Cast iron, cast steel, and malleable iron, which we have previously considered, are three forms of the same metal - iron. The difference in their physical characteristics is due solely to a variation in the proportions of certain elements or metaloids combined with the iron.
The metals to be dealt with in this section are termed alloys - that is, mixtures of two or more separate metals. The common alloys in use in the foundry, for casting various machine parts, are made from combinations of copper, tin, and zinc, and are called brass, or bronze.
Although the term brass is held by some authorities to cover any of these combinations, the general classification accepts brass as an alloy of copper and zinc, and bronze as an alloy of copper and tin. In some sections the latter is spoken of as composition.
Bronze has been used by man in all ages. Centuries before the Christian Era the Egyptians employed it for making coin, armor, and weapons, as well as household utensils, and statuettes of their gods. Analyses of many of these ancient relics show the composition to be almost identical with the bronzes of the present day. Brass also was in use before the time of Christ, but unquestionably bronze was of earlier origin.
A short discussion of the separate metals will help in understanding the properties of their alloys.
Copper has a red color; it is hard, ductile, and very tough. It melts at about 2000 degrees F.; but it is difficult to make castings of the pure metal. Copper does not rust as does iron, and is one of the best conductors of heat and electricity. For this reason it is largely used in sheet form as a sheathing metal, and in the form of wire or rods for electrical transmission. Casting copper is put on the market in ingots of special form weighing from 18 to 25 pounds each.
Tin is a white lustrous metal, very malleable, but lacking tenacity. It may be reduced extremely thin by rolling, as is shown by tin foil. It melts at 450 degrees F. When a bar of tin is bent it will give a crackling sound known as the cry, which at once distinguishes it from other metals such as solder, lead, etc., which have similar external appearance. It is put on the market in pigs weighing about 30 pounds and also in bars of about 1 pound each. Its cost is approximately 1 1/2 times that of copper and 5 times as much as zinc. Tin may be cast unalloyed, and is sometimes used to run pattern letters or small duplicate patterns cast in zinc chill molds. The addition of 1/3 to 1/2 by weight of lead gives a cheaper metal, however, and one that will run equally well.
Tin mixed with copper gives greater fluidity, lower melting point, and greater strength, changing the color from red to bright yellow. Serviceable alloys may contain as high as 20 per cent of tin. This gives a metal of golden yellow color, very hard, tough, and difficult to work. With larger percentages of tin the color shades to gray, the metal is hard, brittle, and has little strength, and has no value for engineering purposes.
Zinc has a bluish white color; it is hard, but weak and brittle. The fracture shows very large crystals of characteristic shape. It melts at about 700 degrees F., and shrinks but little in cooling. For this reason it may be used to cast directly for small metal patterns to form chills from which soft-metal castings may be made for duplicating these patterns. If exposed to the air at high temperatures, zinc will volatilize, that is, turn to a gas and burn. It burns with a bluish flame, and throws off clouds of dense white smoke. For this reason great care must be used to keep the air away from it as much as possible when being melted or mixed in an alloy, for, aside from the loss of metal, an oxide is formed in the mixture which impairs the quality of the alloy.
Zinc is known in commerce under two names: when rolled into sheets it is called zinc; when in ingot form for casting, it is called spelter. These ingots are flat, approximately 8 by 1 by 17 inches, and weigh about 30 pounds. In this form they may be easily broken in small pieces for convenience in charging.
Zinc may be added to copper in a very wide range of proportions, the alloy increasing in hardness and losing ductility with the increase in the proportion of zinc. The color changes from the red of the copper to a full yellow when 1/3 zinc is used. Further additions of zinc change the color to red, yellow, violet, and gray. The alloys are serviceable up to 40 or 50 per cent of zinc.
When zinc is mixed with melted metal, considerable reaction or boiling takes place, which tends to make a more thorough mixture and to drive impurities to the surface. For this reason a small proportion of zinc - 2 or 3 per cent - is often stirred into bronze mixtures after the pot is drawn.
Lead has a bluish white color, and considerable luster when freshly cut. It is malleable, soft, and tough, but very weak. It melts at about 600 degrees F.
Lead is not used by itself as an alloy with copper. A very small proportion may be added to the standard mixture for brass or bronze.
Gun metal - for bearings; very tough hard mixture
Steam or valve metal - cute freely; very tough; resists corrosion
Composition metal - for general use on small machine parts
Art bronze - rich color; runs fluid at comparatively low heat
Common yellow brass - for general run of machine castings
Brass - to machine easier than the above; for same purposes
Antifriction metal - for journal boxes
Mixture - for small patterns; runs well; shrinks little
It will cause them to run more fluidly in pouring, and be softer for machining. For this reason, lead is added to bearing mixtures to advantage. But it tends to deaden the color and reduce the conductivity of the metal for electrical purposes.