Iron.--Iron, in its pure state, is a soft, white, easily worked metal.
It is the most important of all the metallic elements, and is, next to
aluminum, the commonest metal found in the earth.
Mechanically speaking, we have three kinds of iron: wrought iron, cast
iron and steel. Wrought iron is very nearly pure iron; cast iron
contains
carbon and silicon, also chemical impurities; and steel contains a
definite proportion of carbon, but in smaller quantities than cast iron.
Pure iron is never obtained commercially, the metal always being mixed
with various proportions of carbon, silicon, sulphur, phosphorus, and
other elements, making it more or less suitable for different purposes.
Iron is magnetic to the extent that it is attracted by magnets, but it
does not retain magnetism itself, as does steel. Iron forms, with other
elements, many important combinations, such as its alloys, oxides, and
sulphates.
Cast Iron.--Metallic iron is separated from iron ore in the blast
furnace
(Figure 1), and when allowed to run into moulds is called cast iron.
This form is used for engine cylinders and pistons, for brackets,
covers,
housings and at any point where its brittleness is not objectionable.
Good cast iron breaks with a gray fracture, is free from blowholes or
roughness, and is easily machined, drilled, etc. Cast iron is slightly
lighter than steel, melts at about 2,400 degrees in practice, is about
one-eighth as good an electrical conductor as copper and has a tensile
strength of 13,000 to 30,000 pounds per square inch. 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.
Wrought iron is being gradually replaced in use by mild rolled steels.
Wrought iron is slightly heavier than cast iron, is a much better
electrical conductor than either cast iron or steel, has a tensile
strength of 40,000 to 60,000 pounds per square inch and costs slightly
more than steel. Unlike either steel or cast iron, wrought iron does not
harden when cooled 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.
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 makes the cheapest steel.
Open hearth steel is made by placing the molten iron in a receptacle
while currents of air pass over it, this air having itself been highly
heated by just passing over white hot brick (Figure. 3). Open hearth
steel is considered more uniform and reliable than Bessemer, and is used
for springs, bar steel, tool steel, steel plates, etc.
Aluminum is one of the commonest industrial metals. It is used for gear
cases, engine crank cases, covers, fittings, and wherever lightness and
moderate strength are desirable.
Aluminum is about one-third the weight of iron and about the same weight
as glass and porcelain; it is a good electrical conductor (about
one-half
as good as copper); is fairly strong itself and gives great strength to
other metals when alloyed with them. One of the greatest advantages of
aluminum is that it will not rust or corrode under ordinary conditions.
The granular formation of aluminum makes its strength very unreliable
and
it is too soft to resist wear.
Copper is one of the most important metals used in the trades, and the
best commercial conductor of electricity, being exceeded in this respect
only by silver, which is but slightly better. Copper is very malleable
and ductile when cold, and in this state may be easily worked under the
hammer. Working in this way makes the copper stronger and harder, but
less ductile. Copper is not affected by air, but acids cause the
formation of a green deposit called verdigris.
Copper is one of the best conductors of heat, as well as electricity,
being used for kettles, boilers, stills and wherever this quality is
desirable. Copper is also used in alloys with other metals, forming an
important part of brass, bronze, german silver, bell metal and gun
metal.
It is about one-eighth heavier than steel and has a tensile strength of
about 25,000 to 50,000 pounds per square inch.
Lead.--The peculiar properties of lead, and especially its quality of
showing but little action or chemical change in the presence of other
elements, makes it valuable under certain conditions of use. Its
principal use is in pipes for water and gas, coverings for roofs and
linings for vats and tanks. It is also used to coat sheet iron for
similar uses and as an important part of ordinary solder.
Lead is the softest and weakest of all the commercial metals, being very
pliable and inelastic. It should be remembered that lead and all its
compounds are poisonous when received into the system. Lead is more than
one-third heavier than steel, has a tensile strength of only about 2,000
pounds per square inch, and is only about one-tenth as good a conductor
of electricity as copper.
Zinc.--This is a bluish-white metal of crystalline form. It is brittle
at
ordinary temperatures and becomes malleable at about 250 to 300 degrees
Fahrenheit, but beyond this point becomes even more brittle than at
ordinary temperatures. Zinc is practically unaffected by air or moisture
through becoming covered with one of its own compounds which immediately
resists further action. Zinc melts at low temperatures, and when heated
beyond the melting point gives off very poisonous fumes.
The principal use of zinc is as an alloy with other metals to form
brass,
bronze, german silver and bearing metals. It is also used to cover the
surface of steel and iron plates, the plates being then called
galvanized.
Zinc weighs slightly less than steel, has a tensile strength of 5,000
pounds per square inch, and is not quite half as good as copper in
conducting electricity.
Tin resembles silver in color and luster. Tin is ductile and malleable
and slightly crystalline in form, almost as heavy as steel, and has a
tensile strength of 4,500 pounds per square inch.
The principal use of tin is for protective platings on household
utensils
and in wrappings of tin-foil. Tin forms an important part of many alloys
such as babbitt, Britannia metal, bronze, gun metal and bearing metals.
Nickel is important in mechanics because of its combinations with other
metals as alloys. Pure nickel is grayish-white, malleable, ductile and
tenacious. It weighs almost as much as steel and, next to manganese, is
the hardest of metals. Nickel is one of the three magnetic metals, the
others being iron and cobalt. The commonest alloy containing nickel is
german silver, although one of its most important alloys is found in
nickel steel. Nickel is about ten per cent heavier than steel, and has a
tensile strength of 90,000 pounds per square inch.
Platinum.--This metal is valuable for two reasons: it is not affected by
the air or moisture or any ordinary acid or salt, and in addition to
this
property it melts only at the highest temperatures. It is a fairly good
electrical conductor, being better than iron or steel. It is nearly
three
times as heavy as steel and its tensile strength is 25,000 pounds per
square inch.
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Of course you'll find the full welding story and all the techniques and
hundreds of diagrams included in Welding Secrets Revealed...
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