_Cast Iron._--The essential constituents of cast iron are iron and
carbon, the latter forming from 2 to 5 per cent. of the total weight.
Cast iron, however, usually contains varying small amounts of silicon,
sulphur, phosphorus, and manganese.
In cast iron the carbon may exist partly in the free state and partly in
chemical combination with the iron.
In _white cast iron_ the whole of the carbon is in chemical combination
with the iron, while in _grey cast iron_ the carbon is principally in
the free state, that is, simply mixed mechanically with the iron. It is
the free carbon which gives the grey iron its dark appearance. A mixture
of the white and grey varieties of cast iron when melted produces
_mottled cast iron_. The greater the amount of carbon chemically
combined with the iron, the whiter, harder, and more brittle does it
become.
The white cast iron is stronger than the grey, but being more brittle it
is not so suitable for resisting suddenly applied loads. White iron
melts at a lower temperature than grey iron, but after melting it does
not flow so well, or is not so liquid as the grey iron. White iron
contracts while grey iron expands on solidifying. The grey iron,
therefore, makes finer castings than the white. Castings after
solidifying contract in cooling about 1/8 of an inch per foot. Castings
possessing various degrees of strength and hardness are produced by
melting mixtures of various proportions of white and grey cast irons.
White cast iron has a higher specific gravity than grey cast iron.
Cast iron gives little or no warning before breaking. The thickness of
the metal throughout a casting in cast iron should be as uniform as
possible, so that it may cool and therefore contract uniformly
throughout; otherwise some parts may be in a state of initial strain
after the casting has cooled, and will therefore be easier to fracture.
Re-entrant angles should be avoided; such should be rounded out with
fillets.
The presence of phosphorus in cast iron makes it more fusible, and also
more brittle. The presence of sulphur diminishes the strength
considerably.
The grey varieties of cast iron are called _foundry irons_ or _foundry
pigs_, while the white varieties are called _forge irons_ or _forge
pigs_, from the fact that they are used for conversion into wrought
iron.
Amongst iron manufacturers the different varieties of cast iron are
designated by the numbers 1, 2, 3, &c., the lowest number being applied
to the greyest variety.
_Chilled Castings._--When grey cast iron is melted a portion of the free
carbon combines chemically with the iron; this, however, separates out
again if the iron is allowed to cool slowly; but if it is suddenly
cooled a greater amount of the carbon remains in chemical combination,
and a whiter and harder iron is produced. Advantage is taken of this in
making _chilled castings_. In this process the whole or a part of the
mould is lined with cast iron, which, being a comparatively good
conductor of heat, chills a portion of the melted metal next to it,
changing it into a hard white iron to a depth varying from 1/8 to 1/2 an
inch. To protect the cast-iron lining of the mould from the molten metal
it is painted with loam.
_Malleable Cast Iron._--This is prepared by imbedding a casting in
powdered red hematite (an oxide of iron), and keeping it at a bright red
heat for a length of time varying from several hours to several days
according to the size of the casting. By this process a portion of the
carbon in the casting is removed, and the strength and toughness of the
latter become more like the strength and toughness of wrought or
malleable iron.
_Wrought or Malleable Iron._--This is nearly pure iron, and is made from
cast iron by the puddling process, which consists chiefly of raising the
cast iron to a high temperature in a reverberatory furnace in the
presence of air, which unites with the carbon and passes off as gas. In
other words the carbon is burned out. The iron is removed from the
puddling furnace in soft spongy masses called _blooms_, which are
subjected to a process of squeezing or hammering called _shingling_.
These shingled blooms still contain enough heat to enable them to be
rolled into rough _puddled bars_. These puddled bars are of very
inferior quality, having less than half the strength of good wrought
iron. The puddled bars are cut into pieces which are piled together,
reheated, and again rolled into bars, which are called _merchant bars_.
This process of piling, reheating, and re-rolling may be repeated
several times, depending on the quality of iron required. Up to a
certain point the quality of the iron is improved by reheating and
rolling or hammering, but beyond that a repetition of the process
diminishes the strength of the iron.
The process of piling and rolling gives wrought iron a fibrous
structure. When subjected to vibrations for a long time, the structure
becomes crystalline and the iron brittle. The crystalline structure
induced in this way may be removed by the process of _annealing_, which
consists in heating the iron in a furnace, and then allowing it to cool
slowly.
_Forging and Welding._--The process of pressing or hammering wrought
iron when at a red or white heat into any desired shape is called
_forging_. If at a white heat two pieces of wrought iron be brought
together, their surfaces being clean, they may be pressed or hammered
together, so as to form one piece. This is called _welding_, and is a
very valuable property of wrought iron.
_Steel._--This is a compound of iron with a small per-centage of carbon,
and is made either by adding carbon to wrought iron, or by removing some
of the carbon from cast iron.
In the _cementation_ process, bars of wrought iron are imbedded in
powdered charcoal in a fireclay trough, and kept at a high temperature
in a furnace for several days. The iron combines with a portion of the
carbon to form _blister steel_, so named because of the blisters which
are found on the surface of the bars when they are removed from the
furnace.
The bars of blister steel are broken into pieces about 18 inches long,
and tied together in bundles by strong steel wire. These bundles are
raised to a welding heat in a furnace, and then hammered or rolled into
bars of _shear steel_.
To form _cast steel_ the bars of blister steel are broken into pieces
and melted into crucibles.
In the _Siemens-Martin_ process for making steel, cast and wrought iron
are melted together on the hearth of a regenerative gas-furnace.
_Bessemer steel_ is made by pouring melted cast iron into a vessel
called a converter, through which a blast of air is then urged. By this
means the carbon is burned out, and comparatively pure iron remains. To
this is added a certain quantity of 'spiegeleisen,' which is a compound
of iron, carbon, and manganese.
_Hardening and Tempering of Steel._--Steel, if heated to redness and
cooled suddenly, as by immersion in water, is hardened. The degree of
hardness produced varies with the rate of cooling; the more rapidly the
heated steel is cooled, the harder does it become. Hardened steel is
softened by the process of _annealing_, which consists in heating the
hardened steel to redness, and then allowing it to cool slowly. Hardened
steel is _tempered_, or has its degree of hardness lowered, by being
heated to a temperature considerably below that of a red heat, and then
cooling suddenly. The higher the temperature the hardened steel is
raised to, the lower does its 'temper' become.
_Case-hardening._--This is the name given to the process by which the
surfaces of articles made of wrought iron are converted into steel, and
consists in heating the articles in contact with substances rich in
carbon, such as bone-dust, horn shavings, or yellow prussiate of potash.
This process is generally applied to the articles after they are
completely finished by the machine tools or by hand. The coating of
steel produced on the article by this process is hardened by cooling the
article suddenly in water.
_Copper._--This metal has a reddish brown colour, and when pure is very
malleable and ductile, either when cold or hot, so that it may be rolled
or hammered into thin plates, or drawn into wire. Slight traces of
impurities cause brittleness, although from 2 to 4 per cent. of
phosphorus increases its tenacity and fluidity. Copper is a good
conductor of heat and of electricity. Copper is largely used for making
alloys.
_Alloys._--_Brass_ contains two parts by weight of copper to one of
zinc. _Muntz metal_ consists of three parts of copper to two of zinc.
Alloys consisting of copper and tin are called _bronze_ or _gun-metal_.
Bronze is harder the greater the proportion of tin which it contains;
five parts of copper to one of tin produce a very hard bronze, and ten
of copper to one of tin is the composition of a soft bronze. _Phosphor
bronze_ contains copper and tin with a little phosphorus; it has this
advantage over ordinary bronze, that it may be remelted without
deteriorating in quality. This alloy also has the advantage that it may
be made to possess great strength accompanied with hardness, or less
strength with a high degree of toughness.
_Wood._--In the early days of machines wood was largely used in their
construction, but it is now used to a very limited extent in that
direction. _Beech_ and _hornbeam_ are used for the cogs of mortise
wheels. _Yellow pine_ is much used by pattern-makers. _Box_, a heavy,
hard, yellow-coloured wood, is used for the sheaves of pulley blocks,
and sometimes for bearings in machines. _Lignum-vitae_ is a very hard
dark-coloured wood, and remarkable for its high specific gravity, being
1-1/3 times the weight of the same volume of water. This wood is much
used for bearings of machines which are under water.