Alloys
An alloy is a relatively homogeneous metallic material which is not a
chemical element. The constituents of any alloy must be soluble in each
other in the molten state, otherwise the resulting solid will have no
uniformity of structure. Gold and lead dissolve in each other when molten,
but are practically insoluble in each other when solid. On the other hand,
copper and lead are insoluble in each other in the liquid state and
separate into distinct layers. Copper and lead alloys are used successfully
for bearings in railway service. In this case, other elements present in
small amounts promote a fair distribution of the lead particles throughout
the copper matrix.
Constituents of Alloys. When an alloy is examined under the microscope, it
will be found to have a definite appearance, depending upon the way in which
the elements constituting the alloy are combined. The constituents of an
alloy must be soluble in the liquid state. This solubility may continue
even after solidification has taken place, in which case the structure will
appear homogeneous. If, on the other hand, the two constituents are
insoluble in the solid state, microscope examination will reveal two
distinct constituents. Under certain conditions three constituents may
appear. The constituents of an alloy may be classified as follows:
(a) Pure metals; (b) metalloids.
Chemical compounds: (a) of pure metals; (b) of metals and metalloids; (c) of
metals and non-metals.
3. Metallic solid solutions of the above: (a) soluble in each other in all
proportions; (b) partially soluble in each other.
Therefore, an alloy may be made up of a single homogeneous solid solution,
or it may be made up of pure metals, or chemical compounds, or solid
solutions, or of various mixtures of these constituents.
Steel. Steel is an alloy of iron and carbon in which the carbon content does
not exceed 1.7 per cent. It is produced from pig iron and scrap by processes
which remove some carbon and a certain amount of sulphur, phosphorus,
manganese, and silicon. Steels containing only iron and carbon, together
with a small amount of impurities, are classed as plain carbon steels, while
those containing certain elements intentionally added are classed as alloy
steels. In general, plain carbon steels are classified according to the
carbon content, the method of manufacture, and the uses for which their
properties fit them.
Fundamental Considerations in the Manufacture of Steel. The manufacture of
steel is a process by which a melted charge of pig iron or pig and scrap has
the bulk of its impurities removed by oxidation.
The method and extent of removal of these impurities depend upon the process
employed in refining the pig iron. The various methods of refining may be
classified according to the chemical nature of the process and according to
the type of furnace used in the process.
Alloy Steels. In many cases, the engineer requires steel with a tensile
strength, yield point, hardness, response to heat treatment, etc., that can
not be obtained with plain carbon steels. Higher tensile strength, yield
point, endurance limit, and impact strength can be obtained by adding
elements such as nickel, chromium, molybdenum, and vanadium to the steel.
Alloy steel may be defined as steel to which elements other than carbon are
added in sufficient amount to impart desirable mechanical properties. To
obtain full advantage of these properties, alloy steels should be
heat-treated. The alloying elements affect the properties of the steels in
several ways:
By changing the critical temperatures at which austenite is trans-formed
into ferrite with consequent precipitation of iron carbide;
By slowing up the rate of transformation;
By forming solid solutions of the alloying element and the iron to produce
a stronger ferrite (M, u, Mn);
By producing carbides of the alloying elements (Cr, V, Mo, W).
There are many different alloy steels and it is very difficult in many cases
for the engineer to make a selection that will satisfy his requirements. In
many cases, alloy steels are used where good plain carbon steel would be
equally satisfactory.
Bronze is usually an alloy of copper and tin. Bronzes are commercially
important because of their relatively high strength, their ability to resist
corrosion, and the ease with which they may be cast and machined. Brass is
usually an alloy of copper and zinc. Brasses have most of the desirable
characteristics of bronzes with the added advantage that they may be drawn
and rolled as well as cast. Standard brass is an alloy of 65% copper with
zinc; yellow brass contains 62% copper, 2 to 4% lead with zinc and traces of
tin and iron. Yellow brass is inexpensive, gives excellent castings, and is
readily machined.