important of these contained elements is carbon, which is present to a
degree varying from 2 to 5-1/2 per cent. When iron containing much carbon
is quickly cooled and then broken, the fracture is nearly white in color
and the metal is found to be hard and brittle. When the iron is slowly
cooled and then broken the fracture is gray and the iron is more malleable
and less brittle. If cast iron contains sulphur or phosphorus, it will show
a white fracture regardless of the rapidity of cooling, being brittle and
less desirable for general work.

_Steel._--Steel is composed of extremely minute particles of iron and
carbon, forming a network of layers and bands. This carbon is a smaller
proportion of the metal than found in cast iron, the percentage being from
3/10 to 2-1/2 per cent.

Carbon steel is specified according to the number of "points" of carbon, a
point being one one-hundredth of one per cent of the weight of the steel.
Steel may contain anywhere from 30 to 250 points, which is equivalent to
saying, anywhere from 3/10 to 2-1/2 per cent, as above. A 70-point steel
would contain 70/100 of one per cent or 7/10 of one per cent of carbon by
weight. The percentage of carbon determines the hardness of the steel, also
many other qualities, and its suitability for various kinds of work. The
more carbon contained in the steel, the harder the metal will be, and, of
course, its brittleness increases with the hardness. The smaller the grains
or particles of iron which are separated by the carbon, the stronger the
steel will be, and the control of the size of these particles is the object
of the science of heat treatment.

In addition to the carbon, steel may contain the following:

Silicon, which increases the hardness, brittleness, strength and difficulty