Its
compressive strength, or resistance to crushing, is very great. It has
excellent wearing qualities and is not easily warped and deformed by heat.
Chilled iron is cast into a metal mould so that the outside is cooled
quickly, making the surface very hard and difficult to cut and giving great
resistance to wear. It is used for making cheap gear wheels and parts that
must withstand surface friction.
_Malleable Cast Iron._--This is often called simply malleable iron. It
is a form of cast iron obtained by removing much of the carbon from cast
iron, making it softer and less brittle. It has a tensile strength of
25,000 to 45,000 pounds per square inch, is easily machined, will stand a
small amount of bending at a low red heat and is used chiefly in making
brackets, fittings and supports where low cost is of considerable
importance. It is often used in cheap constructions in place of steel
forgings. The greatest strength of a malleable casting, like a steel
forging, is in the surface, therefore but little machining should be done.
_Wrought Iron._--This grade is made by treating the cast iron to
remove almost all of the carbon, silicon, phosphorus, sulphur, manganese
and other impurities. This process leaves a small amount of the slag from
the ore mixed with the wrought iron.
Wrought iron is used for making bars to be machined into various parts. If
drawn through the rolls at the mill once, while being made, it is called
"muck bar;" if rolled twice, it is called "merchant bar" (the commonest
kind), and a still better grade is made by rolling a third time.
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