n the furnace by a gas
flame. The silicon and carbon are slowly burned away, and when a test
shows that the desired percentage of carbon is present the steel is run
out of the furnace. _Steel may therefore be defined as the product of
the Bessemer or open-hearth processes._

~Properties of steel.~ Bessemer and open-hearth steel usually contain only
a few tenths of a per cent of carbon, less than 0.1% silicon, and a very
much smaller quantity of phosphorus and sulphur. Any considerable amount
of the latter elements makes the steel brittle, the sulphur affecting it
when hot, and the phosphorus when cold. This kind of steel is used for
structural purposes, for rails, and for nearly all large steel articles.
It is hard, malleable, ductile, and melts at a lower temperature than
wrought iron. It can be forged into shape, rolled into sheets, or cast
in molds.

~Relation of the three varieties of iron.~ It will be seen that wrought
iron is usually very nearly pure iron, while steel contains an
appreciable amount of alloy material, chiefly carbon, and cast iron
still more of the same substances. It is impossible, however, to assign
a given sample of iron to one of these three classes on the basis of its
chemical composition alone. A low carbon steel, for example, may contain
less carbon than a given sample of wrought iron. The real distinction
between the three is the process by which they are made. The product of
the blast furnace is cast iron; that of the puddling furnace is wrought
iron; that of the Bessemer and open-hearth methods is steel.

~Tool steel.~ Steel designed for use in the manufacture of edged tools and
similar articles should be relatively free from silicon and phosphorus,
but should contain from 0.5 to 1.5% carbon. The percentage of carbon
should be regulated by the exact use to which the steel is to be put.
Steel of this character is usually made in small lots from either
Bessemer or open-hearth steel in the following way.

A charge of melted steel is placed in a large crucible and the
calculated quantity of pure carbon is added. The carbon dissolves in the
steel, and when the solution is complete the metal is poured out of the
crucible. This is sometimes called crucible steel.

~Tempering of steel.~ Steel containing from 0.5 to 1.5% carbon is
characterized by the property of "taking temper." When the hot steel is
suddenly cooled by plunging it into water or oil it becomes very hard
and brittle. On carefu