The alloys of metals are, almost without exception, more important from an
industrial standpoint than the metals themselves. There are innumerable
possible combinations, the most useful of which are here classed under the
head of the principal metal entering into their composition.

_Steel._--Steel may be alloyed with almost any of the metals or
elements, the combinations that have proven valuable numbering more than a
score. The principal ones are given in alphabetical order, as follows:

Aluminum is added to steel in very small amounts for the purpose of
preventing blow holes in castings.

Boron increases the density and toughness of the metal.

Bronze, added by alloying copper, tin and iron, is used for gun metal.

Carbon has already been considered under the head of steel in the section
devoted to the metals. Carbon, while increasing the strength and hardness,
decreases the ease of forging and bending and decreases the magnetism and
electrical conductivity. High carbon steel can be welded only with
difficulty. When the percentage of carbon is low, the steel is called "low
carbon" or "mild" steel. This is used for rods and shafts, and called
"machine" steel. When the carbon percentage is high, the steel is called
"high carbon" steel, and it is used in the shop as tool steel. One-tenth
per cent of carbon gives steel a tensile strength of 50,000 to 65,000
pounds per square inch; two-tenths per cent gives from 60,000 to 80,000;
four-tenths per cent gives 70,000 to 100,000, and six-tenths per cent
gives 90,000 to 120,000.

Chromium forms chrome steel, and with the further addition of nickel is