomes solid, graphite separates out in fine particles.
Think of a fine-grained sponge with pure iron substituted for rubber in making it. Throughout this sponge a great number of thin sheets of very hard material crisscross through each other. All the space not occupied by the pure iron and the thin sheets of hard material is filled with finely powdered graphite. Roughly, this imaginary sponge will illustrate the structure of a gray-iron casting. The thin sheets of hard material are called carbide of iron because they are formed of iron combined with carbon.
When molten iron is first poured into a mold, the carbon is combined with the iron. As the metal cools and becomes solid, the carbon separates from the iron. This carbon is the graphite we find in the casting. The graphite needs space, so as it deposits out of the solution, the casting swells. The swelling of the casting causes all the small spaces to be filled and produces what foundry men call " sharp " castings.
There are, then, three principal parts to a gray-iron casting:
The ferrite binds the whole structure together. The cementite furnishes the rigidity so valuable in machine structures where the slightest deflection is reflected in inaccuracy. The graphite produces the swelling that makes for sharp castings and makes castings easier to machine.
In lathes, shapers, and milling machines, the castings which hold the moving parts are made of cast iron, and it is the presence of the thin sheets of carbide of iron that makes these castings so rigid. Automobile engines are made mostly of it. Tools such as lathes, planers, and milling machines are more than ninety per cent cast iron by weight. The making of gray-iron castings is one of the great branches of industry.
There is, of course, very much more to know about it. Substances such as silicon, manganese, vanadium, and sulfur impart characteristics which vary with the degree to which these substances are present. The size of a casting, its shape, the rate at which it is cooled, and many other factors are studied by specialists who devote their entire time to the making of gray-iron castings.
Malleable iron meets the needs for a material which has properties about half way between those of steel and of cast iron. Its chief virtue is its ability to resist repeated shock. Its manufacture consists of two distinct steps. First, the casting is made of white iron.
White iron is the same substance which appears in gray iron as carbide of iron or cementite. In white iron, care is taken to have just the right amount of carbon present; and by control of the melt, this carbon remains combined with the iron so that the entire casting contains only cementite when it cools.
After cleaning comes the second step. The castings are packed in a suitable material such as iron scale, sand, and lime in various proportions and then heated to about 1400° F. This temperature is maintained for about 60 hours. The castings are then allowed to cool very slowly, during a period of from 2 to 5 days.
During the heat treatment, the carbon in the castings works out to the surface and is oxidized. Some remains in the form of temper carbon. The slow cooling accomplishes two things: it oxidizes the carbon and it thoroughly anneals, or toughens, the casting.
Malleable castings are widely used. In small castings such asthose used in locks, stales, and gun parts, the process is especially effective because decarbonization is more complete than in parts which have thick sections. Thin malleable castings can be bent into a U-shape while cold without fracture.
Malleable castings should not be welded. They can be welded, but the melting which takes place during the welding undoes the effects of the annealing and the resulting joint will be of uncertain quality. However, malleable castings can be successfully brazed, or soldered with hard solder or brass.
Annealed gray iron
In recent years a material closely resembling malleable iron, and sometimes confused with it, has been coming into use. It is called annealed gray iron. It is extensively used in vise castings and to replace malleable iron in farm machinery. It can be made much more rapidly than malleable iron, and in some ways is superior to it. The process is simple.
Gray-iron castings are placed in an uncovered, wrought-iron box, called an open muffle. A gas flame is used to raise the temperature to about 1550° F. A lid is then placed on the box and the flame turned off. In about 15 minutes the castings are cooled to black heat and may be removed. Castings treated in this way stand much bending and possess many of the properties of a mild steel.
Low-carbon steels have displaced wrought iron for many uses, but it stilt is used to meet many special requirements. Some designers specify its use in chains. One type of work for which it is specially fitted is ornamental ironwork, such as grills and fireplace equipment. It lends itself to this work because it is easily welded on the anvil or with a torch. It can be shaped and bent into intricate forms. It differs from mild steel chiefly in the f act that it contains evenly diffused slag.
The hand process used in making it is puddling. In this process, pig iron is melted in a furnace so constructed that the hot gases coming from a fire are deflected down upon the top of the charge of iron, which has been placed on a hearth separated from the fire by a wall of heat-resisting brick.
The hearth is coated with iron oxide and the charge covered with the same material. As soon as the metal melts it is stirred by a heavy iron bar called a puddle. As carbon and impurities are burned out, the molten mass becomes pasty. When in this condition it is formed into balls, lifted from the furnace, and hammered or rolled into bars.
Chemical processes, such as the Ashton process, are coming into use because they produce the same results as puddling.
Inferior wrought iron is made by binding scrap together, heating it to a welding temperature, and then hammering or rolling it into bars. Wrought iron does not become fluid, so it cannot be used for casting.
If all the carbon and impurities are removed from cast iron and then an amount of carbon up to 2 per cent added in such a way that it combines with the iron, steel will be produced. If a very small amount of carbon is so combined, the steel will be very soft and so closely resemble wrought iron that only expert inspection could detect the difference.
This steel of low-carbon content is the steel used in building bridges and making the frames of skyscrapers. It can be forged, rolled, or welded. If more carbon is added, the steel takes on a new property; it can then be made very hard by heating it to red heat and cooling it quickly. It is then called tool steel. This is the steel used for making files, chisels, and cutting tools. It has a remarkable property in that after it has been hardened it can be tempered to intermediate degrees of hardness.
This property of carbon steel is employed to increase the usefulness of the hammer head. It is first hardened and then, by a reheating process which the blacksmith calls drawing, it is slightly softened. Somewhat different treatment is given to that part of the hammer head through which the handle passes, called the eye. It is drawn' by the blacksmith until it is much softer than the face and peen. The result of this treatment is that the face and peen are hard enough to resist the effect of blows and the eye is left tough enough to keep from splitting open from the effects of repeated shock.
A very hard cold chisel could not be used; it has to be tempered. A simple way to temper the chisel is to brighten its surf ace with a piece of emery cloth and then heat it. As it becomes hotter, the color of the brightened surf ace changes. This change in color is produced by oxidation, and indicates the temperature. If the chisel is to be used for ordinary work, it is plunged or cooled just as the cutting edge starts to turn blue, for it can then be tempered. Some tools would be cooled at dark-straw color and some at a light-straw or yellow, depending upon the use to which they are to be put.
Tool steel, because of its properties, has become one of the most useful materials employed in industry. It can be annealed so that it is easy to work with files or other metal-cutting tools. Annealing consists of heating a metal red-hot and then letting it cool very slowly. After the annealed piece has been shaped as desired, it is hardened and tempered. Dies, punches, and a great variety of tools used in manufacture are made in this way.
For many years steels containing combined carbon in varying amounts served the needs of industry. As these needs increased, experiments were made to increase the uses of steel. Great achievements have been made as a result of these experiments.
Tungsten, vanadium, nickel, chromium, and other metals have been alloyed with steel, both separately and in combination. The high-speed steels used in making lathe tools, drills, reamers, and other tools are examples of these steels. Chromium has been used to make steel resist corrosion. New alloys are being developed constantly to meet special needs.
Copper, zinc, lead, tin, aluminum, and brass are the nonferrous metals most commonly used.
Copper can be worked hot or cold. It can be drawn through dies to produce wires so fine that the eye can hardly see them. It is easy to solder. Copper presents some difficulty in welding because when hot it absorbs gases. When it is hammered or rolled it becomes quite hard. When this occurs, it may be heated and then plunged into water to soften it.
As a metal, zinc's most common use is as an alloy of copper and aluminum. We seldom make things of solid zinc, but it can be cast and soldered. It is used in its sheet form for lining tanks and for roofing.
The most familiar uses of lead are for water pipes and storage batteries. It is easy to cut and shape. It is generally joined by a process which has come to be called lead burning, but is really welding. Lead is also added to brass as an alloy to make the brass machine easily, and it is alloyed with tin to make soft solder.
Tin is used extensively to coat steel; tin cans, for example, are really tin-coated steel cans. We have just mentioned the use of tin in combination with lead to make solder. It alloys with copper to form bronze.
We seldom use aluminum in its pure form. The metal which we commonly call aluminum is really an alloy of aluminum and copper or zinc. When alloyed with manganese or magnesium, aluminum has special properties. It can then be forged, and by special heat treatment can be given great tensile strength; it is then used in airplane construction. Aluminum alloy castings are made in sand and in metal molds. They can be machined very rapidly. Sheet aluminum can be stamped and drawn into intricate shapes.
Aluminum and magnesium metals will be playing an increasingly important part in the industrial and economy life of America. It has been estimated that America's capacity to produce aluminum will be six times as great at the close of World War II as it was before and that her capacity to produce magnesium will be 89 times as great.
Brass is an alloy of copper and zinc, with lead added to make it machine easily. Brass is easy to solder; in sheets it can be formed and stamped; and when hammered or rolled, it becomes hard. Brass wire is rolled to make it suitable for use in making springs. If you should wish to soften brass that has been hardened by hammering, you could heat it and then plunge it into water.