Oxy-Acetylene Welding and Cutting | Page 3

suddenly from a red heat.
_Grades of Irons._--The mechanical properties of cast iron differ
greatly according to the amount of other materials it contains. The most
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 of working if from 2 to 3 per cent is present.
Phosphorus, which hardens and weakens the metal but makes it easier
to cast. Three-tenths per cent of phosphorus serves as a hardening agent
and may be present in good steel if the percentage of carbon is low.
More than this weakens the metal.
Sulphur, which tends to make the metal hard and filled with small
holes.
Manganese, which makes the steel so hard and tough that it can with
difficulty be cut with steel tools. Its hardness is not lessened by
annealing, and it has great tensile strength.
Alloy steel has a varying but small percentage of other elements mixed
with it to give certain desired qualities. Silicon steel and manganese
steel are sometimes classed as alloy steels. This subject is taken up in
the latter part of this chapter under _Alloys_, where the various
combinations and their characteristics are given consideration.
Steel has a tensile strength varying from 50,000 to 300,000 pounds per
square inch, depending on the carbon percentage and the other alloys
present, as well as upon the texture of the grain. Steel is heavier than
cast iron and weighs about the same as wrought iron. It is about
one-ninth as good a conductor of electricity as copper.
Steel is made from cast iron by three principal processes: the crucible,
Bessemer and open hearth.
Crucible steel is made by placing pieces of iron in a clay or graphite
crucible, mixed with charcoal and a small amount of any desired alloy.
The crucible is then heated with coal, oil or gas fires until the iron
melts, and, by absorbing the desired elements and giving up or
changing its percentage of carbon, becomes steel. The molten steel is
then poured from the crucible into moulds or bars for use. Crucible
steel may also be made by placing crude steel in the crucibles in place
of the iron. This last method gives the finest grade of metal and the

crucible process in general gives the best grades of steel for mechanical
use.
[Illustration: Figure 2.--A Bessemer Converter]
Bessemer steel is made by heating iron until all the undesirable
elements are burned out by air blasts which furnish the necessary
oxygen. The iron is placed in a large retort called a converter, being
poured, while at a melting heat, directly from the blast furnace into the
converter. While the iron in the converter is molten, blasts of air are
forced through the liquid, making it still hotter and burning out the
impurities together with the carbon and manganese. These two
elements are then restored to the iron by adding spiegeleisen (an alloy
of iron, carbon and manganese). A converter holds from 5 to 25 tons of
metal and requires about 20 minutes to finish a charge. This