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impurities. Pig iron may be cast into molded shapes to produce

cast iron. There are two types of cast iron, depending upon the rate

of cooHng. Gray cast iron, formed by slow cooling, is cheap, readily

machined, but only half as strong as steel. White cast iron, formed

by quick chilling, is strong, hard, but too brittle for some purposes.

A softer malleable iron is produced by properly heat-treating a special

grade of white cast iron. The mechanical treatment of the highmelting

product is an important part of its production. Wrought

iron, used for pipes, bolts, chains, and grate bars, is produced from

cast iron by reducing the amount of impurities and removing excess

slag, then rolling and hammering the resulting product.

Cast iron, malleable iron, and wrought iron cannot be annealed,

hardened, or tempered as is steel; for many purposes, only steel can

be used. Steel is obtained from pig iron by removing carbon and

some of the impurities and then adding definite proportions of carbon,

manganese, or other elements to obtain the properties desired. There

are four chief types of steel

(1) Crucible steel is made by melting wrought iron with charcoal in

a graphite or fire-clay crucible. Carbon is thus added to the iron to

produce a uniform, hard, brittle product especially adapted for use in

such articles as watch springs, razor and knife blades, and other tools

in which the selling price per pound of steel is high.

(2) Bessemer steel is obtained from molten pig iron by oxidizing

the impurities with a blast of air and then adding the desired amount

of carbon and manganese. It may be produced very cheaply. The

invention of the Bessemer process was perfected just before the American

Civil War and has been largely responsible for the tremendous

expansion of railroads since that time.

(3) Open-hearth steel is superior to Bessemer steel in that impurities

are more completely removed by oxidation in a large open hearth

heated with producer gas or natural gas. It yields a more uniform

product than does the Bessemer process, and today about 80 per

cent of the steel is produced by the open-hearth process. It is used

for rails, bridge girders, armor plate, and other products requiring

a strong but cheap material in large quantity.

(4) Electric steel is obtained by use of the electric furnace, which

permits the iron to be kept in a fluid condition in a controlled atmosphere

while other substances are added as desired. The best alloy

steels are made in this way. Such steels are used when a very tough,

dependable product is needed, regardless of cost, as in the manufacture

of certain parts of modern automobiles and machinery.

Literally thousands of different mixtures of metals (i.e., alloys)

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